Role of microglia in neuronal cell death in prion disease

Role of cytokines and chemokines in prion infections of the central nervous system

Prion infections of the central nervous system (CNS) are characterised by a reactive gliosis and the subsequent degeneration of neuronal tissue. The activation of glial cells, which precedes neuronal death, is likely to be initially caused by the deposition of misfolded, proteinase K-resistant, isoforms (termed PrP(res)) of the prion protein (PrP) in the brain. Cytokines and chemokines released by PrP(res)-activated glia cells may contribute directly or indirectly to the disease development by enhancement and generalisation of the gliosis and via cytotoxicity for neurons. However, the actual role of prion-induced glia activation and subsequent cytokine/chemokine secretion in disease development is still far from clear. In the present work, we review our present knowledge concerning the functional biology of cytokines and chemokines in prion infections of the CNS.

The cellular prion protein modulates phagocytosis and inflammatory response

The cellular prion protein (PrPc) is a glycoprotein anchored by glycosylphosphatidylinositol (GPI) to the cell surface and is abundantly expressed in the central nervous system. It is also expressed in a variety of cell types of the immune system. We investigated the role of PrPc in the phagocytosis of apoptotic cells and other particles. Macrophages from mice with deletion of the Prnp gene showed higher rates of phagocytosis than wild-type macrophages in in vitro assays. The elimination of GPI-anchored proteins from the cell surface of macrophages from wild-type mice rendered these cells as efficient as macrophages derived from knockout mice. In situ detection of phagocytosis of apoptotic bodies within the retina indicated augmented phagocytotic activity in knockout mice. In an in vivo assay of acute peritonitis, knockout mice showed more efficient phagocytosis of zymosan particles than wild-type mice. In addition, leukocyte recruitment was altered in knockout mice, as compared with wild type. The data show that PrPc modulates phagocytosis in vitro and in vivo. This activity is described for the first time and may be important for normal macrophage functions as well as for the pathogenesis of prion diseases.

Identification of differentially expressed genes in scrapie-infected mouse brains by using global gene expression technology

The pathogenesis of prion diseases, a class of transmissible fatal neurodegenerative diseases in humans and animals, is still unclear. The aim of this study was to identify the differentially regulated genes that correlate with the development of prion diseases for a better understanding of their pathological mechanisms. We employed Affymetrix Mouse Expression Arrays 430A containing >22,000 transcripts and compared the global gene expression profiles from brains of mice who were intracerebrally inoculated with scrapie strains ME7 and RML with those from brains of uninfected and mock-infected mice. The microarray data were analyzed by Significance Analysis of Microarrays, revealing 121 genes whose expression increased at least twofold in both ME7- and RML-infected mouse brains, with an estimated false discovery rate of < or =5%. These genes encode proteins involved in proteolysis, protease inhibition, cell growth and maintenance, the immune response, signal transduction, cell adhesion, and molecular metabolism. The time course of expression generally showed up-regulation of these genes from 120 days postinoculation (dpi) for ME7-inoculated mouse brains and from 90 dpi for RML-inoculated mouse brains. The onset of elevated expression correlated temporally with the onset of PrP(Sc) accumulation and the activation of glia, which may have contributed to neuronal cell death. Among the differentially regulated genes reported in the present study, the emergence of genes for several cathepsins and S100 calcium binding proteins was conspicuous. These and other genes reported here may represent novel potential diagnostic and therapeutic targets for prion disease.

BSE and vCJD cause disturbance to uric acid levels

Bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD) are two new members of the family of neurodegenerative conditions termed prion diseases. Oxidative damage has been shown to occur in prion diseases and is potentially responsible for the rapid neurodegeneration that is central to the pathogenesis of these diseases. An important nonenzymatic antioxidant in the brain is uric acid. Analysis of uric acid in the brain and cerebrospinal fluid (CSF) of cases of BSE and CJD showed a specific reduction in CSF levels for both BSE and variant CJD, but not sporadic CJD. Further studies based on cell culture experiments suggested that uric acid in the brain was produced by microglia. Uric acid was also shown to inhibit neurotoxicity of a prion protein peptide, production of the abnormal prion protein isoform (PrP(Sc)) by infected cells, and polymerization of recombinant prion protein. These findings suggest that changes in uric acid may aid differential diagnosis of vCJD. Uric acid could be used to inhibit cell death or PrP(Sc) formation in prion disease.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.

Prions can infect primary cultured neurons and astrocytes and promote neuronal cell death

Transmissible spongiform encephalopathies arise as a consequence of infection of the central nervous system by prions, where neurons and glial cells are regarded as primary targets. Neuronal loss and gliosis, associated with the accumulation of misfolded prion protein (PrP), are hallmarks of prion diseases; yet the mechanisms underlying such disorders remain unclear. Here we introduced a cell system based on primary cerebellar cultures established from transgenic mice expressing ovine PrP and then exposed to sheep scrapie agent. Upon exposure to low doses of infectious agent, such cultures, unlike cultures originating from PrP null mice, were found to accumulate de novo abnormal PrP and infectivity, as assessed by mouse bioassay. Importantly, using astrocyte and neuron/astrocyte cocultures, both cell types were found capable of sustaining efficient prion propagation independently, leading to the production of proteinase K-resistant PrP of the same electrophoretic profile as in diseased brain. Moreover, contrasting with data obtained in chronically infected cell lines, late-occurring apoptosis was consistently demonstrated in the infected neuronal cultures. Our results provide evidence that primary cultured neural cells, including postmitotic neurons, are permissive to prion replication, thus establishing an approach to study the mechanisms involved in prion-triggered neurodegeneration at a cellular level.

Neurons and astrocytes respond to prion infection by inducing microglia recruitment

The accumulation and activation of microglial cells at sites of amyloid prion deposits or plaques have been documented extensively. Here, we investigate the in vivo recruitment of microglial cells soon after intraocular injection of scrapie-infected cell homogenate (hgtsc+) using immunohistochemistry on retinal sections. A population of CD11b/CD45-positive microglia was specifically detected within the ganglion and internal plexiform retinal cell layers by 2 d after intravitreal injection of hgtsc+. Whereas no chemotactism properties were ascribed to hgtsc+ alone, a massive migration of microglial cells was observed by incubating primary cultured neurons and astrocytes with hgtsc+ in a time- and concentration-dependent manner. hgtsc+ triggered the recruitment of microglial cells by interacting with both neurons and astrocytes by upregulation of the expression levels of a broad spectrum of neuronal and glial chemokines. We show that, in vitro and in vivo, the microglia migration is at least partly under the control of chemokine receptor-5 (CCR-5) activation, because highly specific CCR-5 antagonist TAK-779 significantly reduced the migration rate of microglia. Activated microglia recruited in the vicinity of prion may, in turn, cause neuronal cell damage by inducing apoptosis. These findings provide insight into the understanding of the cell-cell communication that takes place during the development of prion diseases.

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.

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.

Protofibrils, pores, fibrils, and neurodegeneration: separating the responsible protein aggregates from the innocent bystanders

Many neurodegenerative diseases, including Alzheimer's and Parkinson's and the transmissible spongiform encephalopathies (prion diseases), are characterized at autopsy by neuronal loss and protein aggregates that are typically fibrillar. A convergence of evidence strongly suggests that protein aggregation is neurotoxic and not a product of cell death. However, the identity of the neurotoxic aggregate and the mechanism by which it disables and eventually kills a neuron are unknown. Both biophysical studies aimed at elucidating the precise mechanism of in vitro aggregation and animal modeling studies support the emerging notion that an ordered prefibrillar oligomer, or protofibril, may be responsible for cell death and that the fibrillar form that is typically observed at autopsy may actually be neuroprotective. A subpopulation of protofibrils may function as pathogenic amyloid pores. An analogous mechanism may explain the neurotoxicity of the prion protein; recent data demonstrates that the disease-associated, infectious form of the prion protein differs from the neurotoxic species. This review focuses on recent experimental studies aimed at identification and characterization of the neurotoxic protein aggregates.

Impairment of superoxide dismutase activation by N-terminally truncated prion protein (PrP) in PrP-deficient neuronal cell line

Previous studies have reported a neuroprotective role for cellular prion protein (PrP(C)) against apoptosis induced by serum deprivation in an immortalized prion protein gene (Prnp)-deficient neuronal cell line, but the mechanisms remain unclear. In this study, to investigate the mechanisms by which PrP(C) prevents apoptosis, the authors compared apoptosis of Prnp(-/-) cells with that of Prnp(-/-) cells expressing the wild-type PrP(C) or PrP(C) lacking N-terminal octapeptide repeat region under serum-free conditions. Re-introduction of Prnp rescued cells from apoptosis, upregulated superoxide dismutase (SOD) activity, enhanced superoxide anion elimination, and inhibited caspase-3/9 activation. On the other hand, N-terminally truncated PrP(C) enhanced apoptosis accompanied by potentiation of superoxide production and caspase-3/9 activation due to inhibition of SOD. These results suggest that PrP(C) protects Prnp(-/-) cells from apoptosis via superoxide- and caspase-3/9-dependent pathways by upregulating SOD activity. Furthermore, the octapeptide repeat region of PrP(C) plays an essential role in regulating apoptosis and SOD activity.

Different chromogranin immunoreactivity between prion and a-beta amyloid plaque

Brain lesions in Creutzfeldt-Jakob disease (CJD) include spongiform change, neuronal loss, amyloid plaques, astrogliosis and microglial activation. Microglia are thought to play a key role in prion-induced neurodegeneration. However, the intermediate molecules supporting relationships between neurons and microglia are still unknown. Chromogranins (Cg) are soluble glycophosphoproteins that can activate microglial cells leading to a neurotoxic phenotype. The immunoreactive patterns of CgA and CgB were investigated in CJD and compared to those observed in Alzheimer's disease. We found that CgB, but not CgA, immunoreactivity was selectively associated with prion protein deposits, whereas CgA was only seen in Abeta plaques. This suggests a specific influence of the constitutive amyloid protein on chromogranin secretion and a role of CgB in the CJD neurodegenerative process.

Contribution of two conserved glycine residues to fibrillogenesis of the 106-126 prion protein fragment. Evidence that a soluble variant of the 106-126 peptide is neurotoxic

The fibrillogenic peptide corresponding to the residues 106-126 of the prion protein sequence (PrP 106-126) is largely used to explore the neurotoxic mechanisms underlying the prion disease. However, whether the neuronal toxicity of PrP 106-126 is caused by a soluble or fibrillar form of this peptide is still unknown. The aim of this study was to correlate the structural state of this peptide with its neurotoxicity. Here we show that the two conserved Gly114 and Gly119 residues, in force of their intrinsic flexibility, prevent the peptide assuming a structured conformation, favouring its aggregation in amyloid fibrils. The substitution of both Gly114 and Gly119 with alanine residues (PrP 106-126 AA mutated peptide) reduces the flexibility of this prion fragment and results in a soluble, beta-structured peptide. Moreover, PrP 106-126 AA fragment was highly toxic when incubated with neuroblastoma cells, likely behaving as a neurotoxic protofibrillar intermediate of the wild-type PrP 106-126. These data further confirm that the fibrillar aggregation is not necessary for the induction of the toxic effects of PrP 106-126.

In vivo and in vitro neurotoxicity of the human prion protein (PrP) fragment P118-135 independently of PrP expression

We recently demonstrated that the 118-135 putative transmembrane domain of prion protein (PrP) exhibited membrane fusogenic properties and induced apoptotic neuronal cell death of rat cortical neurons, independently of its aggregation state. The aim of the present study was to analyze the in vivo neurotoxicity of the prion fragment P118-135 and to evaluate the potential role of the physiological isoform of PrP in the P118-135-induced cell death. Here, we demonstrate that the nonfibrillar P118-135 is cytotoxic to retinal neurons in vivo as monitored by intravitreal inoculation and recording of the electrical activity of retina and tissue examination. Moreover, knock-out PrP gene mice exhibit similar sensitivity to the nonfibrillar P118-135-induced cell death and electrical perturbations, strongly suggesting that cell death occurs independently of PrP expression. Interestingly, a variant nonfusogenic P118-135 peptide (termed P118-135theta) had no effects on in vivo neuronal viability, suggesting that the P118-135-induced cell death is mediated by its membrane destabilizing properties. These data have further been confirmed in vitro. We show that the fusogenic peptide P118-135 induces death of cultured neurons from both wild-type and knock-out PrP gene mice via an apoptotic-mediated pathway, involving early caspase activation and DNA fragmentation. Altogether these results emphasize the neurotoxicity of the fusogenic nonfibrillar PrP transmembrane domain and indicate that fibril formation and PrP expression are not obligatory requirements for neuronal cell death. The use of synthetic prion peptides could provide insights into the understanding of neuronal loss mechanisms that take place during the development of the various types of spongiform encephalopathies.

In vivo and in vitro neurotoxicity of the human prion protein (PrP) fragment P118-135 independently of PrP expression

We recently demonstrated that the 118-135 putative transmembrane domain of prion protein (PrP) exhibited membrane fusogenic properties and induced apoptotic neuronal cell death of rat cortical neurons, independently of its aggregation state. The aim of the present study was to analyze the in vivo neurotoxicity of the prion fragment P118-135 and to evaluate the potential role of the physiological isoform of PrP in the P118-135-induced cell death. Here, we demonstrate that the nonfibrillar P118-135 is cytotoxic to retinal neurons in vivo as monitored by intravitreal inoculation and recording of the electrical activity of retina and tissue examination. Moreover, knock-out PrP gene mice exhibit similar sensitivity to the nonfibrillar P118-135-induced cell death and electrical perturbations, strongly suggesting that cell death occurs independently of PrP expression. Interestingly, a variant nonfusogenic P118-135 peptide (termed P118-135theta) had no effects on in vivo neuronal viability, suggesting that the P118-135-induced cell death is mediated by its membrane destabilizing properties. These data have further been confirmed in vitro. We show that the fusogenic peptide P118-135 induces death of cultured neurons from both wild-type and knock-out PrP gene mice via an apoptotic-mediated pathway, involving early caspase activation and DNA fragmentation. Altogether these results emphasize the neurotoxicity of the fusogenic nonfibrillar PrP transmembrane domain and indicate that fibril formation and PrP expression are not obligatory requirements for neuronal cell death. The use of synthetic prion peptides could provide insights into the understanding of neuronal loss mechanisms that take place during the development of the various types of spongiform encephalopathies.

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.

Microglia from Creutzfeldt-Jakob disease-infected brains are infectious and show specific mRNA activation profiles

Neurons are often assumed to be the principal sites for replication of the infectious agents causing Creutzfeldt-Jakob disease (CJD), scrapie, and bovine spongiform encephalopathy because they express high levels of normal and pathological prion protein (PrP). However, isolated brain cell types have not been evaluated for either infection or gene expression. Microglia purified from CJD-infected mice showed infectivity comparable to that of starting brain homogenate but expressed approximately 50-fold less PrP. CJD-infected microglia also displayed morphological changes indicative of cellular activation. To determine the molecular pathways of activation, we evaluated pertinent transcripts, including those linked to inflammation. Semiquantitative reverse transcription-PCR showed a >4-fold increase in cathepsin S, an enzyme important in antigen presentation, the cytokine interleukin-1beta, and the chemokine B-lymphocyte chemoattractant. The profile of microglial changes induced by the CJD agent differed substantially from activation induced by bacterial lipopolysaccharide or by beta-amyloid, a structure comparable to pathological PrP. These microglial studies emphasize migratory hematopoietic cells in the dispersion, and possibly replication, of the CJD agent. The low PrP levels in these highly infectious and activated cells further support the concept that pathological PrP is the result of infection rather than the infectious agent itself. Because microglia develop a specific pattern of responses to the CJD agent, microglial markers may be exploited in the diagnosis of these spongiform encephalopathies.

Phenotyping of protein-prion (PrPsc)-accumulating cells in lymphoid and neural tissues of naturally scrapie-affected sheep by double-labeling immunohistochemistry

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases characterized by amyloid deposition of protein-prion (PrPsc), the pathogenic isoform of the host cellular protein PrPc, in the immune and central nervous systems. In the absence of definitive data on the nature of the infectious agent, PrPsc immunohistochemistry (IHC) constitutes one of the main methodologies for pathogenesis studies of these diseases. In situ PrPsc immunolabeling requires formalin fixation and paraffin embedding of tissues, followed by post-embedding antigen retrieval steps such as formic acid and hydrated autoclaving treatments. These procedures result in poor cellular antigen preservation, precluding the phenotyping of cells involved in scrapie pathogenesis. Until now, PrPsc-positive cell phenotyping relied mainly on morphological criteria. To identify these cells under the PrPsc IHC conditions, a new, rapid, and highly sensitive PrPsc double-labeling technique was developed, using a panel of screened antibodies that allow specific labeling of most of the cell subsets and structures using paraffin-embedded lymphoid and neural tissues from sheep, leading to an accurate identification of ovine PrPsc-accumulating cells. This technique constitutes a useful tool for IHC investigation of scrapie pathogenesis and may be applicable to the study of other ovine infectious diseases.

Copper-dependent functions for the prion protein

Prion diseases such as bovine spongiform encephalopathy and Creutzfeldt-Jakob disease are fatal neurodegenerative diseases. These diseases are characterized by the conversion of a normal cellular protein, the prion protein, to an abnormal isoform that is thought to be responsible for both pathogenesis in the disease and the infectious nature of the disease agent. Understanding the biology and metabolism of the normal prion protein is therefore important for understanding the nature of these diseases. This review presents evidence for the normal function of the cellular prion protein, which appears to depend on its ability to bind copper (Cu). There is now considerable evidence that the prion protein is an antioxidant. Once the prion protein binds Cu, it may have an activity like that of a superoxide dismutase. Conversion of the prion protein to an abnormal isoform might lead to a loss of antioxidant protection that could be responsible for neurodegeneration in the disease.

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.

Synaptic pathology and cell death in the cerebellum in Creutzfeldt-Jakob disease

Prion protein (PrP(c)) is a cell membrane glycoprotein particularly abundant in the synapses. Prion diseases are characterized by the replacement of the normal PrPc by a protease-resistant, sheet-containing isoform (PrP(CJD), PrP(Sc), PrP(BSE)) that is pathogenic. Creutzfeldt-Jakob disease (CJD) in humans, scrapie (Sc) in sheep and goats, and bovine spongiform encephalopathy (BSE) in cattle are typical prion diseases. Classical CJD can be presented as sporadic, infectious or familial, whereas the new variant of CJD (nvCJD) is considered a BSE-derived human disease. Spongiform degeneration, glial proliferation, involving astrocytes and microglia, neuron loss and abnormal PrP deposition are the main neuopathological findings in most human and animal prion diseases. Yet recent data point to synapses as principal targets of abnormal PrP deposition. Loss of synapses is an early abnormality in experimental scrapie. Decreased expression of crucial proteins linked to exocytosis and neurotransmission, covering synaptophysin, synaptosomal-associated protein of 25,000 mol wt (SNAP-25), synapsins, syntaxins and Rab3a occurs in the cerebral cortex and cerebellum in sporadic CJD. Moreover, impairment of glomerular synapses and attenuation of parallel fiber pre-synaptic terminals on Purkinje cell dendrites is a cardinal consequence of abnormal PrP metabolism in CJD. Accumulation of synaptic proteins in the soma and axonal torpedoes of Purkinje cells suggests additional impairment of axonal transport. Increase in nuclear DNA vulnerability leading to augmented numbers of cells bearing nuclear DNA fragments is a common feature in the brains of humans affected by prion diseases examined at post-mortem, but also in archival biopsy samples processed with the method of in situ end-labeling of nuclear DNA fragmentation. This form of cell death is reminiscent of apoptosis found in experimental scrapie in rodents. It is not clear that all forms of cell death in human and animal prion diseases are due to apoptosis. Yet new observations have shown cleaved (active) caspase-3 (17 kDa), a main executioner of apoptosis, expressed in scattered cells in the brains of mice with experimental scrapie and in the cerebellum of patients with sporadic CJD. Together, these data suggest activation of the caspase pathway of apoptosis in human and animal prion diseases.

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.

Cellular prion protein transduces neuroprotective signals

To test for a role for the cellular prion protein (PrP(c)) in cell death, we used a PrP(c)-binding peptide. Retinal explants from neonatal rats or mice were kept in vitro for 24 h, and anisomycin (ANI) was used to induce apoptosis. The peptide activated both cAMP/protein kinase A (PKA) and Erk pathways, and partially prevented cell death induced by ANI in explants from wild-type rodents, but not from PrP(c)-null mice. Neuroprotection was abolished by treatment with phosphatidylinositol-specific phospholipase C, with human peptide 106-126, with certain antibodies to PrP(c) or with a PKA inhibitor, but not with a MEK/Erk inhibitor. In contrast, antibodies to PrP(c) that increased cAMP also induced neuroprotection. Thus, engagement of PrP(c) transduces neuroprotective signals through a cAMP/PKA-dependent pathway. PrP(c) may function as a trophic receptor, the activation of which leads to a neuroprotective state.

Ablation of the chemokine monocyte chemoattractant protein-1 delays retrograde neuronal degeneration, attenuates microglial activation, and alters expression of cell death molecules

The mechanisms regulating retrograde neuronal degeneration and subsequent death of thalamic neurons following cortical injury are not well understood. However, the delay in the onset of retrograde cell death and observed morphological changes are consistent with apoptosis. Our previous studies demonstrated that monocyte chemoattractant protein-1 (MCP-1), a beta-chemokine that attracts cells of monocytic origin to sites of injury, is rapidly and specifically expressed in the lateral geniculate nucleus following visual cortical lesions. To determine the potential role of MCP-1 in retrograde degeneration, the present study examined the effect of genetic deletion of MCP-1 (MCP-1 KO or -/-) or its high affinity receptor CCR2 (CCR2 KO or -/-) on thalamic microglial activation and neuronal cell death following aspiration lesions of the visual cortex in adult mice. Deletion of the MCP-1 gene delayed microglial activation and transiently improved the survival of thalamic neurons. Deletion of the CCR2 receptor resulted in a significant increase in apoptosis as measured by nucleosomal fragmentation after injury compared to wild-type mice, but did not alter neuron survival, suggesting that glial apoptosis is increased in the receptor knockout mice. Investigation of Bcl-2, Bax, Fas, Fas ligand (FasL) and activated caspase-3, key regulators of apoptosis that can be modulated by cytokines, revealed complex alterations of mRNA and protein levels in MCP-1(-/-) and CCR2(-/-) mice. As examples, Bcl-2 protein was detected in wild-type, but not in MCP-1(-/-) mice. Caspase-3 activity was higher in MCP-1(-/-) mice compared to wild-type and CCR2(-/-) mice at 5 days after injury. High levels of activated caspase-3 correlate with the beginning of a period of delayed, but rapid cell death in the thalami of MCP-1(-/-) mice. In summary, our data strongly suggest that MCP-1 is involved in early microglial response to axotomy and that modulation of this chemokine could provide a novel strategy for improved neuronal survival following injury to the central nervous system.

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.

Molecular advances in understanding inherited prion diseases

The prion diseases are neurodegenerative disorders that have attracted great interest because of the possible link between bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (CTD) in humans. Possible transmission of these diseases has been linked to a single protein termed the prion protein. This protein is an abnormal isoform of a normal synaptic glycoprotein. The majority of prion diseases does not appear to be caused by transmission of an infectious agent but occur spontaneously with no known cause. The strongest supporting evidence that the prion protein is the causative agent in prion disease comes from specific inheritable forms of prion disease which are linked to single point mutations in the prion protein gene. Paradoxically, these point mutations, although autosomal dominant with 100% penetrance do not lead to disease until late in life. Molecular techniques are now being used extensively to determine how these point-mutations alter the prion protein's normal structure and activity. This review deals with the latest insights into how inherited mutations in the prion protein gene lead to neurodegenerative disease.

Metal imbalance and compromised antioxidant function are early changes in prion disease

The prion protein (PrP) has been shown to bind copper. In the present study we have investigated whether prion disease in a mouse scrapie model resulted in modification of metal concentrations. We found changes in the levels of copper and manganese in the brains of scrapie-infected mice prior to the onset of clinical symptoms. Interestingly, we noted a major increase in blood manganese in the early stages of disease. Analysis of purified PrP from the brains of scrapie-infected mice also showed a reduction in copper binding to the protein and a proportional decrease in antioxidant activity between 30 and 60 days post-inoculation. We postulate that alterations in trace-element metabolism as a result of changes in metal binding to PrP are central to the pathological modifications in prion disease.

p38 MAP kinase mediates the cell death induced by PrP106-126 in the SH-SY5Y neuroblastoma cells

Prion diseases are neurodegenerative pathologies characterized by the accumulation in the brain of a protease-resistant form of the prion protein (PrP(c)), named PrP(Sc). A synthetic peptide homologous to residues 106-126 of PrP (PrP106-126) maintains many PrP(Sc) characteristics. We investigated the intracellular signaling responsible for the PrP106-126-dependent cell death of SH-SY5Y, a cell line derived from a human neuroblastoma. In this cell line, PrP106-126 induced apoptotic cell death and caused activation of caspase-3, although the blockade of this enzyme did not inhibit cell death. The p38 MAP kinase blockers, SB203580 and PD169316, prevented the apoptotic cell death evoked by PrP106-126 and Western blot analysis revealed that the exposure of the cells to the peptide induced p38 phosphorylation. Taken together, our data suggest that the p38 MAP kinase pathway can mediate the SH-SY5Y cell death induced by PrP106-126.

Adult human microglia secrete cytokines when exposed to neurotoxic prion protein peptide: no intermediary role for prostaglandin E2

Prion diseases are characterized by accumulation of protease resistant isoforms of prion protein (termed PrP(SC)), glial activation and neurodegeneration. The time course of PrP deposition, appearance of activated microglia, and of neuronal apoptosis in experimentally-induced prion disease suggests that microglial activation precedes the process of neuronal loss. Activated microglia and inflammatory mediators, including cytokines and prostaglandin E2 (PGE2) co-localize with PrP deposits. In vitro, mouse microglia secrete neurotoxic agents and interleukins (IL)-1 and IL-6, when exposed to synthetic peptides representing the neurotoxic fragment of PrP. In this study, adult human microglia were found to secrete IL-6 and TNF-alpha upon exposure to synthetic fibrillar PrP105-132, the putative transmembrane domain of PrP. Little cytokine release occurred following exposure of microglia to C-terminally amidated, nonfibrillar PrP105-132, suggesting that the degree of fibrillarity of PrP peptides affects their biological properties. Non-steroidal anti-inflammatory drugs (NSAIDs) are thought to exert beneficial effects in neurodegenerative disorders through suppressive effects on microglial activation and on cyclooxygenase (COX) activity. Since microglial COX-2 expression and PGE(2) synthesis are increased in human and experimental prion diseases, we investigated the effects of the NSAIDs indomethacin and BF389, an experimental COX-2 selective inhibitor, on the PrP105-132-induced microglial IL-6 and TNF-alpha synthesis in vitro. No inhibitory effects of the NSAIDs were observed. Furthermore, PrP105-132 did not stimulate microglial PGE(2) synthesis. We conclude that, unlike IL-1beta-induced IL-6 synthesis in astrocytes, the PrP-induced IL-6 synthesis in human adult microglia is not PGE2 mediated.

Lack of prion protein expression results in a neuronal phenotype sensitive to stress

The prion protein is a highly conserved glycoprotein expressed most highly in the synapse. Evidence has recently been put forward to suggest that the prion protein is an antioxidant. However, the functional importance of the prion protein has been disputed; it is claimed that mice genetically ablated to lack prion protein expression are normal and have no specific phenotype. We have reexamined the phenotype of prion protein knockout mice and found that there are multiple biochemical changes in the mice, including increased levels of nuclear factor NF-kappaB and Mn superoxide dismutase, COX-IV decreased levels of Cu/Zn superoxide dismutase activity, decreased p53, and altered melatonin levels. Additionally, cultured cells from these mice are more sensitive to a range of insults, all linked to increased neuronal sensitivity to oxidative stress. These results imply that prion protein knockout mice are more sensitive to oxidative stress and have an altered phenotype that must be taken into account when considering the additional effects of increased levels of proteins such as Doppel. The implication of these results is that the consequence of genetic ablation of genes must include biochemical analysis as well as analyses of possible developmental and behavioral changes.

Toxicity of novel C-terminal prion protein fragments and peptides harbouring disease-related C-terminal mutations

Mice expressing a C-terminal fragment of the prion protein instead of wild-type prion protein die from massive neuronal degeneration within weeks of birth. The C-terminal region of PrPc (PrP121-231) expressed in these mice has an intrinsic neurotoxicity to cultured neurones. Unlike PrPSc, which is not neurotoxic to neurones lacking PrPc expression, PrP121-231 was more neurotoxic to PrPc-deficient cells. Human mutations E200K and F198S were found to enhance toxicity of PrP121-231 to PrP-knockout neurones and E200K enhanced toxicity to wild-type neurones. The normal metabolic cleavage point of PrPc is approximately amino-acid residue 113. A fragment of PrPc corresponding to the whole C-terminus of PrPc (PrP113-231), which is eight amino acids longer than PrP121-231, lacked any toxicity. This suggests the first eight amino residues of PrP113-121 suppress toxicity of the toxic domain in PrP121-231. Addition to cultures of a peptide (PrP112-125) corresponding to this region, in parallel with PrP121-231, suppressed the toxicity of PrP121-231. These results suggest that the prion protein contains two domains that are toxic on their own but which neutralize each other's toxicity in the intact protein. Point mutations in the inherited forms of disease might have their effects by diminishing this inhibition.

Activation of microglia cells is dispensable for the induction of rat retroviral spongiform encephalopathy

In the course of retroviral CNS infections, microglia activation has been observed frequently, and it has been hypothesized that activated microglia produce and secrete neurotoxic products like proinflammatory cytokines, by this promoting brain damage. We challenged this hypothesis in a rat model for neurodegeneration. In a kinetic study, we found that microglia cells of rats neonatally inoculated with neurovirulent murine leukemia virus (MuLV) NT40 became infected in vivo to maximal levels within 9-13 days postinoculation (d.p.i.). Beginning from 13 d.p.i., degenerative alterations, i.e., vacuolization of neurons and neuropil were found in cerebellar and other brain-stem nuclei. Elevated numbers of activated microglia cells--as revealed by immunohistochemical staining with monoclonal antibody ED1--were first detected at 19 d.p.i. and were always locally associated with degenerated areas but not with nonaltered, yet infected, brain regions. Both neuropathological changes and activated microglia cells increased in intensity and numbers, respectively, with ongoing infection but did not spread to other than initially affected brain regions. By ribonuclease protection assays, we were unable to detect differences in the expression levels of tumor-necrosis-factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and interleukin-6 (IL-6) in microglia cells nor in total brains from infected versus uninfected rats. Our results suggest that the activation of microglia in the course of MuLV neurodegeneration is rather a reaction to, and not the cause of, neuronal damage. Furthermore, overt expression of the proinflammatory cytokines TNF-alpha, IL-1beta, and IL-6 within the CNS is not required for the induction of retroviral associated neurodegeneration in rats.

Phenothiazines: potential management of Creutzfeldt-Jacob disease and its variants

Creutzfeldt-Jakob disease acquired from bovines (nvCJD) has been responsible for nearly 100 deaths in the UK and thousands more may die in the years to come. New variant CJD (nvCJD) is incurable and although clinical diagnosis is becoming more precise, the diagnosis is only certain at autopsy. Phenothiazine derivatives inhibit production of prions, the disease causing agent, in cultured neuroblastoma cells, and an advanced case of nvCJD was recently brought to remission by the use of these agents in combination with an antimalarial. In this review we present direct and circumstantial evidence in support of a model describing the manner by which the intracellular antimicrobial activity of phenothiazines might cause the destruction of intracellular prions.

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.

Prion diseases: what is the neurotoxic molecule?

A great deal of effort has been devoted during the past 20 years to defining the chemical nature of prions, the infectious agents responsible for transmissible spongiform encephalopathies. In contrast, much less attention has been paid to elucidating how prions actually damage the central nervous system. Although it is commonly assumed that PrP(Sc), the protein constituent of infectious prions, is the primary culprit, increasing evidence indicates that this may not be the case. Several alternative molecular forms of PrP are reasonable candidates for the neurotoxic species in prion diseases, although it is still too early to tell whether these or other ones will turn out to be the true instigating factors. The cellular pathways activated by neurotoxic forms of PrP that ultimately result in neuronal death are also being investigated, and several possible mechanisms have been uncovered, including the operation of quality control processes in the endoplasmic reticulum. Elucidating the distinction between the infectious and neurotoxic forms of PrP has important implications for designing therapy of prion diseases, as well as for understanding pathogenic mechanisms operative in other neurodegenerative disorders and the role of prion-like states in biology.

Killing of prion-damaged neurones by microglia

The loss of neurones that occurs in the transmissible spongiform encephalopathies, or prion diseases, can be reproduced in vitro by incubating neuronal cultures with either peptides derived from the prion protein or with partially purified prion preparations. In the present studies, the extent of neuronal loss on exposure to these prions or prion peptides was increased by the addition of microglia, a process that was dependent upon the number of microglia added, the concentration of prions/peptides present and the degree of fibrillarity of the prion peptides. Microglia also killed scrapie-infected neuroblastoma cells expressing infectious PrP(SC). Microglia secreted low amounts of interleukin (IL)-6 when incubated with peptides alone but up to 10 times as much IL-6 when incubated with peptide-treated neurones, suggesting that microglia recognise peptide-induced changes in neurones.

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.

Prion-induced neuronal damage--the mechanisms of neuronal destruction in the subacute spongiform encephalopathies

Prion diseases are characterized by the accumulation of a specific disease-associated isoform of the prion protein (PrP), termed PrPSc, which is the main, if not the only, component of the infectious agent termed prion. PrPSc is derived by an autocatalytic post-translational process involving conformational changes from the normal host-encoded isoform of the prion protein, termed PrPC. PrPC is a copper-binding glycoprotein attached to the cell membrane of neurons and other cells by means of a GPI anchor. The pattern of neurodegeneration differs between variants of prion disease and is related to the pattern of PrPSc deposition and differences in susceptibility of different cell types to the disease process. The pattern of PrPSc deposition depends on the strain of the agent and the PrP genotype of the host. Strain properties of prions appear to be related to different pathological conformations of PrPSc. Neuronal cell death is a salient feature in the pathology of prion diseases. Histological and electron microscopical studies have shown that cell death in prion disease occurs by apoptosis. Apoptosis of neuronal cells can also be induced in vitro by exposure to PrPSc or a neurotoxic peptide fragment corresponding to amino acids 106-126 of human prion protein (PrP106-126). Both in vitro and in vivo, the toxicity of PrPSc and PrP fragments appears to depend on neuronal expression of PrPC and on microglial activation. Activated microglial cells release pro-inflammatory cytokines and reactive oxygen species. Cell culture experiments suggest an important role of microglia-mediated oxidative stress in the induction of neuronal cell death. Only limited data are available on direct effects of PrPSc on neuronal cells. Potential effects include increased formation of an aberrant transmembrane form of PrP, termed CtmPrP, and changes in plasma membrane properties. In addition to direct and indirect toxic effects of PrPSc, a loss of function of PrPC may contribute to neuronal cell death. Potential mechanisms include disturbances in cerebral copper metabolism and antioxidative defense mechanisms. A better understanding of the pathogenesis of neuronal cell death in prion diseases may also have important therapeutic implications in the future.

Copper and prion disease

The prion protein is a cell surface glyco-protein expressed by neurones. Its function has remained elusive until it was recently shown to be a copper binding protein. There is now strong evidence that the prion protein has a role in normal brain copper metabolism. Prion protein expression alters copper uptake into cells and enhances copper incorporation into superoxide dismutase. Furthermore the prion protein itself can act as a superoxide dismutase. One aspect of prion disease is the conversion of functional prion protein into an aggregated amyloid. This conversion may alter the function of the prion protein or abolish it. These results suggest that prion disease may involve disturbance to brain copper homeostasis.

Neurotoxicity but not infectivity of prion proteins can be induced reversibly in vitro

Prion diseases include Creutzfeldt-Jakob disease in humans, scrapie in sheep and bovine spongiform encephalopathy. The hallmark of prion diseases is the accumulation of an abnormal isoform (PrP(Sc)) of the cellular prion protein accompanied by neuronal cell death and astroglial proliferation. To characterize the correlation between PrP secondary and quarternary structure and their biological effects we assayed soluble and aggregated forms of PrP 27-30, the N-terminal truncated form of PrP(Sc), as well as the corresponding recombinant PrP(90-231) for their neurotoxicity and infectivity. PrP was kept soluble in 0.2% SDS and subsequently re-aggregated either by diluting the SDS or by adding acetonitril. The neurotoxicity of the re-aggregated states were comparable to that of prion rods (PrP 27-30) whereas the soluble forms had no neurotoxic effects. The solubilized PrP 27-30 showed no significant infection upon re-aggregation as determined by bioassays in Syrian golden hamsters. The recombinant PrP did not exhibit infectivity in any state.

Sublethal concentrations of prion peptide PrP106-126 or the amyloid beta peptide of Alzheimer's disease activates expression of proapoptotic markers in primary cortical neurons

Neurodegenerative disorders such as prion diseases and Alzheimer's disease (AD) are characterized by neuronal dysfunction and accumulation of amyloidogenic protein. In vitro studies have demonstrated that these amyloidogenic proteins can induce cellular oxidative stress and therefore may contribute to the neuronal dysfunction observed in these illnesses. Although the neurotoxic pathways are not fully elucidated, recent studies in AD have demonstrated up-regulation of caspases in neurons treated with amyloid beta (Abeta) peptide, suggesting involvement of apoptotic processes. To examine the role of proapoptotic pathways in prion diseases we treated primary mouse cortical neurons with the toxic prion protein peptide PrP106-126 and measured caspase activation and annexin V binding. We found that PrP106-126 induced a rapid and marked elevation in caspase 3, 6, and 8-like activity in neuronal cultures. Increased annexin V binding was observed predominantly on cortical cell neurites in peptide-treated cultures. Interestingly, these effects were induced by sublethal (5-50 microM) or lethal (100-200 microM) concentrations of PrP106-126. Sublethal concentrations of PrP106-126 maintained elevated caspase activation for at least 10 days with no loss of cell viability. Abeta1-40 also up-regulated caspase 3 activity and annexin V binding at both sublethal (5 microM) and lethal (25 microM) concentrations. There were no changes to proapoptotic marker expression in cultures treated with scrambled PrP106-126 (200 microM) or Abeta1-28 (25 microM) peptides. These studies demonstrate that amyloidogenic peptides can induce prolonged activation of proapoptotic marker expression in cultured neurons even at sublethal concentrations. These effects could contribute to chronic neuronal dysfunction and increase susceptibility to additional metabolic insults in neurodegenerative disorders. If so, targeting of therapeutic strategies against neuronal caspase activation early in the disease course could be beneficial in AD and prion diseases.

Antioxidant activity related to copper binding of native prion protein

We have developed a method to affinity-purify mouse prion protein (PrP(c)) from mouse brain and cultured cells. PrP(c) from mouse brain bound three copper atoms; PrP(c) from cultured cells bound between one and four copper atoms depending on the availability of copper in the culture medium. Purified PrP(c) exhibited antioxidant activity, as determined by spectrophotometric assay. Incubation of PrP(c) with the neurotoxic peptide, PrP106-126, inactivated the superoxide dismutase-like activity. Culture experiments showed that PrP(c) protects cells against oxidative stress relative to the amount of copper it binds. These results suggest that PrP(c) is a copper-binding protein which can incorporate varying amounts of copper and exhibit protective antioxidant activity.