The stimulation of inducible nitric-oxide synthase by the prion protein fragment 106--126 in human microglia is tumor necrosis factor-alpha-dependent and involves p38 mitogen-activated protein kinase

Systematic validation and assessment of immunohistochemical markers for central nervous system pathology in cetaceans, with emphasis on auditory pathways

Cetacean neuropathology is a developing field that aims to assess structural and neurochemical changes involved in neurodegenerative, infectious and traumatic processes, however markers used previously in cetaceans have rarely undergone systematic validation. This is a prerequisite to investigating the potential damage inflicted on the cetacean auditory system by anthropogenic noise. In order to assess apoptotic, neuroinflammatory and structural aberrations on a protein level, the baseline expression of biomarker proteins has to be characterized, implementing a systematic approach to validate the use of anti-human and anti-laboratory animal antibodies in dolphin tissues. This approach was taken to study 12 different antibodies associated with hypoxic-ischemic, inflammatory, plastic and excitatory-inhibitory changes implicated in acoustic trauma within the ventral cochlear nuclei and inferior colliculi of 20 bottlenose dolphins (Tursiops truncatus). Out of the 12 tested antibodies, pro-apoptotic protease factor 1 (Apaf-1), diacylglycerolkinase-ζ (DGK-ζ), B-cell lymphoma related protein 2 (Bcl-2), amyloid-β peptide (Aβ) and neurofilament 200 (NF200) were validated employing Western blot analyses and immunohistochemistry (IHC). The results of the validation process indicate specific patterns of immunoreactivity that are comparable to those reported in other mammals, thus suggesting a key panel of IHC biomarkers of pathological processes in the cetacean brain. As a consequence, the antibodies tested in this study may constitute a valid tool for supporting existing diagnostic methods in neurological diseases. The approach of systematic validation of IHC markers in cetaceans is proposed as a standard practice, in order for results to be transparent, reliable and comparable.

Microglia in Prion Diseases: Angels or Demons?

Prion diseases are rare transmissible neurodegenerative disorders caused by the accumulation of a misfolded isoform (PrP^Sc) of the cellular prion protein (PrP^C) in the central nervous system (CNS). Neuropathological hallmarks of prion diseases are neuronal loss, astrogliosis, and enhanced microglial proliferation and activation. As immune cells of the CNS, microglia participate both in the maintenance of the normal brain physiology and in driving the neuroinflammatory response to acute or chronic (e.g., neurodegenerative disorders) insults. Microglia involvement in prion diseases, however, is far from being clearly understood. During this review, we summarize and discuss controversial findings, both in patient and animal models, suggesting a neuroprotective role of microglia in prion disease pathogenesis and progression, or—conversely—a microglia-mediated exacerbation of neurotoxicity in later stages of disease. We also will consider the active participation of PrP^C in microglial functions, by discussing previous reports, but also by presenting unpublished results that support a role for PrP^C in cytokine secretion by activated primary microglia.

Translational Research in Alzheimer's and Prion Diseases

Translational neuroscience integrates the knowledge derived by basic neuroscience with the development of new diagnostic and therapeutic tools that may be applied to clinical practice in neurological diseases. This information can be used to improve clinical trial designs and outcomes that will accelerate drug development, and to discover novel biomarkers which can be efficiently employed to early recognize neurological disorders and provide information regarding the effects of drugs on the underlying disease biology. Alzheimer’s disease (AD) and prion disease are two classes of neurodegenerative disorders characterized by incomplete knowledge of the molecular mechanisms underlying their occurrence and the lack of valid biomarkers and effective treatments. For these reasons, the design of therapies that prevent or delay the onset, slow the progression, or improve the symptoms associated to these disorders is urgently needed. During the last few decades, translational research provided a framework for advancing development of new diagnostic devices and promising disease-modifying therapies for patients with prion encephalopathies and AD. In this review, we provide present evidence of how supportive can be the translational approach to the study of dementias and show some results of our preclinical studies which have been translated to the clinical application following the ‘bed-to-bench-and-back’ research model.

The Brain NO Levels and NOS Activities Ascended in the Early and Middle Stages and Descended in the Terminal Stage in Scrapie-Infected Animal Models

The infections of prion agents may cause progressive and fatal neurodegenerative diseases in humans and a serial of animal species. Previous studies have proposed that the levels of nitric oxide (NO) and nitric oxide synthase (NOS) in the brains of some neurodegeneration diseases changed, while S-nitrosylation (SNO) of many brain proteins altered in prion diseases. To elucidate the potential changes of brain NO levels during prion infection, the NO levels and NOS activities in the brain tissues of three scrapie experimental rodents were measured, including scrapie agent 263 K-infected hamsters and 139A- and ME7-infected mice. Both NO levels and NOS activities, including total NOS (TNOS) and inducible NOS (iNOS), were increased at the terminal stages of scrapie-infected animals. Assays of the brain samples collected at different time points during scrapie infection showed that the NO levels and NOS activities started to increase at early stage, reached to the peak in the middle stage, and dropped down at late stage. Western blots for brain iNOS revealed increased firstly and decreased late, especially in the brains of 139A- and ME7-infected mice. In line with those alterations, the levels of the SNO forms of several selected brain proteins such as aquaporin-1 (AQP1), calcium/calmodulin-dependent protein kinase II (CaMKII), neurogranin, and opalin, underwent similar changing trends, while their total protein levels did not change obviously during scrapie infection. Our data here for the first time illustrate the changing profile of brain NO and NOS during prion infection. Time-dependent alterations of brain NO level and the associated protein S-nitrosylation process may contribute greatly to the neuropathological damage in prion diseases.

Pathogenic prion protein fragment (PrP106-126) promotes human immunodeficiency virus type-1 infection in peripheral blood monocyte-derived macrophages

Transfusion of blood and blood products contaminated with the pathogenic form of prion protein Prp(sc), thought to be the causative agent of variant a Creutzfeldt-Jakob disease (vCJD), may result in serious consequences in recipients with a compromised immune system, for example, as seen in HIV-1 infection. In the present study, we demonstrate that treatment of peripheral blood monocyte-derived macrophages (MDM) with PrP106-126, a synthetic domain of PrP(sc) that has intrinsic functional activities related to the full-length protein, markedly increased their susceptibility to HIV-1 infection, induced cytokine secretion, and enhanced their migratory behavior in response to N-formyl-l-methionyl-l-leucyl-l-phenylalanine (fMLP). Live-cell imaging of MDM cultured in the presence of PrP106-126 showed large cell clusters indicative of cellular activation. Tyrosine kinase inhibitor STI-571, protein kinase C inhibitor K252B, and cyclin-dependent kinase inhibitor olomoucine attenuated PrP106-126-induced altered MDM functions. These findings delineate a previously undefined functional role of PrP106-126-mediated host cell response in promoting HIV-1 pathogenesis.

Celecoxib Inhibits Prion Protein 90-231-Mediated Pro-inflammatory Responses in Microglial Cells

Activation of microglia is a central event in the atypical inflammatory response occurring during prion encephalopathies. We report that the prion protein fragment encompassing amino acids 90-231 (PrP90-231), a model of the neurotoxic activity of the pathogenic prion protein (PrP(Sc)), causes activation of both primary microglia cultures and N9 microglial cells in vitro. This effect was characterized by cell proliferation arrest and induction of a secretory phenotype, releasing prostaglandin E2 (PGE2) and nitric oxide (NO). Conditioned medium from PrP90-231-treated microglia induced in vitro cytotoxicity of A1 mesencephalic neurons, supporting the notion that soluble mediators released by activated microglia contributes to the neurodegeneration during prion diseases. The neuroinflammatory role of COX activity, and its potential targeting for anti-prion therapies, was tested measuring the effects of ketoprofen and celecoxib (preferential inhibitors of COX1 and COX2, respectively) on PrP90-231-induced microglial activation. Celecoxib, but not ketoprofen significantly reverted the growth arrest as well as NO and PGE2 secretion induced by PrP90-231, indicating that PrP90-231 pro-inflammatory response in microglia is mainly dependent on COX2 activation. Taken together, these data outline the importance of microglia in the neurotoxicity occurring during prion diseases and highlight the potentiality of COX2-selective inhibitors to revert microglia as adjunctive pharmacological approach to contrast the neuroinflammation-dependent neurotoxicity.

Analyses of the similarity and difference of global gene expression profiles in cortex regions of three neurodegenerative diseases: sporadic Creutzfeldt-Jakob disease (sCJD), fatal familial insomnia (FFI), and Alzheimer's disease (AD)

Neurodegenerative disease is a general designation for the disorders that are progressive loss of structure or function and final death of neurons, including Alzheimer's, Parkinson's, Huntington's, prion diseases, etc. In this study, we comparatively analyzed 21 individual microarray data sets of the cortex tissues from 11 sporadic Creutzfeldt-Jakob disease (sCJD), 3 fatal familial insomnia (FFI), 3 Alzheimer's disease (AD), and 4 normal controls. After normalization, a collection of 730 differently expressed sets (DESets) were obtained by comparison of the data of three diseases with their original controls. Principal component analysis (PCA) showed a background-related distribution within the groups of FFI, AD, and normal control, but two apparently different subgroups within the group of sCJD were observed. Review of the clinical materials of 11 sCJD patients identified the difference in brain PrP(Sc) deposits between two subgroups. Hierarchical cluster analysis illustrated the relatively independent clusters of normal controls, FFIs, six sCJD cases (subgroup 1) with more PrP(Sc) deposits, respectively, while an overlapped cluster of five cases of sCJD2 (subgroup 2) with less PrP(Sc) deposits and AD patients. Despite of the presence of special gene expressions, many common features were found among those neurodegenerative diseases. The most commonly changed biological processes (BPs) were signal transduction, synaptic transmission, and neuropeptide signaling pathway. The most commonly changed pathways were MAPK signaling pathway, Parkinson's disease, and oxidative phosphorylation. Our data here provide the similarity and difference in global gene expressions among the patients with sCJD, FFI, and AD, which may help to understand the common mechanism of neurodegenerative diseases.

PP2 and piceatannol inhibit PrP106-126-induced iNOS activation mediated by CD36 in BV2 microglia

Prion diseases are a group of transmissible fatal neurodegenerative disorders of humans and animals, including bovine spongiform encephalopathy, scrapie, and Creutzfeldt-Jakob disease. Microglia, the resident macrophages of the central nervous system, are exquisitely sensitive to pathological tissue alterations, altering their morphology and phenotype to adopt a so-called activated state and perform immunological functions in response to pathophysiological brain insults. Although recent findings have provided valuable insights into the role microglia play in the proinflammatory events observed in prion, the intracellular signaling molecules responsible for the initiation of these responses remain to be elucidated. It seems that microglial activation involve PrP106-126 binding and the activation of cell surface immune and adhesion molecules such as CD36 and integrins, with the subsequent recruitment of Src family tyrosine kinases such as Fyn, Lyn, and Syk kinases. In the present study, we show that CD36 is involved in PrP106-126-induced microglial activation and that PP2 and piceatannol (Pic) can abrogate neurotoxic prion peptides-induced inducible nitric oxide synthase activation in microglia. These findings unveil a previously unrecognized role of PP2 and Pic as Src family kinase Fyn and the tyrosine kinase Syk inhibitor involved in neurotoxic prion peptides-microglia interactions, thus providing new insights into mechanisms underlying the activation of microglia by neurotoxic prion peptides.

Prion peptide uptake in microglial cells--the effect of naturally occurring autoantibodies against prion protein

In prion disease, a profound microglial activation that precedes neurodegeneration has been observed in the CNS. It is still not fully elucidated whether microglial activation has beneficial effects in terms of prion clearance or whether microglial cells have a mainly detrimental function through the release of pro-inflammatory cytokines. To date, no disease-modifying therapy exists. Several immunization attempts have been performed as one therapeutic approach. Recently, naturally occurring autoantibodies against the prion protein (nAbs-PrP) have been detected. These autoantibodies are able to break down fibrils of the most commonly used mutant prion variant PrP106-126 A117V and prevent PrP106-126 A117V-induced toxicity in primary neurons. In this study, we examined the phagocytosis of the prion peptide PrP106-126 A117V by primary microglial cells and the effect of nAbs-PrP on microglia. nAbs-PrP considerably enhanced the uptake of PrP106-126 A117V without inducing an inflammatory response in microglial cells. PrP106-126 A117V uptake was at least partially mediated through scavenger receptors. Phagocytosis of PrP106-126 A117V with nAbs-PrP was inhibited by wortmannin, a potent phosphatidylinositol 3-kinase inhibitor, indicating a separate uptake mechanism for nAbs-PrP mediated phagocytosis. These data suggest the possible mechanisms of action of nAbs-PrP in prion disease.

Comparative analysis of gene expression profiles between cortex and thalamus in Chinese fatal familial insomnia patients

Fatal familial insomnia (FFI) is a special subtype of genetic human prion diseases that is caused by the D178N mutation of the prion protein gene (PRNP). According to the surveillance data from 2006, FFI accounts for about half of all genetic prion disease cases in China. In this study, global expression patterns of the thalamus and parietal cortex from three patients with FFI were analyzed by Affymetrix Human Genome U133+ 2.0 chip. A total of 1,314 genes in the thalamus and 332 ones in the parietal lobe were determined to be differentially expressed genes (DEGs). The percentage of upregulated DEGs is much less in the thalamus (19.3 %) than that in the parietal lobe (42.8 %). Moreover, 255 of those DEGs showed the same altering tendencies in both tested regions, including 99 upregulated and 156 downregulated ones. The reliability of the results was confirmed by the real-time RT-PCR assays. There were 1,152 and 531 biological processes affected in the thalamus and the parietal lobe, respectively, as well as 391 overlapping ones in both regions. The most significantly changed molecular functions included transcription and DNA-dependent regulation of transcription, RNA splicing, mitochondrial electron transport, etc. The changed functions in the thalamus contained more numbers of DEGs than parietal lobe. According to KEGG classification, there were 167 and 115 different pathways changed in the thalamus and the parietal lobe, respectively, while 102 were changed in both. Interestingly, the top three changed pathways in the three groups mentioned above were Parkinson's disease, Alzheimer's disease, and oxidative phosphorylation. These results demonstrate the greater damage in the thalamus than in the parietal lobe during FFI pathogenesis, which is consistent with previous pathological observations. This study aims to describe the global expression profiles in various brain regions of FFI while proposing useful clues for understanding the pathogenesis of FFI and selecting potential biomarkers for diagnostic and therapeutic tools.

Global transcriptional profiling of the postmortem brain of a patient with G114V genetic Creutzfeldt-Jakob disease

Familial or genetic Creutzfeldt-Jakob disease (fCJD or gCJD) is an inherent human prion disease caused by mutation of the prion protein gene (PRNP). In the present study, global expression patterns of the parietal cortex from a patient with G114V gCJD were analyzed using the Affymetrix Human Genome U133+ 2.0 chip with a commercial normal human parietal cortex RNA pool as a normal control. In total, 8,774 genes showed differential expression; among them 2,769 genes were upregulated and 6,005 genes were downregulated. The reliability of the results was confirmed using real-time RT-PCR assays. The most differentially expressed genes (DEGs) were involved in transcription regulation, ion transport, transcription, cell adhesion, and signal transduction. The genes associated with gliosis were upregulated and the genes marked for neurons were downregulated, while the transcription of the PRNP gene remained unaltered. A total of 169 different pathways exhibited significant changes in the brain of G114V gCJD. The most significantly regulated pathways included Alzheimer's and Parkinson's disease, oxidative phosphorylation, regulation of actin cytoskeleton, MAPK signaling and proteasome, which have previously been linked to prion diseases. In addition, we found some pathways that have rarely been explored in regards to prion diseases that were also significantly altered in G114V gCJD, such as axon guidance, gap junction and purine metabolism. The majority of the genes in the 10 most altered pathways were downregulated. The data of the present study provide useful insights into the pathogenesis of G114V gCJD and potential biomarkers for diagnostic and therapeutic purposes.

Induction of ligand-specific PrP (C) signaling in human neuronal cells

Cellular prion protein (PrP (C) ) has attracted considerable attention for its role in transmissible spongiform encephalopathies (TSEs). In spite of being a point of intense research effort critical questions still remain regarding the physiological function of PrP (C) and how these functions may change with the conversion of the protein into the infectious and pathological conformation (PrP (Sc) ). While emerging evidence suggests PrP (C/Sc) are involved in signal transduction there is little consensus on the signaling pathways associated with the normal and diseased states. The purported involvement of PrP (C) in signal transduction, and the association of TSEs with neural pathology, makes kinome analysis of human neurons an interesting and appropriate model to characterize patterns of signal transduction following activation of PrP (C) by two commonly employed experimental ligands; antibody-induced dimerization by 6H4 and the amino acids 106-126 PrP peptide fragment (PrP 106-126). Analysis of the induced kinome responses reveals distinct patterns of signaling activity following each treatment. Specifically, stimulation of human neurons with the 6H4 antibody results in alterations in mitogen activated protein kinase (MAPK) signaling pathways while the 106-126 peptide activates growth factor related signaling pathways including vascular endothelial growth factor (VEGF) signaling and the phosphoinositide-3 kinase (PI3K) pathway. These pathways were validated through independent functional assays. Collectively these results indicate that stimulation of PrP (C) with distinct ligands, even within the same cell type, results in unique patterns of signaling. While this investigation highlights the apparent functional versatility of PrP (C) as a signaling molecule and may offer insight into cellular mechanisms of TSE pathology it also emphasizes the potential dangers associated with attributing activation of specific intracellular events to particular receptors through artificial models of receptor activation.

CD36 participates in PrP(106-126)-induced activation of microglia

Microglial activation is a characteristic feature of the pathogenesis of prion diseases. The molecular mechanisms that underlie prion-induced microglial activation are not very well understood. In the present study, we investigated the role of the class B scavenger receptor CD36 in microglial activation induced by neurotoxic prion protein (PrP) fragment 106-126 (PrP(106-126)). We first examined the time course of CD36 mRNA expression upon exposure to PrP(106-126) in BV2 microglia. We then analyzed different parameters of microglial activation in PrP(106-126)-treated cells in the presence or not of anti-CD36 monoclonal antibody (mAb). The cells were first incubated for 1 h with CD36 monoclonal antibody to block the CD36 receptor, and were then treated with neurotoxic prion peptides PrP(106-126). The results showed that PrP(106-126) treatment led to a rapid yet transitory increase in the mRNA expression of CD36, upregulated mRNA and protein levels of proinflammatory cytokines (IL-1β, IL-6 and TNF-α), increased iNOS expression and nitric oxide (NO) production, stimulated the activation of NF-κB and caspase-1, and elevated Fyn activity. The blockade of CD36 had no effect on PrP(106-126)-stimulated NF-κB activation and TNF-α protein release, abrogated the PrP(106-126)-induced iNOS stimulation, downregulated IL-1β and IL-6 expression at both mRNA and protein levels as well as TNF-α mRNA expression, decreased NO production and Fyn phosphorylation, reduced caspase-1 cleavage induced by moderate PrP(106-126)-treatment, but had no effect on caspase-1 activation after treatment with a high concentration of PrP(106-126). Together, these results suggest that CD36 is involved in PrP(106-126)-induced microglial activation and that the participation of CD36 in the interaction between PrP(106-126) and microglia may be mediated by Src tyrosine kinases. Our findings provide new insights into the mechanisms underlying the activation of microglia by neurotoxic prion peptides and open perspectives for new therapeutic strategies for prion diseases by modulation of CD36 signaling.

Neuronal phosphorylated RNA-dependent protein kinase in Creutzfeldt-Jakob disease

The mechanisms of neuronal apoptosis in Creutzfeldt-Jakob disease (CJD) and their relationship to accumulated prion protein (PrP) are unclear. A recent cell culture study showed that intracytoplasmic PrP may induce phosphorylated RNA-dependent protein kinase (PKR(p))-mediated cell stress. The double-stranded RNA protein kinase PKR is a proapoptotic and stress kinase that accumulates in degenerating neurons in Alzheimer disease. To determine whether neuronal apoptosis in human CJD is associated with activation of the PKR(p) signaling pathway, we assessed in situ end labeling and immunocytochemistry for PrP, glial fibrillary acidic protein, CD68, activated caspase 3, and phosphorylated PKR (Thr451) in samples of frontal, occipital, and temporal cortex, striatum, and cerebellum from 6 patients with sporadic CJD and 5 controls. Neuronal immunostaining for activated PKR was found in all CJD cases. The most staining was in nuclei and, in contrast to findings in Alzheimer disease, cytoplasmic labeling was not detected. Both the number and distribution of PKR(p)-positive neurons correlated closely with the extent of neuronal apoptosis, spongiosis, astrocytosis, and microglial activation and with the phenotype and disease severity. There was no correlation with the type, topography, or amount of extracellular PrP deposits. These findings suggest that neuronal apoptosis in human CJD may result from PKR(p)-mediated cell stress and are consistent with recent studies supporting a pathogenic role for intracellular or transmembrane PrP.

Aspirin inhibits cytotoxicity of prion peptide PrP106-126 to neuronal cells associated with microglia activation in vitro

The synthetic peptide consisting of amino acid residues 106-126 of the human prion protein PrP106-126 has been demonstrated to generate fibrils, which damage neurons either directly by interacting with components of the cell surface to trigger cell apoptosis signaling or indirectly by activating microglia to produce inflammatory mediators. In our study, rat microglia cells were treated with PrP106-126 or scramble PrP106-126 (Scr PrP). Activated nuclear factor kappaB (NF-kappaB) was determined using immunofluorescence staining and the expression of TNF-alpha and IL-1beta mRNA was measured by quantitative RT-PCR. Inhibitory activity of aspirin on neurotoxicity of PrP106-126 associated with microglia activation was determined using an apoptosis detection kit. Treatment of microglia with 25 microM PrP106-126, but not Scr PrP, resulted in activation and translocation of NF-kappaB, which peaked after 20 min of treatment. The activation of NF-kappaB was followed by increased mRNA expression of TNF-alpha and IL-1beta peaking at about 20 h. In the presence of microglia, aspirin significantly inhibited neuro-2a cell death induced by PrP106-126. The number of neuro-2a cells in apoptosis and necrosis with 5 mM aspirin was about 3-fold lower than the cell culture without aspirin (P<0.05). These data suggest that increased production of cytokines by microglia cells in prion disease is probably regulated by NF-kappaB translocation and may contribute to neurotoxicity of prions, and neurotoxicity of PrP106-126 may be inhibited by aspirin.

In vitro effect of prion peptide PrP 106–126 on mouse macrophages: Possible role of macrophages in transport and proliferation for prion protein

While there is a growing consensus on the understanding that the immune system plays an important role in facilitating the spread of prion infections from the periphery to the central nervous system, little is known about the key players in the first steps of the infection and about the sites of the disease development. Owing to their subepithelial location and their migratory capacity, macrophages could be early targets for prion transportation or propagation during the later stages of disease. In order to investigate the role of macrophages, we studied in vitro the effect of exposing primary peritoneal macrophages to a synthetic peptide homologous to residues 106-126 of the human prion protein (PrP), PrP 106-126. As shown by MTT assay, macrophage viability treated with less than 50 microM PrP 106-126 for 72 h was not inhibited but slightly stimulated at 10 and 25 microM, while there was significant decrease when exposed to 100 microM PrP 106-126 for 72 h. The expressions of PrP at mRNA and protein level were up-regulated following treatment with PrP 106-126 for 72 h. Cytokine TNF-alpha production were elevated by the PrP peptide in a time-dependent manner, which demonstrated a proinflammatory response linked to the presence and progression of prion disease took place in macrophages. These findings suggested that macrophages may play roles in the transportation and replication of the infectious agent.

ERK1/2 and p38 MAP kinases control prion protein fragment 90-231-induced astrocyte proliferation and microglia activation

Astrogliosis and microglial activation are a common feature during prion diseases, causing the release of chemoattractant and proinflammatory factors as well as reactive free radicals, involved in neuronal degeneration. The recombinant protease-resistant domain of the prion protein (PrP90-231) displays in vitro neurotoxic properties when refolded in a beta-sheet-rich conformer. Here, we report that PrP90-231 induces the secretion of several cytokines, chemokines, and nitric oxide (NO) release, in both type I astrocytes and microglial cells. PrP90-231 elicited in both cell types the activation of ERK1/2 MAP kinase that displays, in astrocytes, a rapid kinetics and a proliferative response. Conversely, in microglia, PrP90-231-dependent MAP kinase activation was delayed and long lasting, inducing functional activation and growth arrest. In microglial cells, NO release, dependent on the expression of the inducible NO synthase (iNOS), and the secretion of the chemokine CCL5 were Ca(2+) dependent and under the control of the MAP kinases ERK1/2 and p38: ERK1/2 inhibition, using PD98059, reduced iNOS expression, while p38 blockade by PD169316 inhibited CCL5 release. In summary, we demonstrate that glial cells are activated by extracellular misfolded PrP90-231 resulting in a proliferative/secretive response of astrocytes and functional activation of microglia, both dependent on MAP kinase activation. In particular, in microglia, PrP90-231 activated a complex signalling cascade involved in the regulation of NO and chemokine release. These data argue in favor of a causal role for misfolded prion protein in sustaining glial activation and, possibly, glia-mediated neuronal death.

Alteration of NF-kappaB activity leads to mitochondrial apoptosis after infection with pathological prion protein

Nuclear factor kappa B (NF-kappaB) is a key regulator of the immune response, but in almost the same manner it is involved in induction of inflammation, proliferation and regulation of apoptosis. In the central nervous system activated NF-kappaB plays a neuroprotective role. While in some neurodegenerative disorders the role of NF-kappaB is well characterized, there is poor knowledge on the role of NF-kappaB in prion disease. We found binding but no transcriptional activity of the transcription factor in vitro. Characterizing the mechanism of cell death after infection with pathological prion protein increased caspase-9 and caspase-3 activity was detected and the lack of NF-kappaB activity resulted in the inability to activate target genes that usually play an important role in neuroprotection. Additionally, we investigated the role of NF-kappaB after prion infection of Nfkb1(-/-), Nfkb2(-/-) and Bcl3(-/-) mice and central nervous system-specific p65-deleted mice revealing an accelerated prion disease in NF-kappaB2- and Bcl-3-deficient mice, which is in line with a reduced neuroprotective activity in prion infection. Based on our findings, we propose a model whereby the alteration of NF-kappaB activity at the early stages of infection with pathological prion protein leads to neuronal cell death mediated by mitochondrial apoptosis.

Amino-terminally truncated prion protein PrP90-231 induces microglial activation in vitro

The conversion of the prion protein (PrP) into a protease-resistant isoform (PrP(Res)) is considered the pathogenic event responsible for prion encephalopathies. Microglia activation accompanies PrP(Res) deposition representing an early event in the progression of these diseases. It is now believed that microglial cells play a worsening, if not causative, role in prion-induced neuronal death, through the release of proinflammatory and neurotoxic molecules. Indeed, in vitro observations have demonstrated that PrP(Res) and the synthetic prion fragment PrP106-126 induce neuronal death by activating microglial to migrate in the lesion area and secrete cytokines. Recently, we and others have demonstrated that the recombinant peptide, corresponding to the protease-resistant portion of PrP encompassing the amino acids 90-231 (PrP90-231), when beta-structured, is toxic for neuronal cells, in vitro. Here we report that PrP90-231 induces activation of N9 microglial cells, characterized by cell proliferation arrest and increased secretion of different cytokines (RANTES, GCSF, and IL-12). Moreover, the treatment of N9 cells with PrP90-231 elicited inducible nitric oxide synthase (i-NOS) expression, nitric oxide release, and a delayed (15 min to 1 h of treatment) extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation/activation. Although ERK1/2 is known to regulate proliferative and differentiative events, we show that its blockade, using the specific MEK inhibitor PD98059, did not prevent PrP90-231-induced inhibition of N9 cell proliferation. To our knowledge, this is the first evidence that a recombinant PrP(Res)-like peptide elicits microglial activation in vitro, thus representing a potentially important tool to develop possible therapeutic strategies to target prion-induced brain inflammation.

Overstimulation of PrPC signaling pathways by prion peptide 106-126 causes oxidative injury of bioaminergic neuronal cells

Transmissible spongiform encephalopathies, also called prion diseases, are characterized by neuronal loss linked to the accumulation of PrP(Sc), a pathologic variant of the cellular prion protein (PrP(C)). Although the molecular and cellular bases of PrP(Sc)-induced neuropathogenesis are not yet fully understood, increasing evidence supports the view that PrP(Sc) accumulation interferes with PrP(C) normal function(s) in neurons. In the present work, we exploit the properties of PrP-(106-126), a synthetic peptide encompassing residues 106-126 of PrP, to investigate into the mechanisms sustaining prion-associated neuronal damage. This peptide shares many physicochemical properties with PrP(Sc) and is neurotoxic in vitro and in vivo. We examined the impact of PrP-(106-126) exposure on 1C11 neuroepithelial cells, their neuronal progenies, and GT1-7 hypothalamic cells. This peptide triggers reactive oxygen species overflow, mitogen-activated protein kinase (ERK1/2), and SAPK (p38 and JNK1/2) sustained activation, and apoptotic signals in 1C11-derived serotonergic and noradrenergic neuronal cells, while having no effect on 1C11 precursor and GT1-7 cells. The neurotoxic action of PrP-(106-126) relies on cell surface expression of PrP(C), recruitment of a PrP(C)-Caveolin-Fyn signaling platform, and overstimulation of NADPH-oxidase activity. Altogether, these findings provide actual evidence that PrP-(106-126)-induced neuronal injury is caused by an amplification of PrP(C)-associated signaling responses, which notably promotes oxidative stress conditions. Distorsion of PrP(C) signaling in neuronal cells could hence represent a causal event in transmissible spongiform encephalopathy pathogenesis.

Inducible-NOS but not neuronal-NOS participate in the acute effect of TNF-α on hypothalamic insulin-dependent inhibition of food intake

TNF-alpha acts on the hypothalamus modulating food intake and energy expenditure through mechanisms incompletely elucidated. Here, we explore the hypothesis that, to modulate insulin-induced anorexigenic signaling in hypothalamus, TNF-alpha requires the synthesis of NO. TNF-alpha activates signal transduction through JNK and p38 in hypothalamus, peaking at 10(-8) M. This is accompanied by the induction of expression of the inducible and neuronal forms of NOS, in both cases peaking at 10(-12) M. In addition, TNF-alpha stimulates NOS catalytic activity. Pre-treatment with TNF-alpha at a low dose (10(-12) M) inhibits insulin-dependent anorexigenic signaling, and this effect is abolished in iNOS but not in nNOS knockout mice.

Distinct mono- and dinucleotide-specific P2Y receptors in A549 lung epithelial cells: different control of arachidonic acid release and nitric oxide synthase expression

P2Y nucleotide receptors activated by mono- and dinucleotides have already been found in lung tissue. Here, we compare effects of dinucleotides and mononucleotides on arachidonic acid release, intracellular calcium mobilization, and inducible nitric oxide synthase (iNOS) expression in the alveolar lung cell line A549. Both types of nucleotides were effective. Diadenosine polyphosphates (Ap(n)A, n=2 to 5) increased arachidonic acid release and raised intracellular calcium concentration ([Ca(2+)](i)), albeit with lower potency than mononucleotides (ATP, UTP, UDP). Among the dinucleotides only diadenosine tetraphosphate (Ap(4)A) was a potent agonist. Arachidonic acid release induced by Ap(4)A was almost completely abolished in the presence of the P2 receptor antagonists suramin and Reactive blue 2, whereas arachidonic acid release evoked by ATP, UTP or UDP was hardly reduced by these antagonists. Both, the mononucleotides ATP and UDP and the dinucleotide Ap(4)A induced the expression of iNOS in the cytoplasm around the nucleus, similar to the expression of iNOS evoked by lipopolysaccharide. iNOS is barely detectable in unstimulated cells. Suramin selectively blocked the capacity of Ap(4)A to induce iNOS, but not that of ATP or UDP. Thus, we find the same pharmacology for nucleotide-induced arachidonic acid release and iNOS expression. Therefore, we suggest that a distinct P2Y receptor subtype specifically activated by Ap(4)A exists in A549 cells, which is sensitive to the antagonist suramin, in contrast to other P2Y receptor subtypes activated by mononucleotides which are suramin-insensitive. Distinct P2Y receptors activated by mononucleotides or by Ap(4)A could play a role in inflammatory conditions by affecting the release of arachidonic acid and the expression of iNOS. Therefore, these receptors present a promising target in inflammatory diseases.

Regulation of inducible nitric oxide synthase gene in glial cells

Elevated levels of NO produced within the central nervous system (CNS) are associated with the pathogenesis of neuroinflammatory and neurodegenerative human diseases such as multiple sclerosis, HIV dementia, brain ischemia, trauma, Parkinson's disease, and Alzheimer's disease. Resident glial cells in the CNS (astroglia and microglia) express inducible nitric oxide synthase (iNOS) and produce high levels of NO in response to a wide variety of proinflammatory and degenerative stimuli. Although pathways resulting in the expression of iNOS may vary in two different glial cells of different species, the intracellular signaling events required for the expression of iNOS in these cells are slowly becoming clear. Various signaling cascades converge to activate several transcription factors that control the transcription of iNOS in glial cells. The present review summarizes different results and discusses current understandings about signaling mechanisms for the induction of iNOS expression in activated glial cells. A complete understanding of the regulation of iNOS expression in glial cells is expected to identify novel targets for therapeutic intervention in NO-mediated neurological disorders.

Endoplasmic Reticulum Stress Features Are Prominent in Alzheimer Disease but Not in Prion Diseases In Vivo

Prion diseases and Alzheimer disease (AD) share a variety of clinical and neuropathologic features (e.g. progressive dementia, accumulation of abnormally folded proteins in diseased tissue, and pronounced neuronal loss) as well as pathogenic mechanisms like generation of oxidative stress molecules and complement activation. Recently, it was suggested that neuronal death in AD may have its origin in the endoplasmic reticulum (ER). Cellular stress conditions can interfere with protein folding and subsequently cause accumulation of unfolded or misfolded proteins in the ER lumen. The ER responds to this by the activation of adaptive pathways, which are termed unfolded protein response (UPR). The UPR transducer PERK, which launches the most immediate response to ER stress (i.e. the transient attenuation of mRNA translation), and the downstream effector of PERK, eIF2alpha, were shown to be activated in AD. We demonstrate that neither in sporadic nor in infectiously acquired or inherited human prion diseases can the activated forms of PERK and eIF2alpha be detected, except when concomitant neurofibrillary pathology is present; whereas the distribution of phosphorylated PERK correlates with abnormally phosphorylated tau in AD. In brains of scrapie-affected mice and mice infected with sporadic or variant Creutzfeldt-Jakob disease, activated PERK is only very faintly expressed. The lack of prominent activation of the PERK-eIF2alpha pathway in prion diseases suggests that, in contrast to AD, ER stress does not play a crucial role in neuronal death in prion disorders.

Neurotoxicity from innate immune response is greatest with targeted replacement of E4 allele of apolipoprotein E gene and is mediated by microglial p38MAPK

Inheritance of APOE alleles is associated with varying clinical outcomes in several neurodegenerative diseases that are associated with innate immune response in brain. We tested the hypothesis that inheritance of different APOE alleles would significantly modulate neurotoxicity arising from glial innate immune response. We first used dissociated cultures of wild-type (wt) murine neurons and glia derived from mice with targeted replacement (TR) of the epsilon2, epsilon3, or, epsilon4 APOE allele. Our results showed that the vast majority of bystander damage to wt neurons derived from microglia was greatest with TR APOE4 glia, intermediate from TR APOE3 glia, and least from TR APOE2 glia and preceded detectable NO secretion. Microglial p38MAPK-dependent cytokine secretion followed a similar pattern of TR APOE dependence. In hippocampal slice cultures, innate immune activation had a similar pattern of TR APOE-dependence and produced postsynaptic neuronal damage in TR APOE4 and TR APOE3 but not TR APOE2 cultures that was p38MAPK dependent. These findings suggest a new mechanism by which inheritance of different APOE alleles may influence the outcome of neurodegenerative diseases associated with microglial innate immune response.

Comparative study of microglia activation induced by amyloid-beta and prion peptides: Role in neurodegeneration

The inflammatory responses in Alzheimer's disease (AD) and prion-related encephalopathies (PRE) are dominated by microglia activation. Several studies have reported that the amyloid-beta (Abeta) peptides, which are associated with AD, and the pathogenic isoform of prion protein (PrPSc) have a crucial role in neuronal death and gliosis that occur in both of these disorders. In this study, we investigate whether Abeta and PrPSc cause microglia activation per se and whether these amyloidogenic peptides differentially affect these immunoeffector cells. In addition, we also determined whether substances released by Abeta- and PrP-activated microglia induce neuronal death. Cultures of rat brain microglia cells were treated with the synthetic peptides Abeta1-40, Abeta1-42 and PrP106-126 for different time periods. The lipopolysaccharide was used as a positive control of microglia activation. Our results show that Abeta1-40 and PrP106-126 caused similar morphological changes in microglia and increased the production of nitric oxide and hydroperoxides. An increase on inducible nitric oxide synthase expression was also observed in microglia treated with Abeta1-40 or PrP106. However, these peptides affected in a different manner the secretion of interleukin-1beta (IL-1beta) and interleukin-6 (IL-6) secretion. In cocultures of microglia-neurons, it was observed that microglia treated with Abeta1-40 or PrP106-126 induced a comparable extent of neuronal death. The neutralizing antibody for IL-6 significantly reduced the neuronal death induced by Abeta- or PrP-activated microglia. Taken together, the data indicate that Abeta and PrP peptides caused microglia activation and differentially affected cytokine secretion. The IL-6 released by reactive microglia caused neuronal injury.

Nitric oxide induces prion protein via MEK and p38 MAPK signaling

The prion diseases or transmissible spongiform encephalopathy, such as human Creutzfeldt-Jakob disease (CJD) and so-called mad cow disease, are attributed to the causative agent, the scrapie variant of prion protein (PrP(Sc)) which causes fatal neurodegeneration. To investigate if stresses such as nitric oxide (NO) induced the cellular isoform of prion protein (PrP(C)), lipopolysaccharide, and sodium nitroprusside were used to treat N2a and NT2 cells, which resulted in elevated levels of the PRNP mRNA and prion protein. The signaling pathway for the NO-induced PrP(C) production involved guanylyl cyclase, MEK, and p38 MAPK as shown by the effect of specific pharmacological inhibitors ODQ, PD98059, and SB203580, respectively. Knowing the PrP induction by the biologically existing stimulus, this study provides useful information about the possible cellular mechanism and strategies for the treatment of CJD.

Microglia and multiple sclerosis

Microglia participate in all phases of the multiple sclerosis (MS) disease process. As members of the innate immune system, these cells have evolved to respond to stranger/danger signals; such a response within the central nervous system (CNS) environment has the potential to induce an acute inflammatory response. Engagement of Toll-like receptors (TLRs), a major family of pattern-recognition receptors (PRRs), provides an important mechanism whereby microglia can interact with both exogenous and endogenous ligands within the CNS. Such interactions modulate the capacity of microglia to present antigens to cells of the adaptive immune system and thus contribute to the initiation and propagation of the more sophisticated antigen-directed responses. This inflammatory response introduces the potential for bidirectional feedback between CNS resident and infiltrating systemic cells. Such interactions acquire particular relevance in the era of therapeutics for MS because the infiltrating cells can be subjected to systemic immunomodulatory therapies known to change their functional properties. Phagocytosis by microglia/macrophages is a hallmark of the MS lesion; however, the extent of tissue damage and the type of cell death will dictate subsequent innate responses. Microglia/macrophages are armed with a battery of effector molecules, such as reactive nitrogen species, that may contribute to CNS tissue injury, specifically to the injury of oligodendrocytes that is associated with MS. A therapeutic challenge is to modulate the dynamic properties of microglia/macrophages so as to limit potentially damaging innate responses, to protect the CNS from injury, and to promote local recovery.

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.

NF-κB in the Survival and Plasticity of Neurons

The transcription factor nuclear factor kappa-B (NF-kappaB) is involved in regulating responses of neurons to activation of several different signaling pathways in a variety of physiological and pathological settings. During development of the nervous system NF-kappaB is activated in growing neurons by neurotrophic factors and can induce the expression of genes involved in cell differentiation and survival. In the mature nervous system NF-kappaB is activated in synapses in response to excitatory synaptic transmission and may play a pivotal role in processes such as learning and memory. NF-kappaB is activated in neurons and glial cells in acute neurodegenerative conditions such as stroke and traumatic injury, as well as in chronic neurodegenerative conditions such as Alzheimer's disease. Activation of NF-kappaB in neurons can promote their survival by inducing the expression of genes encoding anti-apoptotic proteins such as Bcl-2 and the antioxidant enzyme Mn-superoxide dismutase. On the other hand, by inducing the production and release of inflammatory cytokines, reactive oxygen molecules and excitotoxins, activation of NF-kappaB in microglia and astrocytes may contribute to neuronal degeneration. Emerging findings suggest roles for NF-kappaB as a mediator of effects of behavioral and dietary factors on neuronal plasticity. NF-kappaB provides an attractive target for the development of novel therapeutic approaches for a range of neurological disorders.

Activation of alpha7 nicotinic acetylcholine receptor by nicotine selectively up-regulates cyclooxygenase-2 and prostaglandin E2 in rat microglial cultures

Background

Nicotinic acetylcholine (Ach) receptors are ligand-gated pentameric ion channels whose main function is to transmit signals for the neurotransmitter Ach in peripheral and central nervous system. However, the α7 nicotinic receptor has been recently found in several non-neuronal cells and described as an important regulator of cellular function. Nicotine and ACh have been recently reported to inhibit tumor necrosis factor-α (TNF-α) production in human macrophages as well as in mouse microglial cultures. In the present study, we investigated whether the stimulation of α7 nicotinic receptor by the specific agonist nicotine could affect the functional state of activated microglia by promoting and/or inhibiting the release of other important pro-inflammatory and lipid mediator such as prostaglandin E₂.

Methods

Expression of α7 nicotinic receptor in rat microglial cell was examined by RT-PCR, immunofluorescence staining and Western blot. The functional effects of α7 receptor activation were analyzed in resting or lipopolysaccharide (LPS) stimulated microglial cells pre-treated with nicotine. Culture media were assayed for the levels of tumor necrosis factor, interleukin-1β, nitric oxide, interleukin-10 and prostaglandin E₂. Total RNA was assayed by RT-PCR for the expression of COX-2 mRNA.

Results

Rat microglial cells express α7 nicotinic receptor, and its activation by nicotine dose-dependently reduces the LPS-induced release of TNF-α, but has little or no effect on nitric oxide, interleukin-10 and interleukin-1β. By contrast, nicotine enhances the expression of cyclooxygenase-2 and the synthesis of one of its major products, prostaglandin E₂.

Conclusions

Since prostaglandin E₂ modulates several macrophage and lymphocyte functions, which are instrumental for inflammatory resolution, our study further supports the existence of a brain cholinergic anti-inflammatory pathway mediated by α7 nicotinic receptor that could be potentially exploited for novel treatments of several neuropathologies in which local inflammation, sustained by activated microglia, plays a crucial role.

Conformational properties of peptide fragments homologous to the 106–114 and 106–126 residues of the human prion protein: a CD and NMR spectroscopic study

Two peptide fragments, corresponding to the amino acid residues 106-126 (PrP[Ac-106-126-NH(2)]) and 106-114 (PrP[Ac-106-114-NH(2)]) of the human prion protein have been synthesised in the acetylated and amide form at their N- and C-termini, respectively. The conformational preferences of PrP[Ac-106-126-NH(2)] and PrP[Ac-106-114-NH(2)] were investigated using CD and NMR spectroscopy. CD results showed that PrP[Ac-106-126-NH(2)] mainly adopts an alpha-helical conformation in TFE-water mixture and in SDS micelles, while a predominantly random structure is observed in aqueous solution. The shorter PrP[Ac-106-114-NH(2)] fragment showed similar propensities when investigated under the same experimental conditions as those employed for PrP[Ac-106-126-NH(2)]. From CD experiments at different SDS concentrations, an alpha-helix/beta-sheet conformational transition was only observed in the blocked PrP[Ac-106-126-NH(2)] sequence. The NMR analysis confirmed the helical nature of PrP[Ac-106-126-NH(2)] in the presence of SDS micelles. The shorter PrP[Ac-106-114-NH(2)] manifested a similar behaviour. The results as a whole suggest that both hydrophobic effects and electrostatic interactions play a significant role in the formation and stabilisation of ordered secondary structures in PrP[Ac-106-126-NH(2)].

Neurokinin-1 receptor interacts with PrP106–126-induced dendritic cell migration and maturation

Circulating monocytic cells may mediate neuroinvasion in transmissible spongiform encephalopathies by transporting prion protein (PrP) from sites of entry to the nervous system. Factors regulating monocyte-derived dendritic cell (DC) functions in neuronal tissue include neurogenic mediators, but their interactions with prion-infected DCs are unknown. Here, we report that neuropeptides regulate the interaction of DCs exposed to PrP(106-126). Inhibition of neurokinin-1-receptors activation with neurokinin-1-receptor antagonist or antibody attenuates substance P-induced enhancement of DC migration induced by PrP(106-126) and prion-protein-induced DC maturation. Other neuropeptides arrest chemotaxis. Data support a priming role of the neuropeptide substance P in PrP(106-126)-induced migration of DCs involving neurokinin-1-receptors.

Inducible form of nitric oxide synthase expression in rat cortical neuronal cells in vitro

The inducible form of nitric oxide synthase (iNOS) is an essential element of the immune response, which is expressed primarily in microglial cells within the CNS. Exposure of rat cortical neuronal cells to the pro-inflammatory bacterial endotoxin lipopolysaccharide (LPS) resulted in a significant increase in the expression of the cellular iNOS protein expression and NO generation (which serves as an indirect measure of NOS catalytic activity). These effects were potentiated by costimulation with interferon-gamma (IFNgamma) and the increase in NO generation was abolished by the iNOS selective inhibitor 1400W, although this did not attenuate the toxin-induced increase in the enzyme expression. As the cortex is one of the principal areas to be targeted in Alzheimer's disease (AD), the present findings may help to further our understanding of the biochemical events associated with the neurodegenerative process.

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.

Prolonged exposure of microglia to lipopolysaccharide modifies the intracellular signaling pathways and selectively promotes prostaglandin E2 synthesis

During inflammatory or degenerative processes microglial cells are likely to be exposed to activating agents that persist in brain parenchyma for prolonged periods. As our knowledge on microglial activation is largely based on in vitro studies in which microglial cultures are activated by a single administration of pro-inflammatory stimuli, we investigated the effects of repeated endotoxin (LPS) challenges on microglial functional state. Primary rat microglial cultures were subjected to one, two or three consecutive LPS-stimulation and the production of tumor necrosis factor-alpha (TNF-alpha), nitric oxide (NO), prostaglandin E2 (PGE2) and 15-deoxy-Delta12,14-PGJ2 (15d-PGJ2) measured. The ability of microglial cells to produce NO, TNF-alpha and 15d-PGJ2 upon the first LPS challenge rapidly declined after the second and the third stimulations, whereas PGE2 synthesis remained constantly elevated. Accordingly, the expression of inducible NO synthase decreased whereas cyclooxygenase-2 and microsomal PGE synthase remained up-regulated. The signaling pathways evoked by single or multiple LPS-stimulation were also profoundly different, when considering the activation of the transcription factors nuclear factor-kappa B and CREB, and of the p38 MAPK. Our observations suggest that prolonged exposure to LPS, and likely other activating agents, induces in microglia a functional state clearly distinct from that triggered by acute stimulation. The progressive down-regulation of pro-inflammatory molecules and the sustained release of PGE2 could have important implications for the resolution of brain inflammation.

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.

Inflammatory neurodegeneration mediated by nitric oxide, glutamate, and mitochondria

In inflammatory, infectious, ischemic, and neurodegenerative pathologies of the central nervous system (CNS) glia become "activated" by inflammatory mediators, and express new proteins such as the inducible isoform of nitric oxide synthase (iNOS). Although these activated glia have benefi- cial roles, in vitro they potently kill cocultured neurons, and there is increasing evidence that they contribute to pathology in vivo. Nitric oxide (NO) from iNOS appears to be a key mediator of such glial-induced neuronal death. The high sensitivity of neurons to NO is partly due to NO causing inhibition of respiration, rapid glutamate release from both astrocytes and neurons, and subsequent excitotoxic death of the neurons. NO is a potent inhibitor of mitochondrial respiration, due to reversible binding of NO to cytochrome oxidase in competition with oxygen, resulting in inhibition of energy production and sensitization to hypoxia. Activated astrocytes or microglia cause a potent inhibition of respiration in cocultured neurons due to glial NO inhibiting cytochrome oxidase within the neurons, resulting in ATP depletion and glutamate release. In some conditions, glutamate- induced neuronal death can itself be mediated by N-methyl-D-aspartate (NMDA)-receptor activation of the neuronal isoform of NO synthase (nNOS) causing mitochondrial damage. In addition NO can be converted to a number of reactive derivatives such as peroxynitrite, NO2, N2O3, and S-nitrosothiols that can kill cells in part by inhibiting mitochondrial respiration or activation of mitochondrial permeability transition, triggering neuronal apoptosis or necrosis.

Differential effects of lowering culture temperature on mediator release from lipopolysaccharide-stimulated neonatal rat microglia

OBJECTIVE

Therapeutic moderate hypothermia has the potential for neuronal protection against brain injury. Microglia, a type of immune-related cell in the brain, may play a certain role in neuronal damage subsequent to injury. We examined the effects of culture temperature changes from 37 degrees C to 33 degrees C or 30 degrees C on mediator release, including nitric oxide, interleukin-6, and tumor necrosis factor-alpha from lipopolysaccharide-stimulated microglia harvested from neonatal rats.

DESIGN

Laboratory study.

SETTING

University medical school.

SUBJECTS

Microglial cells isolated from primary cultures of rat brains.

INTERVENTIONS

The production of nitric oxide was measured by a nitrite accumulation method in a culture medium, whereas cytokines, interleukin-6, and tumor necrosis factor-alpha were measured by enzyme-linked immunosorbent assay.

MEASUREMENT AND MAIN RESULTS

At 30 degrees C and 33 degrees C, nitric oxide production stimulated by lipopolysaccharide decreased to 10 and 30% of control (37 degrees C), respectively, 24 hrs after the stimulation, and the decrease was sustained for 48 hrs. Interleukin-6 production at 30 degrees C and 33 degrees C was also reduced to 30% of control 6 hrs after the activation. Such responses lasted throughout the study. However, tumor necrosis factor-alpha release at 30 degrees C and 33 degrees C was depressed for only 6 hrs after stimulation, followed by subsequent elevation to concentrations similar to those at 37 degrees C. Microglial morphologic activation, showing changes from round to bipolar, reached a peak at 6 hrs in the 37 degrees C group, returning to round 12 hrs after lipopolysaccharide application. In 30 degrees C and 33 degrees C, the zenith was detected at 6 hrs, with activation remaining even 12 hrs after the stimulation, suggesting prolongation of the microglial response to lipopolysaccharide, which was inconsistent with changes in tumor necrosis factor release.

CONCLUSIONS

Decreasing culture temperature inhibits the production of nitric oxide and interleukin-6 from activated microglia. Differences were found in the degree or time course change between tumor necrosis factor-alpha and the other mediators. Also, the time course of morphologic changes in microglia was dependent on culture temperature. Further studies are required to define the mechanisms for such differences in mediator release from cooled microglia and also to clarify the inconsistency between morphologic change and its function in the cell.

(-)-Epigallocatechin gallate protects against NO stress-induced neuronal damage after ischemia by acting as an anti-oxidant

The present study investigated the effects of (-)-epigallocatechin gallate (EGCG), which is the major component of polyphenol in green tea, on nitric oxide (NO) stress-induced neuronal damage, by monitoring NO mobilizations in the intact rat hippocampus and assaying the viability of cultured rat hippocampal neurons. A 10-min ischemia increased NO (NO(3)(-)/NO(2)(-)) concentrations in the intact rat hippocampus, while EGCG (50 mg/kg i.p.) inhibited the increase by 77% without affecting hippocampal blood flow. The NO donor, sodium nitroprusside (SNP; 50 microM), produced NO (NO(3)(-)/NO(2)(-)), while EGCG inhibited it in a dose-dependent manner at concentrations ranging from 50 to 200 microM. Treatment with SNP (100 microM) reduced the viability of cultured rat hippocampal neurons to 22% of control levels, while EGCG caused it to recover to 51% for 10 microM, 73% for 20 microM, and 70% for 50 microM. Taken together, it appears that EGCG could protect against ischemic neuronal damage by deoxidizing peroxynitrate/peroxynitrite, which is converted to NO radical or hydroxy radical.

Role of p38 and p44/42 mitogen-activated protein kinases in microglia

Although microglial cells are thought to play a beneficial role in the regeneration and plasticity of the central nervous system (CNS), recent studies have indicated that at least some molecules released by microglia may be harmful in acute brain insults and neurodegenerative diseases. Therefore, the pathways mediating the synthesis and release of these neurotoxic compounds are of importance. p38 and p44/42 families of mitogen-activated protein kinases (MAPKs) in microglia respond strongly to various extracellular stimuli, such as ATP, thrombin, and beta-amyloid, a peptide thought to be responsible for the neuropathology in Alzheimer's disease. In this review we describe in vivo evidence implicating that p38 and p44/42 MAPKs may play a critical role in harmful microglial activation in acute brain injury, such as stroke, and in more chronic neurodegenerative diseases, such as Alzheimer's disease. We also clarify the extracellular signals responsible for activation of p38 and p44/42 MAPK in microglia and review the responses so far reported to be mediated by these kinases.

Tetracyclines affect prion infectivity

Prion diseases are transmissible neurodegenerative disorders of humans and animals for which no effective treatment is available. Conformationally altered, protease-resistant forms of the prion protein (PrP) termed PrP(Sc) are critical for disease transmissibility and pathogenesis, thus representing a primary target for therapeutic strategies. Based on previous findings that tetracyclines revert abnormal physicochemical properties and abolish neurotoxicity of PrP peptides in vitro, we tested the ability of these compounds to interact with PrP(Sc) from patients with the new variant of Creutzfeldt-Jakob disease (vCJD) and cattle with bovine spongiform encephalopathy (BSE). The incubation with tetracycline hydrochloride or doxycycline hyclate at concentrations ranging from 10 microM to 1 mM resulted in a dose-dependent decrease in protease resistance of PrP(Sc). This finding prompted us to investigate whether tetracyclines affect prion infectivity by using an animal model of disease. Syrian hamsters were injected intracerebrally with 263K scrapie-infected brain homogenate that was coincubated with 1 mM tetracycline hydrochloride, 1 mM doxycycline hyclate, or vehicle solution before inoculation. Hamsters injected with tetracycline-treated inoculum showed a significant delay in the onset of clinical signs of disease and prolonged survival time. These effects were paralleled by a delay in the appearance of magnetic-resonance abnormalities in the thalamus, neuropathological changes, and PrP(Sc) accumulation. When tetracycline was preincubated with highly diluted scrapie-infected inoculum, one third of hamsters did not develop disease. Our data suggest that these well characterized antibiotics reduce prion infectivity through a direct interaction with PrP(Sc) and are potentially useful for inactivation of BSE- or vCJD-contaminated products and prevention strategies.

Regulating factors for microglial activation

Microglia, residential macrophages in the central nervous system, can release a variety of factors including cytokines, chemokines, etc. to regulate the communication among neuronal and other types of glial cells. Microglia play immunological roles in mechanisms underlying the phagocytosis of invading microorganisms and removal of dead or damaged cells. When microglia are hyperactivated due to a certain pathological imbalance, they may cause neuronal degeneration. Pathological activation of microglia has been reported in a wide range of conditions such as cerebral ischemia, Alzheimer's disease, prion diseases, multiple sclerosis, AIDS dementia, and others. Nearly 5000 papers on microglia can be retrieved on the Web site PubMed at present (November 2001) and half of them were published within the past 5 years. Although it is not possible to read each paper in detail, as many factors as possible affecting microglial functions in in vitro culture systems are presented in this review. The factors are separated into "activators" and "inhibitors," although it is difficult to classify many of them. An overview on these factors may help in the development of a new strategy for the treatment of various neurodegenerative diseases.

PACAP protects neuronal PC12 cells from the cytotoxicity of human prion protein fragment 106-126

Misfolding of the prion protein yields amyloidogenic isoforms, and it shows exacerbating neuronal damage in neurodegenerative disorders including prion diseases. Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) potently stimulate neuritogenesis and survival of neuronal cells in the central nervous system. Here, we tested these neuropeptides on neurotoxicity in PC12 cells induced by the prion protein fragment 106-126 [PrP (106-126)]. Concomitant application of neuropeptide with PrP(106-126) (5x10(-5) M) inhibited the delayed death of neuron-like PC12 cells. In particular, PACAP27 inhibited the neurotoxicity of PrP(106-126) at low concentrations (>10(-15) M), characterized by the deactivation of PrP(106-126)-stimulated caspase-3. The neuroprotective effect of PACAP27 was antagonized by the selective PKA inhibitor, H89, or the MAP kinase inhibitor, U0126. These results suggest that PACAP27 attenuates PrP(106-126)-induced delayed neurotoxicity in PC12 cells by activating both PKA and MAP kinases mediated by PAC1 receptor.

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.

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.