Role of microglia in neuronal cell death in prion disease

PrPSc-like prion protein peptide inhibits the function of cellular prion protein

Mice lacking expression of the prion protein are protected against infection with prion disease. Neurodegeneration in prion disease requires the formation of the abnormal isoform of the prion protein (PrP(Sc)) from host prion protein. Therefore expression of normal host prion protein is necessary for prion disease. In the present investigation, it was demonstrated that PrP(Sc) and a peptide resembling PrP(Sc), PrP106-126, both bind to cellular prion protein at amino acid residues 112-119. Interaction between PrP106-126 and the prion protein strips the prion protein from cells. Direct interaction of PrP106-126 with the prion protein was found to make cells more susceptible to copper toxicity, inhibited copper uptake into cells and inhibited the superoxide dismutase-like activity of the prion protein. Direct inhibition of prion protein function by PrP(Sc) may be necessary for neurodegeneration in prion disease.

Identification of upregulated genes in scrapie-infected brain tissue

The pathogenesis of scrapie, and of neurodegenerative diseases in general, is still insufficiently understood and is therefore being intensely researched. There is abundant evidence that the activation of glial cells precedes neurodegeneration and may thus play an important role in disease development and progression. The identification of genes with altered expression patterns in the diseased brain may provide insight on the molecular level into the process which ultimately leads to neuronal loss. Differentially expressed genes in scrapie-infected brain tissue were enriched by the suppression subtractive hybridization technique, molecularly cloned, and further characterized. Northern blotting and nucleotide sequencing confirmed the identities of 19 upregulated genes, 11 of which were unknown to be affected by scrapie. A considerable number of these 19 genes, namely those encoding interferon-inducible protein 10 (IP-10), 2',5'-oligo(A) synthetase, Mx protein, IIGP protein, major histocompatibility complex classes I and II, complement, and beta(2)-microglobulin, were inducible by interferons (IFNs), suggesting that an IFN response is a possible mechanism of gene activation in scrapie. Among the newly found genes, that coding for 2',5'-oligo(A) synthetase is of special interest because it could contribute to the apoptotic loss of neuronal cells via RNase L activation. In addition, upregulation of the chemokine IP-10 and B-lymphocyte chemoattractant mRNAs was seen at relatively early stages of the disease and was sustained throughout disease development.

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

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

Pathogenesis of prion diseases: a progress report

Almost 20 years have passed since Stanley Prusiner proposed that the agent causing transmissible spongiform encephalopathies consists exclusively of a protein and termed it prion. A mixed balance can be drawn from the enormous research efforts that have gone into prion research during this time. On the negative side, the protein-only hypothesis has not been conclusively proven yet. On the positive side, our understanding of spongiform encephalopathies has experienced tremendous advances, mostly through human genetics, mouse transgenetics, and biophysical methods. Perhaps the most astonishing development is the realization that many human neurodegenerative diseases for which transmissibility has been more or less stringently excluded, may follow pathogenetic principles similar to those of prion diseases. Also, the hypothesis that prion-like phenomena may underlie certain non-genetic traits observed in yeast has resulted in the surprising recognition that the instructional self-propagating changes in protein conformation may be much more prevalent in nature than previously thought. The latter developments have been astonishingly successful, and one could now argue that the prion principle is much more solidly established in yeast than in mammals.

Microglial activation precedes acute neurodegeneration in Sandhoff disease and is suppressed by bone marrow transplantation

Sandhoff disease is a lysosomal storage disorder characterized by the absence of beta-hexosaminidase and storage of G(M2) ganglioside and related glycolipids in the central nervous system. The glycolipid storage causes severe neurodegeneration through a poorly understood pathogenic mechanism. In symptomatic Sandhoff disease mice, apoptotic neuronal cell death was prominent in the caudal regions of the brain. cDNA microarray analysis to monitor gene expression during neuronal cell death revealed an upregulation of genes related to an inflammatory process dominated by activated microglia. Activated microglial expansion, based on gene expression and histologic analysis, was found to precede massive neuronal death. Extensive microglia activation also was detected in a human case of Sandhoff disease. Bone marrow transplantation of Sandhoff disease mice suppressed both the explosive expansion of activated microglia and the neuronal cell death without detectable decreases in neuronal G(M2) ganglioside storage. These results suggest a mechanism of neurodegeneration that includes a vigorous inflammatory response as an important component. Thus, this lysosomal storage disease has parallels to other neurodegenerative disorders, such as Alzheimer's and prion diseases, where inflammatory processes are believed to participate directly in neuronal cell death.

A model for the mechanism of astrogliosis in prion disease

Astrogliosis is a hallmark of prion diseases. Finding ways of inhibiting astrocyte proliferation may be beneficial to treating these diseases. PrP106-126 a peptide fragment of the prion protein induces proliferation of astrocytes. The mechanism of its action was studied in detail. Induction of astrocyte proliferation in culture requires cytokines interleukin-1 and interleukin-6 released from microglia in the presence of PrP106-126. However, the increased release of these cytokines is insufficient without direct effects of PrP106-126 on astrocytes. PrP106-126 induces increased progression through the cell cycle to late G1 and enhances the level of both p53 and phosphorylated ERKs in astrocytes. PrP106-126-induced proliferation of astrocytes in culture can be inhibited by antibodies to cytokines or by MEK inhibitors.

Apoptotic cell death and impairment of L-type voltage-sensitive calcium channel activity in rat cerebellar granule cells treated with the prion protein fragment 106-126

Prion diseases are neurodegenerative pathologies characterized by the accumulation, in the brain, of altered forms of the prion protein (PrP), named PrP(Sc). A synthetic peptide homologous to residues 106-126 of PrP (PrP106-126) was reported to maintain the neurodegenerative characteristics of PrP(Sc). We investigated the intracellular mechanisms involved in PrP106-126-dependent degeneration of primary cultures of cerebellar granule neurons. Prolonged exposure of such neurons to PrP106-126 induced apoptotic cell death. The L-type voltage-sensitive calcium channel blocker nicardipine reproduced this effect, suggesting that blockade of Ca(2+) entry through this class of calcium channels may be responsible for the granule cell degeneration. Microfluorometric analysis showed that PrP106-126 caused a reduction in cytosolic calcium levels, elicited by depolarizing K(+) concentrations in these neurons. Electrophysiological studies demonstrated that PrP106-126 and nicardipine selectively reduce the L-type calcium channel current. These data demonstrate that PrP106-126 alters the activity of L-type voltage-sensitive calcium channels in rat cerebellar granule cells and suggest that this phenomenon is related to the cell death induced by the peptide.

Immunohistochemical studies of the PrP(CJD) deposition in Creutzfeldt-Jakob disease

The PrP(CJD) deposition in eight brains of sporadic Creutzfeldt-Jakob disease (CJD) was examined immunohistochemically using both hydrolytic autoclaving and formic acid pretreatment in order to understand the pathogenesis of CJD. Synaptic-type PrP immunoreactivity was revealed in the gray matter in all cases and had a tendency to be weaker in devastated areas in cases with a longer duration of illness. However, in one particular case with numerous prion plaques, the degeneration was relatively mild while PrP(CJD) immunoreactivity was intense despite the longest duration of illness among the examined cases. Deep layer accentuation of PrP(CJD) immunoreactivity was observed in the cerebral cortices in most cases. This staining pattern, however, disappeared in a burnt-out lesion exhibiting status spongiosus. The granular layer was damaged mostly in the cerebellum of the advanced cases. PrP(CJD) and synaptophysin immunoreactivities decreased as the tissue degeneration progressed. Interestingly, the Purkinje cells had no positivity for PrP(CJD) in all cases, although the neurons in relatively preserved cerebellum showed apparent positivity for synaptophysin. In the Ammon's horn and subiculum the neurons were well preserved despite the marked immunoreactivity for PrP(CJD) in all cases, although some cases demonstrated severe spongiform change. Approximately half of the cases showed intracytoplasmic inclusion body-like immunoreactivity for PrP(CJD) in neurons of the dentate nucleus. These findings suggest that PrP(CJD) deposition may be an event that precedes neuronal degeneration evolving from deeper layers of the cerebral cortex. Although the Ammon's horn and subiculum showed striking PrP(CJD) deposition and spongiform change, neuronal loss did not take place, suggesting that deposited PrP(CJD) itself seems not to be directly harmful to the neurons. Some investigators have assumed that microglia activated by PrP(CJD) plays an important role in neuronal degeneration. Considering this, we speculate that microglia in the Ammon's horn and subiculum may have a unique characteristic of not responding to PrP(CJD).

Recent advances in the pre-mortem diagnosis of Creutzfeldt-Jakob disease

Included in the spectrum of human transmissible spongiform encephalopathies are Creutzfeldt-Jakob disease (CJD) and the new variant form (vCJD), Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, kuru and various less distinct neuropsychiatric disorders. Progress in our understanding of this group of disorders continues at a prodigious rate, although important vexing practical issues persist. The definitive confirmation of symptomatic prion disease still requires pathological examination, most reliably performed post-mortem. However, paralleling the recent advances in the molecular biological understanding of normal prion protein (PrP(c)) function and the pathophysiology of prion diseases, there have been worthwhile developments in the pre-mortem diagnosis of CJD. Efforts to develop less invasive but very reliable ante-mortem diagnostic tests have received an additional impetus because of the potential epidemic of vCJD. Historically, the ancillary investigation of most merit has been the EEG, whereas the recent advances have encompassed a broader range of technologies, including both magnetic resonance and radioisotopic neuroimaging, and immunoassays for a range of non-specific marker proteins in both CSF, and less commonly, blood. However, given the recent refinement of sophisticated immunoassays, it is envisaged that the pathognomonic, protease-resistant, disease-associated isoforms of the prion protein (PrPres) may soon be directly detectable in the blood and tissues of patients manifesting or incubating a spongiform encephalopathy.

Altered toxicity of the prion protein peptide PrP106-126 carrying the Ala(117)-->Val mutation

The inherited prion diseases such as Gerstmann-Sträussler-Scheinker syndrome (GSS) are linked to point mutations in the gene coding for the cellular isoform of the prion protein (PrP(C)). One particular point mutation A117V (Ala(117)-->Val) is linked to a variable pathology that usually includes deposition of neurofibrillary tangles. A prion protein peptide carrying this point mutation [PrP106-126(117V)] was generated and compared with a peptide based on the normal human sequence [PrP106-126(117A)]. The inclusion of this point mutation increased the toxicity of PrP106-126 which could be linked to an increased beta-sheet content. An assay of microtubule formation in the presence of tau indicated that PrP106-126 decreased the rate of microtubule formation that could be related to the displacement of tau. PrP106-126 carrying the 117 mutation was more efficient at inhibiting microtubule formation. These results suggest a possible mechanism of toxicity for protein carrying this mutation via destabilization of the cytoskeleton and deposition of tau in filaments, as observed in GSS.

Prion protein peptides: optimal toxicity and peptide blockade of toxicity

In prion disease neurodegeneration requires deposition of the abnormal isoform of the prion protein (PrP(Sc)) within nervous tissue. In vitro PrP(Sc) has neurotoxicity that can be mimicked by peptides based on part of its sequence. In this investigation the region of the protein required for maximal neurotoxicity was precisely determined. The optimal neurotoxic peptide was found to contain amino acids 112-126 of the human sequence. The sequence AGAAAAGA was found to be necessary but not sufficient for a neurotoxic effect. The AGAAAAGA peptide blocked the toxicity of PrP106-126, suggesting that this sequence is necessary for the interaction of PrP106-126 with neurons. These results suggest that targeting or use of the AGAAAAGA peptide may represent a therapeutic opportunity for controlling prion disease.

Poly(ADP-ribose) immunostaining to detect apoptosis induced by a neurotoxic fragment of prion protein

PrP106-126 is a synthetic peptide representing codons 106-126 of the prion protein, which spontaneously forms amyloid fibrils and exerts neurotoxic effects on primary mouse brain cell cultures. Neurotoxicity by this peptide is commonly used as a model for the neurotoxicity observed in prion diseases and involves the formation of reactive oxygen species which, in turn, can cause DNA damage, including DNA strand breaks. Strand breaks in nuclear DNA can activate poly(ADP-ribose) polymerase to covalently modify nuclear proteins with poly(ADP-ribose). We, therefore, examined by immunofluorescence whether or not PrP106-126 triggers poly(ADP-ribose) formation. We observed strong poly(ADP-ribose) immunofluorescence signals in a fraction of cells, typically arranged in a clustered pattern, by 30-48 h after peptide addition. A few positive cells were also present in untreated cultures. Cell morphology was suggestive of apoptosis, and this was confirmed by positivity in the terminal deoxynucleotidyltransferase-mediated dUTP nick-end labelling (TUNEL) assay. On the other hand, our immunofluorescence assay did not detect any 'early' activation of poly(ADP-ribose) polymerase in morphologically normal cells that could have resulted from peptide-induced formation of reactive oxygen species. We conclude that poly(ADP-ribose) immunostaining is a convenient and reliable method for visualizing cells undergoing apoptosis induced by PrP106-126.

Prion protein peptide neurotoxicity can be mediated by astrocytes

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

Microglial activation varies in different models of Creutzfeldt-Jakob disease

Progressive changes in host mRNA expression can illuminate crucial pathogenetic pathways in infectious disease. We examined general and specific approaches to mRNA expression in three rodent models of Creutzfeldt-Jakob disease (CJD). Each of these models displays distinctive neuropathology. Although mRNAs for the chemokine receptor CCR5, the lysosomal protease cathepsin S, and the pleiotropic cytokine transforming growth factor beta1 (TGF-beta1) were progressively upregulated in rodent CJD, the temporal patterns and peak magnitudes of each of these transcripts varied substantially among models. Cathepsin S and TGF-beta1 were elevated more than 15-fold in mice and rats infected with two different CJD strains, but not in CJD-infected hamsters. In rats, an early activation of microglial transcripts preceded obvious deposits of prion protein (PrP) amyloid. However, in each of the three CJD models, the upregulation of CCR5, cathepsin S, and TGF-beta1 was variable with respect to the onset of PrP pathology. These results show glial cell involvement varies as a consequence of the agent strain and species infected. Although neurons are generally assumed to be the primary sites for agent replication and abnormal PrP formation, microglia may be targeted by some agent strains. In such instances, microglia can both process PrP to become amyloid and can enhance neuronal destruction. Because microglia can participate in agent clearance, they may also act as chronic reservoirs of infectivity. Finally, the results here strongly suggest that TGF-beta1 can be an essential signal for amyloid deposition.

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

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