Prion replication alters the distribution of synaptophysin and caveolin 1 in neuronal lipid rafts

Prion protein complexed to a DNA aptamer induce behavioral and synapse dysfunction in mice

Conversion of the cellular prion protein (PrP^C) into the scrapie form (PrP^Sc) is the leading step to the development of transmissible spongiform encephalopathies (TSEs), still incurable neurodegenerative disorders. Interaction of PrP^C with cellular and synthetic ligands that induce formation of scrapie-like conformations has been deeply investigated in vitro. Different nucleic acid (NA) sequences bind PrP and convert it to β-sheet-rich or unfolded species; among such NAs, a 21-mer double-stranded DNA, D67, was shown to induce formation of PrP aggregates that were cytotoxic. However, in vivo effects of these PrP-DNA complexes were not explored. Herein, aggregates of recombinant full-length PrP (rPrP^23-231) induced by interaction with the D67 aptamer were inoculated into the lateral ventricle of Swiss mice and acute effects were investigated. The aggregates had no influence on emotional, locomotor and motor behavior of mice. In contrast, mice developed cognitive impairment and hippocampal synapse loss, which was accompanied by intense activation of glial cells in this brain region. Our results suggest that the i.c.v. injection of rPrP:D67 aggregates is an interesting model to study the neurotoxicity of aggregated PrP in vivo, and that glial cell activation may be an important step for behavioral and cognitive dysfunction in prion diseases.

Chronic N-acetylcysteine treatment alleviates acute lipopolysaccharide-induced working memory deficit through upregulating caveolin-1 and synaptophysin in mice

RATIONALE

Working memory (WM) is a dynamic encoding process and an active representation of information over a short time. The ability to guide forthcoming behavior would be disrupted if WM was impaired by various factors including inflammation, stress, free radicals, and disease states such as schizophrenia. However, the mechanism underlying acute working memory impairment remains to be defined.

OBJECTIVES

In this study, we tested the hypothesis that decreased caveolin-1 (Cav-1) and synaptophysin (SYP) accounted for the WM impairment challenged with acute intraperitoneally lipopolysaccharide (LPS), which mimicked neuroinflammation. Delayed alternation T-maze task (DAT) was used to assess working memory of adult male C57BL/6 mice, and western blot and immunostaining were used to detect protein expression and distribution in medial prefrontal cortex (mPFC) and hippocampus.

RESULTS

Our results showed that LPS dose-dependently induced working memory deficit accompanied by the decrease of Cav-1 and SYP in mPFC but not hippocampus. In addition, LPS significantly decreased protein level of Cav-1 and SYP in neurons by activating microglia cells. More important, 2-week N-acetylcysteine (NAC) treatment dose-dependently inhibited LPS-induced working memory deficit by improving the ability to use Lose-shift but not Win-shift strategy and significantly inhibited LPS-induced downregulation of Cav-1 and SYP in mPFC.

CONCLUSIONS

Taken together, our findings demonstrate that chronic NAC treatment alleviates acute LPS-induced working memory deficit through upregulating Cav-1 and SYP in mice.

REST alleviates neurotoxic prion peptide-induced synaptic abnormalities, neurofibrillary degeneration and neuronal death partially via LRP6-mediated Wnt-β-catenin signaling

Prion diseases are a group of infectious neurodegenerative diseases characterized by multiple neuropathological hallmarks including synaptic damage, spongiform degeneration and neuronal death. The factors and mechanisms that maintain cellular morphological integrity and protect against neurodegeneration in prion diseases are still unclear. Here we report that after stimulation with the neurotoxic PrP106-126 fragment in primary cortical neurons, REST translocates from the cytoplasm to the nucleus and protects neurons from harmful effects of PrP106-126. Overexpression of REST reduces pathological damage and abnormal biochemical alterations of neurons induced by PrP106-126 and maintains neuronal viability by stabilizing the level of pro-survival protein FOXO1 and inhibiting the permeability of the mitochondrial outer membrane, release of cytochrome c from mitochondria to cytoplasm and the activation of Capase3. Conversely, knockdown of REST exacerbates morphological damage and inhibits the expression of FOXO1. Additionally, by overexpression or knockdown of LRP6, we further show that LRP6-mediated Wnt-β-catenin signaling partly regulates the expression of REST. Collectively, we demonstrate for the first time novel neuroprotective function of REST in prion diseases and hypothesise that the LRP6-Wnt-β-catenin/REST signaling plays critical and collaborative roles in neuroprotection. This signaling of neuronal survival regulation could be explored as a viable therapeutic target for prion diseases and associated neurodegenerative diseases.

Mitochondrial Respiration Is Impaired during Late-Stage Hamster Prion Infection

Mitochondria are crucial to proper neuronal function and overall brain health. Mitochondrial dysfunction within the brain has been observed in many neurodegenerative diseases, including prion disease. Several markers of decreased mitochondrial activity during prion infection have been reported, yet the bioenergetic respiratory status of mitochondria from prion-infected animals is unknown. Here we show that clinically ill transgenic mice overexpressing hamster prion protein (Tg7) infected with the hamster prion strain 263K suffer from a severe deficit in mitochondrial oxygen consumption in response to the respiratory complex II substrate succinate. Characterization of the mitochondrial proteome of purified brain mitochondria from infected and uninfected Tg7 mice showed significant differences in the relative abundance of key mitochondrial electron transport proteins in 263K-infected animals relative to that in controls. Our results suggest that at clinical stages of prion infection, dysregulation of respiratory chain proteins may lead to impairment of mitochondrial respiration in the brain.IMPORTANCE Mitochondrial dysfunction is present in most major neurodegenerative diseases, and some studies have suggested that mitochondrial processes may be altered during prion disease. Here we show that hamster prion-infected transgenic mice overexpressing the hamster prion protein (Tg7 mice) suffer from mitochondrial respiratory deficits. Tg7 mice infected with the 263K hamster prion strain have little or no signs of mitochondrial dysfunction at the disease midpoint but suffer from a severe deficit in mitochondrial respiration at the clinical phase of disease. A proteomic analysis of the isolated brain mitochondria from clinically affected animals showed that several proteins involved in electron transport, mitochondrial dynamics, and mitochondrial protein synthesis were dysregulated. These results suggest that mitochondrial dysfunction, possibly exacerbated by prion protein overexpression, occurs at late stages during 263K prion disease and that this dysfunction may be the result of dysregulation of mitochondrial proteins.

Functions of the Prion Protein

Although initially disregarded compared to prion pathogenesis, the functions exerted by the cellular prion protein PrP^C have gained much interest over the past two decades. Research aiming at unraveling PrP^C functions started to intensify when it became appreciated that it would give clues as to how it is subverted in the context of prion infection and, more recently, in the context of Alzheimer's disease. It must now be admitted that PrP^C is implicated in an incredible variety of biological processes, including neuronal homeostasis, stem cell fate, protection against stress, or cell adhesion. It appears that these diverse roles can all be fulfilled through the involvement of PrP^C in cell signaling events. Our aim here is to provide an overview of our current understanding of PrP^C functions from the animal to the molecular scale and to highlight some of the remaining gaps that should be addressed in future research.

The Role of Unfolded Protein Response and Mitogen-Activated Protein Kinase Signaling in Neurodegenerative Diseases with Special Focus on Prion Diseases

Prion diseases are neurodegenerative pathologies characterized by the accumulation of a protease-resistant form of the cellular prion protein named prion protein scrapie (PrP^Sc) in the brain. PrP^Sc accumulation in the endoplasmic reticulum (ER) result in a dysregulated calcium (Ca²⁺) homeostasis and subsequent initiation of unfolded protein response (UPR) leading to neuronal dysfunction and apoptosis. The molecular mechanisms for the transition between adaptation to ER stress and ER stress-induced apoptosis are still unclear. Mitogen-activated protein kinases (MAPKs) are serine/threonine protein kinases that rule the signaling of many extracellular stimuli from plasma membrane to the nucleus. However the identification of numerous points of cross talk between the UPR and MAPK signaling pathways may contribute to our understanding of the consequences of ER stress in prion diseases. Indeed the MAPK signaling network is known to regulate cell cycle progression and cell survival or death responses following a variety of stresses including misfolded protein response stress. In this article, we review the UPR signaling in prion diseases and discuss the triad of MAPK signaling pathways. We also describe the role played by MAPK signaling cascades in Alzheimer’s (AD) and Parkinson’s disease (PD). We will also overview the mechanisms of cell death and the role of MAPK signaling in prion disease progression and highlight potential avenues for therapeutic intervention.

Abnormal Prion Accumulation Associated with Retinal Pathology in Experimentally Inoculated Scrapie-Affected Sheep

The purpose of this study was to characterize the patterns of PrPSc immunoreactivity in the retinae of scrapie-affected sheep and to determine the extent of retinal pathology as indicated by glial fibrillary acidic protein immunoreactivity (GFAP-IR) of Müller glia. Sections from the retina of 13 experimentally inoculated scrapie-affected and 2 negative control sheep were examined with immunohistochemical staining for PrPSc, GFAP, and PrPSc/GFAP double staining. GFAP-IR of Müller glia is suggestive of retinal pathology in the absence of morphologic abnormality detected by light microscopy. Sheep with the least amount of PrPSc in the retina have multifocal punctate aggregates of prion staining in the outer half of the inner plexiform layer and rarely in the outer plexiform layer. In these retinae, GFAP-IR is not localized with prion accumulation, but rather is present in moderate numbers of Müller glia throughout the sections of retina examined. The majority of sheep with retinal accumulation of PrPSc have intense, diffuse PrPSc staining in both plexiform layers, with immunoreactivity in the cytoplasm of multiple ganglion cells and lesser amounts in the optic fiber layer and between nuclei in nuclear layers. This intense PrPSc immunoreactivity is associated with diffuse, intense GFAP-IR that extends from the inner limiting membrane to the outer limiting membrane. This is the first report of a prion disease in a natural host that describes the accumulation of PrPSc in retina associated with retinal pathology in the absence of overt morphologic changes indicative of retinal degeneration.

Do prion protein gene polymorphisms induce apoptosis in non-mammals?

Genetic variations such as single nucleotide polymorphisms (SNPs) in prion protein coding gene, Prnp, greatly affect susceptibility to prion diseases in mammals. Here, the coding region of Prnp was screened for polymorphisms in redeared turtle, Trachemys scripta. Four polymorphisms, L203V, N205I, V225A and M237V, were common in 15 out of 30 turtles; in one sample, three SNPs, L203V, N205I and M237V, and in the remaining 14 samples, only L203V and N205I polymorphisms, were investigated. Besides, C658T, C664T, C670A and C823A SNPs were silent mutations. To elucidate the relationship between the SNPs and apoptosis, TUNEL assays and active caspase-3 immunodetection techniques in brain sections of the polymorphic samples were performed. The results revealed that TUNEL-positive cells and active caspase-3-positive cells in the turtles with four polymorphisms were significantly increased compared with those of the turtles with two polymorphisms (P less than 0.01 and P less than 0.05, respectively). In conclusion, this study provides preliminary information about the possible relationship between SNPs within the Prnp locus and apoptosis in a non-mammalian species, Trachemys scripta, in which prion disease has never been reported.

Molecular regulation of dendritic spine dynamics and their potential impact on synaptic plasticity and neurological diseases

The structure and dynamics of dendritic spines reflect the strength of synapses, which are severely affected in different brain diseases. Therefore, understanding the ultra-structure, molecular signaling mechanism(s) regulating dendritic spine dynamics is crucial. Although, since last century, dynamics of spine have been explored by several investigators in different neurological diseases, but despite countless efforts, a comprehensive understanding of the fundamental etiology and molecular signaling pathways involved in spine pathology is lacking. The purpose of this review is to provide a contextual framework of our current understanding of the molecular mechanisms of dendritic spine signaling, as well as their potential impact on different neurodegenerative and psychiatric diseases, as a format for highlighting some commonalities in function, as well as providing a format for new insights and perspectives into this critical area of research. Additionally, the potential strategies to restore spine structure-function in different diseases are also pointed out. Overall, these informations should help researchers to design new drugs to restore the structure-function of dendritic spine, a "hot site" of synaptic plasticity.

Modeling the effects of cyclodextrin on intracellular membrane vesicles from Cos-7 cells prepared by sonication and carbonate treatment

Cholesterol has important functions in the organization of membrane structure and this may be mediated via the formation of cholesterol-rich, liquid-ordered membrane microdomains often referred to as lipid rafts. Methyl-beta-cyclodextrin (cyclodextrin) is commonly used in cell biology studies to extract cholesterol and therefore disrupt lipid rafts. However, in this study we reassessed this experimental strategy and investigated the effects of cyclodextrin on the physical properties of sonicated and carbonate-treated intracellular membrane vesicles isolated from Cos-7 fibroblasts. We treated these membranes, which mainly originate from the trans-Golgi network and endosomes, with cyclodextrin and measured the effects on their equilibrium buoyant density, protein content, represented by the palmitoylated protein phosphatidylinositol 4-kinase type IIα, and cholesterol. Despite the reduction in mass stemming from cholesterol removal, the vesicles became denser, indicating a possible large volumetric decrease, and this was confirmed by measurements of hydrodynamic vesicle size. Subsequent mathematical analyses demonstrated that only half of this change in membrane size was attributable to cholesterol loss. Hence, the non-selective desorption properties of cyclodextrin are also involved in membrane size and density changes. These findings may have implications for preceding studies that interpreted cyclodextrin-induced changes to membrane biochemistry in the context of lipid raft disruption without taking into account our finding that cyclodextrin treatment also reduces membrane size.

The intricate mechanisms of neurodegeneration in prion diseases

Prion diseases are a group of infectious neurodegenerative diseases with an entirely novel mechanism of transmission, involving a protein-only infectious agent that propagates the disease by transmitting protein conformational changes. The disease results from extensive and progressive brain degeneration. The molecular mechanisms involved in neurodegeneration are not entirely known but involve multiple processes operating simultaneously and synergistically in the brain, including spongiform degeneration, synaptic alterations, brain inflammation, neuronal death and the accumulation of protein aggregates. Here, we review the pathways implicated in prion-induced brain damage and put the pieces together into a possible model of neurodegeneration in prion disorders. A more comprehensive understanding of the molecular basis of brain degeneration is essential to develop a much needed therapy for these devastating diseases.

Polo-like kinase 3 (PLK3) mediates the clearance of the accumulated PrP mutants transiently expressed in cultured cells and pathogenic PrP(Sc) in prion infected cell line via protein interaction

Polo-like kinases (PLKs) family has long been known to be critical for cell cycle and recent studies have pointed to new dimensions of PLKs function in the nervous system. Our previous study has verified that the levels of PLK3 in the brain are severely downregulated in prion-related diseases. However, the associations of PLKs with prion protein remain unclear. In the present study, we confirmed that PrP protein constitutively interacts with PLK3 as determined by both in vitro and in vivo assays. Both the kinase domain and polo-box domain of PLK3 were proved to bind PrP proteins expressed in mammalian cell lines. Overexpression of PLK3 did not affect the level of wild-type PrP, but significantly decreased the levels of the mutated PrPs in cultured cells. The kinase domain appeared to be responsible for the clearance of abnormally aggregated PrPs, but this function seemed to be independent of its kinase activity. RNA-mediated knockdown of PLK3 obviously aggravated the accumulation of cytosolic PrPs. Moreover, PLK3 overexpression in a scrapie infected cell line caused notable reduce of PrP(Sc) level in a dose-dependent manner, but had minimal effect on the expression of PrP(C) in its normal partner cell line. Our findings here confirmed the molecular interaction between PLK3 and PrP and outlined the regulatory activity of PLK3 on the degradation of abnormal PrPs, even its pathogenic isoform PrP(Sc). We, therefore, assume that the recovery of PLK3 in the early stage of prion infection may be helpful to prevent the toxic accumulation of PrP(Sc) in the brain tissues.

PrP(C) signalling in neurons: from basics to clinical challenges

The cellular prion protein PrP(C) was identified over twenty-five years ago as the normal counterpart of the scrapie prion protein PrP(Sc), itself the main if not the sole component of the infectious agent at the root of Transmissible Spongiform Encephalopathies (TSEs). PrP(C) is a ubiquitous cell surface protein, abundantly expressed in neurons, which constitute the targets of PrP(Sc)-mediated toxicity. Converging evidence have highlighted that neuronal, GPI-anchored PrP(C) is absolutely required for prion-induced neuropathogenesis, which warrants investigating into the normal function exerted by PrP(C) in a neuronal context. It is now well-established that PrP(C) can serve as a cell signalling molecule, able to mobilize transduction cascades in response to interactions with partners. This function endows PrP(C) with the capacity to participate in multiple neuronal processes, ranging from survival to synaptic plasticity. A diverse array of data have allowed to shed light on how this function is corrupted by PrP(Sc). Recently, amyloid Aβ oligomers, whose accumulation is associated with Alzheimer's disease (AD), were shown to similarly instigate toxic events by deviating PrP(C)-mediated signalling. Here, we provide an overview of the various signal transduction cascades ascribed to PrP(C) in neurons, summarize how their subversion by PrP(Sc) or Aβ oligomers contributes to TSE or AD neuropathogenesis and discuss the ensuing clinical implications.

Design and syntheses of peptides which induce or enhance structural changes of recombinant bovine prion protein (rbPrP) and discovery of peptides from bovine brain which accelerate structural conversions of rbPrP

The co-existence of certain peptides influenced the kinetic rate of aggregation and the lag-time of fibril formation of rbPrP. Using recently developed structural conversion assay system, peptides have been screened from bovine brain peptide library. Peptide sequences of positive components have been elucidated by mass spectrometry and chemically synthesized to confirm actions.

Characterization of peptides obtained from digests of bovine brain which accelerate structural conversions of the recombinant bovine prion protein

Structural changes of proteins are thought to involve specific protein-peptide interactions, thus we hypothesize that certain peptides may contribute to the conformational change of prion proteins. Hence peptide libraries were constructed from partial digests of bovine brain. Using a recently developed conversion assay method, we have screened peptides responsible for structural conversion. Positive components were identified of which amino acid sequences were elucidated by top-down sequencing using mass spectrometry. A database search identified a peptide derived from synaptophysin. This peptide was chemically synthesized to confirm acceleration of the structural change of recombinant bovine prion protein.

Early mechanisms of pathobiology are revealed by transcriptional temporal dynamics in hippocampal CA1 neurons of prion infected mice

Prion diseases typically have long pre-clinical incubation periods during which time the infectious prion particle and infectivity steadily propagate in the brain. Abnormal neuritic sprouting and synaptic deficits are apparent during pre-clinical disease, however, gross neuronal loss is not detected until the onset of the clinical phase. The molecular events that accompany early neuronal damage and ultimately conclude with neuronal death remain obscure. In this study, we used laser capture microdissection to isolate hippocampal CA1 neurons and determined their pre-clinical transcriptional response during infection. We found that gene expression within these neurons is dynamic and characterized by distinct phases of activity. We found that a major cluster of genes is altered during pre-clinical disease after which expression either returns to basal levels, or alternatively undergoes a direct reversal during clinical disease. Strikingly, we show that this cluster contains a signature highly reminiscent of synaptic N-methyl-D-aspartic acid (NMDA) receptor signaling and the activation of neuroprotective pathways. Additionally, genes involved in neuronal projection and dendrite development were also altered throughout the disease, culminating in a general decline of gene expression for synaptic proteins. Similarly, deregulated miRNAs such as miR-132-3p, miR-124a-3p, miR-16-5p, miR-26a-5p, miR-29a-3p and miR-140-5p follow concomitant patterns of expression. This is the first in depth genomic study describing the pre-clinical response of hippocampal neurons to early prion replication. Our findings suggest that prion replication results in the persistent stimulation of a programmed response that is mediated, at least in part, by synaptic NMDA receptor activity that initially promotes cell survival and neurite remodelling. However, this response is terminated prior to the onset of clinical symptoms in the infected hippocampus, seemingly pointing to a critical juncture in the disease. Manipulation of these early neuroprotective pathways may redress the balance between degeneration and survival, providing a potential inroad for treatment.

Neurodegenerative diseases affect an ever-increasing proportion of the population; therefore, there is an urgent need to develop treatments. Prion disorders belong to this group of diseases and although rare and uniquely transmissible, share many features on a sub-cellular level. Central to disease is progressive synaptic impairment that invariably leads to the irreversible loss of neurons. Understanding this process is undoubtedly essential for rational drug discovery. In this study we looked at neurons very early in disease, when prions are barely detectable and there are no clinical symptoms observed. Specifically, we performed a comprehensive analysis of transcriptional changes within a particularly dense area of neurons, the CA1 hippocampus region, from prion-infected and control mice. In this way we were able to enrich our data for molecular changes unique to neurons and minimize those changes characteristic of support cells such as astrocytes and microglia. We detected the activation of a transcriptional program indicative of a protective mechanism within these neurons early in disease. This mechanism diminished as disease progressed and was lost altogether, concurrently with the onset of clinical symptoms. These findings demonstrate the ability of neurons to mount an initial neuroprotective response to prions that could be exploited for therapy development.

Altered Prion protein expression pattern in CSF as a biomarker for Creutzfeldt-Jakob disease

Creutzfeldt-Jakob disease (CJD) is the most frequent human Prion-related disorder (PrD). The detection of 14-3-3 protein in the cerebrospinal fluid (CSF) is used as a molecular diagnostic criterion for patients clinically compatible with CJD. However, there is a pressing need for the identification of new reliable disease biomarkers. The pathological mechanisms leading to accumulation of 14-3-3 protein in CSF are not fully understood, however neuronal loss followed by cell lysis is assumed to cause the increase in 14-3-3 levels, which also occurs in conditions such as brain ischemia. Here we investigated the relation between the levels of 14-3-3 protein, Lactate dehydrogenase (LDH) activity and expression of the prion protein (PrP) in CSF of sporadic and familial CJD cases. Unexpectedly, we found normal levels of LDH activity in CJD cases with moderate levels of 14-3-3 protein. Increased LDH activity was only observed in a percentage of the CSF samples that also exhibited high 14-3-3 levels. Analysis of the PrP expression pattern in CSF revealed a reduction in PrP levels in all CJD cases, as well as marked changes in its glycosylation pattern. PrP present in CSF of CJD cases was sensitive to proteases. The alterations in PrP expression observed in CJD cases were not detected in other pathologies affecting the nervous system, including cases of dementia and tropical spastic paraparesis/HTLV-1 associated myelopathy (HAM/TSP). Time course analysis in several CJD patients revealed that 14-3-3 levels in CSF are dynamic and show a high degree of variability during the end stage of the disease. Post-mortem analysis of brain tissue also indicated that 14-3-3 protein is upregulated in neuronal cells, suggesting that its expression is modulated during the course of the disease. These results suggest that a combined analysis of 14-3-3 and PrP expression pattern in CSF is a reliable biomarker to confirm the clinical diagnosis of CJD patients and follow disease progression.

Signalosomes in the brain: relevance in the development of certain neuropathologies such as Alzheimer's disease

Emerging data suggest that compartmentalization of signaling molecules into particular membrane compartments, or lipid rafts, may be at the basis of numerous activities related to neuronal preservation against different pathologies. These signaling platforms (signalosomes) are formed by complex lipid and protein that may interact to develop a plethora of different physiological responses upon activation by different extracellular stimuli, thereby contributing to neuroprotection. One of the first studied signalosomes involved in neuroprotection against Alzheimer's disease (AD) is constituted by estrogen receptor (ER), in association with scaffolding caveolin-1 and a voltage-dependent anion channel (VDAC). In this complex, ER plays a neuroprotective role partially through the modulation of VDAC activation, a porin involved in amyloid-beta-induced toxicity. Interestingly, ER and VDAC interactions appear to be altered in lipid rafts of AD brains, a phenomenon that may contribute to neuronal impairment. Alterations in lipid components of these subdomains may contribute to destabilization of this macrocomplex. These recent advances in the relevance of signaling platforms related to brain preservation, in particular against AD, are discussed in this work.

Effect of glycans and the glycophosphatidylinositol anchor on strain dependent conformations of scrapie prion protein: improved purifications and infrared spectra

Mammalian prion diseases involve conversion of normal prion protein, PrP(C), to a pathological aggregated state (PrP(res)). The three-dimensional structure of PrP(res) is not known, but infrared (IR) spectroscopy has indicated high, strain-dependent β-sheet content. PrP(res) molecules usually contain a glycophosphatidylinositol (GPI) anchor and large Asn-linked glycans, which can also vary with strain. Using IR spectroscopy, we tested the conformational effects of these post-translational modifications by comparing wild-type PrP(res) with GPI- and glycan-deficient PrP(res) produced in GPI-anchorless PrP transgenic mice. These analyses required the development of substantially improved purification protocols. Spectra of both types of PrP(res) revealed conformational differences between the 22L, ME7, and Chandler (RML) murine scrapie strains, most notably in bands attributed to β-sheets. These PrP(res) spectra were also distinct from those of the hamster 263K scrapie strain. Spectra of wild-type and anchorless 22L PrP(res) were nearly indistinguishable. With ME7 PrP(res), modest differences between the wild-type and anchorless spectra were detected, notably an ∼2 cm(-1) shift in an apparent β-sheet band. Collectively, the data provide evidence that the glycans and anchor do not grossly affect the strain-specific secondary structures of PrP(res), at least relative to the differences observed between strains, but can subtly affect turns and certain β-sheet components. Recently reported H-D exchange analyses of anchorless PrP(res) preparations strongly suggested the presence of strain-dependent, solvent-inaccessible β-core structures throughout most of the C-terminal half of PrP(res) molecules, with no remaining α-helix. Our IR data provide evidence that similar core structures also comprise wild-type PrP(res).

Detection of the abnormal isoform of the prion protein associated with chronic wasting disease in the optic pathways of the brain and retina of Rocky Mountain elk (Cervus elaphus nelsoni)

Eyes and nuclei of the visual pathways in the brain were examined in 30 Rocky Mountain elk (Cervus elaphus nelsoni) representing 3 genotypes of the prion protein gene PRNP (codon 132: MM, ML, or LL). Tissues were examined for the presence of the abnormal isoform of the prion protein associated with chronic wasting disease (PrP(CWD)). Nuclei and axonal tracts from a single section of brain stem at the level of the dorsal motor nucleus of the vagus nerve were scored for intensity and distribution of PrP(CWD) immunoreactivity and degree of spongiform degeneration. This obex scoring ranged from 0 (elk with no PrP(CWD) in the brain stem) to 10 (representing elk in terminal stage of disease). PrP(CWD) was detected in the retina of 16 of 18 (89%) elk with an obex score of > 7. PrP(CWD) was not detected in the retina of the 3 chronic wasting disease-negative elk and 9 elk with an obex score of < 6. PrP(CWD) was found in the nuclei of the visual pathways in the brain before it was found in the retina. Within the retina, PrP(CWD) was first found in the inner plexiform layer, followed by the outer plexiform layer. Intracytoplasmic accumulation of PrP(CWD) was found in a few neurons in the ganglion cell layer in the PRNP 132ML elk but was a prominent feature in the PRNP 132LL elk. Small aggregates of PrP(CWD) were present on the inner surface of the outer limiting membrane in PRNP 132LL elk but not in PRNP 132MM or 132ML elk. This study demonstrates PrP(CWD) accumulation in nuclei of the visual pathways of the brain, followed by PrP(CWD) in the retina.

Immunohistochemical characterisation of classical scrapie neuropathology in sheep

Neuroinflammation elicited by PrP(res) (resistant prion protein [PrP]) deposits in the central nervous system (CNS) has been shown to involve cellular and oxidative stress responses in bovine spongiform encephalopathy (BSE) as well as in several murine models of transmissible spongiform encephalopathy (TSE). Additionally, deregulation of water homeostasis has been suggested to be a further component of the spongiform changes observed in TSEs. The aim of the present study was to characterize the pathogenic events occurring in the CNS of sheep with spontaneously arising classical scrapie. Brains from seven affected animals and two controls were subject to immunohistochemical and histochemical examinations. Semi-quantitative evaluation of PrP(res) deposits and spongiform changes throughout the encephalon confirmed that PrP(res) deposition elicits significant astroglial and microglial reactions, as evidenced by an increase in the number of glial cells and changes in glial cell morphology involving increased expression of vimentin. The altered expression of metallothionein and heat shock protein 25 (HSP25) suggested that this neuroinflammatory reaction entails cellular and oxidative stress responses. In contrast, there was no change in expression of the membrane-associated water channel aquaporin 1 when PrP(res) accumulated in the brain.

Binding of laminin-1 to monosialoganglioside GM1 in lipid rafts is crucial for neurite outgrowth

Laminin-1, an extracellular matrix molecule, promotes neurite outgrowth through the interaction of integrin and actin. Monosialoganglioside GM1 in the lipid rafts associates with and activates the NGF receptor TrkA, and enhances neurite outgrowth. However, the role of GM1 in laminin-1-induced neurite outgrowth was still unclear. Here, we describe that laminin-1 binds to GM1 through a carbohydrate moiety and a specific conformation of GM1, induces focal formation of large clusters of GM1, and enhances the relocation of TrkA in the membrane of dorsal root ganglion (DRG) and PC12 cells. We found that laminin-1-mediated clustering of GM1 causes the translocation and enrichment of beta1 integrin in lipid rafts--where TrkA colocalizes with beta1 integrin--and the activation of Lyn, Akt and MAPK to promote the outgrowth of neurites. Our results suggest that the binding of laminin-1 to GM1 facilitates the formation of a focal microdomain in the membrane, and enhances signal transduction that promotes neurite outgrowth by linking NGF-TrkA signaling with the laminin-integrin signaling pathways.

Unaltered SNARE complex formation in an in vivo model of prion disease

The ME7 model of prion disease is a chronic slowly evolving model of neurodegeneration in which cell death is preceded by synaptic dysfunction. Previous studies in cell culture show that accumulation of misfolded prion inhibits the formation of the SNARE complexes involving synaptobrevin, syntaxin and SNAP-25 that play an essential role in neurotransmitter release. Such observations suggest that similar phenomenon may contribute to synaptic dysfunction observed in vivo. We have thus used detergent extraction of hippocampal tissue to investigate the status of SNARE complexes in the ME7 model. In the presence of increasing PrP(Sc) deposition we failed to see a change in the amount of SNARE complexes directly extracted into SDS and resolved by SDS-PAGE. Conversely pre-extraction in Triton X-100, a treatment that promotes SNARE complexes ex vivo, demonstrated a modest reduction in hippocampal SNARE complexes when homogenates were made from tissue at late stage disease. This suggests that accumulated PrP(Sc), or perhaps fibrillar complexes formed of prion only inhibit SNARE complexes that are formed ex vivo following biochemical extraction. Thus the accumulation of PrP(Sc) although deleterious to synaptic function in vivo, does not exert its synaptic effects by disrupting the formation of SNARE complexes that are core to transmitter release.

Coexpression of wild-type and mutant prion proteins alters their cellular localization and partitioning into detergent-resistant membranes

Transmissible spongiform encephalopathies (TSEs) are a group of diseases of infectious, sporadic and genetic origin, found in higher organisms and caused by the pathological form of the prion protein. The inheritable subgroup of TSEs is linked to insertional or point mutations in the prion gene prnp, which favour its misfolding and are passed on to offspring in an autosomal-dominant fashion. The large majority of patients with these diseases are heterozygous for the prnp gene, leading to the coexpression of the wild-type (wt) (PrP(C)) and the mutant forms (PrPmut) in the carriers of these mutations. To mimic this situation in vitro, we produced Fischer rat thyroid cells coexpressing PrPwt alongside mutant versions of mouse PrP including A117V, E200K and T182A relevant to the human TSE diseases Gestmann-Sträussler-Scheinker (GSS) disease and familial Creutzfeldt-Jakob disease (fCJD). We found that coexpression of mutant PrP with wt proteins does not affect the glycosylation pattern or the biochemical characteristics of either protein. However, FRET and co-immunoprecipitation experiments suggest an interaction occurring between the wt and mutant proteins. Furthermore, by comparing the intracellular localization and detergent-resistant membrane (DRM) association in single- and double-expressing clones, we found changes in the intracellular/surface ratio and an increased sequestration of both proteins in DRMs, a site believed to be involved in the pathological conversion (or protection thereof) of the prion protein. We, therefore, propose that the mutant forms alter the subcellular localization and the membrane environment of the wt protein in co-transfected cells. These effects may play a role in the development of these diseases.

Sequestration of free cholesterol in cell membranes by prions correlates with cytoplasmic phospholipase A2 activation

Background

The transmissible spongiform encephalopathies (TSEs), otherwise known as the prion diseases, occur following the conversion of the normal cellular prion protein (PrP^C) to an alternatively folded isoform (PrP^Sc). The accumulation of PrP^Sc within the brain leads to neurodegeneration through an unidentified mechanism. Since many neurodegenerative disorders including prion, Parkinson's and Alzheimer's diseases may be modified by cholesterol synthesis inhibitors, the effects of prion infection on the cholesterol balance within neuronal cells were examined.

Results

We report the novel observation that prion infection altered the membrane composition and significantly increased total cholesterol levels in two neuronal cell lines (ScGT1 and ScN2a cells). There was a significant correlation between the concentration of free cholesterol in ScGT1 cells and the amounts of PrP^Sc. This increase was entirely a result of increased amounts of free cholesterol, as prion infection reduced the amounts of cholesterol esters in cells. These effects were reproduced in primary cortical neurons by the addition of partially purified PrP^Sc, but not by PrP^C. Crucially, the effects of prion infection were not a result of increased cholesterol synthesis. Stimulating cholesterol synthesis via the addition of mevalonate, or adding exogenous cholesterol, had the opposite effect to prion infection on the cholesterol balance. It did not affect the amounts of free cholesterol within neurons; rather, it significantly increased the amounts of cholesterol esters. Immunoprecipitation studies have shown that cytoplasmic phospholipase A₂ (cPLA₂) co-precipitated with PrP^Sc in ScGT1 cells. Furthermore, prion infection greatly increased both the phosphorylation of cPLA₂ and prostaglandin E₂ production.

Conclusion

Prion infection, or the addition of PrP^Sc, increased the free cholesterol content of cells, a process that could not be replicated by the stimulation of cholesterol synthesis. The presence of PrP^Sc increased solubilisation of free cholesterol in cell membranes and affected their function. It increased activation of the PLA₂ pathway, previously implicated in PrP^Sc formation and in PrP^Sc-mediated neurotoxicity. These observations suggest that the neuropathogenesis of prion diseases results from PrP^Sc altering cholesterol-sensitive processes. Furthermore, they raise the possibility that disturbances in membrane cholesterol are major triggering events in neurodegenerative diseases.

Statins reduce neuronal alpha-synuclein aggregation in in vitro models of Parkinson's disease

Aggregation of alpha-synuclein (alpha-syn) is believed to play a critical role in the pathogenesis of disorders such as dementia with Lewy bodies and Parkinson's disease. The function of alpha-syn remains unclear, although several lines of evidence suggest that alpha-syn is involved in synaptic vesicle trafficking probably via lipid binding. Moreover, interactions with cholesterol and lipids have been shown to be involved in alpha-syn aggregation. In this context, the main objective of this study was to determine if statins--cholesterol synthesis inhibitors--might interfere with alpha-syn accumulation in cellular models. For this purpose, we studied the effects of lovastatin, simvastatin, and pravastatin on the accumulation of alpha-syn in a stably transfected neuronal cell line and in primary human neurons. Statins reduced the levels of alpha-syn accumulation in the detergent insoluble fraction of the transfected cells. This was accompanied by a redistribution of alpha-syn in caveolar fractions, a reduction in oxidized alpha-syn, and enhanced neurite outgrowth. In contrast, supplementation of the media with cholesterol increased alpha-syn aggregation in detergent insoluble fractions of transfected cells and was accompanied by reduced neurite outgrowth. Taken together, these results suggest that regulation of cholesterol levels with cholesterol inhibitors might be a novel approach for the treatment of Parkinson's disease.

Antiprion activity of cholesterol esterification modulators: a comparative study using ex vivo sheep fibroblasts and lymphocytes and mouse neuroblastoma cell lines

Our studies on the role of cholesterol homeostasis in the pathogenesis of scrapie revealed abnormal accumulation of cholesterol esters in ex vivo peripheral blood mononuclear cells (PBMCs) and skin fibroblasts from healthy and scrapie-affected sheep carrying a scrapie-susceptible genotype compared to sheep with a resistant genotype. Similar alterations were observed in mouse neuroblastoma N2a cell lines persistently infected with mouse-adapted 22L and RML strains of scrapie that showed up to threefold-higher cholesterol ester levels than parental N2a cells. We now report that proteinase K-resistant prion protein (PrPres)-producing cell populations of subclones from scrapie-infected cell lines were characterized by higher cholesterol ester levels than clone populations not producing PrPres. Treatments with a number of drugs known to interfere with different steps of cholesterol metabolism strongly reduced the accumulation of cholesterol esters in ex vivo PBMCs and skin fibroblasts from scrapie-affected sheep but had significantly less or no effect in their respective scrapie-resistant or uninfected counterparts. In scrapie-infected N2a cells, inhibition of cholesterol esters was associated with selective antiprion activity. Effective antiprion concentrations of cholesterol modulators (50% effective concentration [EC(50)] range, 1.4 to 40 microM) were comparable to those of antiprion reference compounds (EC(50) range, 0.6 to 10 microM). These data confirm our hypothesis that abnormal accumulation of cholesterol esters may represent a biological marker of susceptibility to prion infection/replication and a novel molecular target of potential clinical importance.

Dendritic pathology in prion disease starts at the synaptic spine

Spine loss represents a common hallmark of neurodegenerative diseases. However, little is known about the underlying mechanisms, especially the relationship between spine elimination and neuritic destruction. We imaged cortical dendrites throughout a neurodegenerative disease using scrapie in mice as a model. Two-photon in vivo imaging over 2 months revealed a linear decrease of spine density. Interestingly, only persistent spines (lifetime > or = 8 d) disappeared, whereas the density of transient spines (lifetime < or = 4 d) was unaffected. Before spine loss, dendritic varicosities emerged preferentially at sites where spines protrude from the dendrite. These results implicate that the location where the spine protrudes from the dendrite may be particularly vulnerable and that dendritic varicosities may actually cause spine loss.

Lipid rafts in health and disease

Lipid rafts are sphingolipid- and cholesterol-rich domains of the plasma membrane which contain a variety of signalling and transport proteins. Different subtypes of lipid rafts can be distinguished according to their protein and lipid composition. Caveolae are types of rafts that are rich in proteins of the caveolin family (caveolin-1, -2 and -3) which present a distinct signalling platform. The importance of lipid raft signalling in the pathogenesis of a variety of conditions, such as Alzheimer's, Parkinson's, cardiovascular and prion diseases, systemic lupus erythematosus and HIV, has been elucidated over recent years and makes these specific membrane domains an interesting target for pharmacological approaches in the cure and prevention of these diseases. This Review analyses the importance of lipid raft proteins and lipids in health and disease, with a focus on the current state of knowledge.

Perturbation of endoplasmic reticulum homeostasis facilitates prion replication

Prion diseases are fatal and infectious neurodegenerative disorders characterized by the accumulation of an abnormally folded form of the prion protein (PrP), termed PrP(Sc). Prion replication triggers endoplasmic reticulum (ER) stress, neuronal dysfunction, and apoptosis. In this study we analyze the effect of perturbations in ER homeostasis on PrP biochemical properties and prion replication. ER stress led to the generation of a mis-folded PrP isoform, which is detergent-insoluble and protease-sensitive. To understand the mechanism by which ER stress generates PrP misfolding, we assessed the contribution of different signaling pathways implicated in the unfolded protein response. Expression of a dominant negative form of IRE1 alpha or XBP-1 significantly increased PrP aggregation, whereas overexpression of ATF4 or an active mutant form of XBP-1 and ATF6 had the opposite affect. Analysis of prion replication in vitro revealed that the PrP isoform generated after ER stress is more efficiently converted into PrP(Sc) compared with the protein extracted from untreated cells. These findings indicate that ER-damaged cells might be more susceptible to prion replication. Because PrP(Sc) induces ER stress, our data point to a vicious cycle accelerating prion replication, which may explain the rapid progression of the disease.

Lipid raft microdomains and neurotransmitter signalling

Lipid rafts are specialized structures on the plasma membrane that have an altered lipid composition as well as links to the cytoskeleton. It has been proposed that these structures are membrane domains in which neurotransmitter signalling might occur through a clustering of receptors and components of receptor-activated signalling cascades. The localization of these proteins in lipid rafts, which is affected by the cytoskeleton, also influences the potency and efficacy of neurotransmitter receptors and transporters. The effect of lipid rafts on neurotransmitter signalling has also been implicated in neurological and psychiatric diseases.

Oxidative stress in the brain at early preclinical stages of mouse scrapie

Oxidative stress has been shown to be involved in the pathogenesis of neurodegenerative diseases including prion diseases. Although a growing body of evidence suggests direct involvement of oxidative stress in the pathogenesis of prion diseases, it is still not clear whether oxidative stress is a causative early event in these conditions or a secondary phenomenon commonly found in the progression of neurodegenerative diseases. Using a mouse scrapie model, we assessed oxidative stress in the brain at various stages of the disease progression and observed significantly increased concentration of lipid peroxidation markers, malondialdehyde and 4-hydroxyalkenals, and mRNA level of an oxidative stress response enzyme, heme oxygenase-1, at early preclinical stages of scrapie. The changes preceded dramatic synaptic loss demonstrated by immunohistochemical staining of a synaptic protein, synaptophysin. These findings imply that the brain undergoes oxidative stress even from an early stage of prion invasion into the brain. Given the well-known deleterious effects of reactive-oxygen-species-mediated damage in the brain, it is considered that the oxidative stress at the preclinical stage of prion diseases may predispose the brain to neurodegenerative mechanisms that characterize the diseases.

Immunohistochemical study of caveolin-1 and -2 in the rat retina

The expression of caveolin-1 and -2 in the retina was examined; Western blot analysis showed that both were present. Immunohistochemistry indicated that caveolin-1 was expressed in the majority of retinal layers, including the ganglion cell layer, inner plexiform layer, outer plexiform layer, and in the vascular endothelial cells of the retina. Caveolin-2 was primarily immunostained in the vessels, but in a few other elements as well. This is the first demonstration of caveolin differential expression in the retina of rats, and suggests that caveolin plays an important role in signal transduction in glial cells and neuronal cells.

Immunohistochemical approach to the pathogenesis of bovine spongiform encephalopathy in its early stages

An immunohistochemical and histochemical study was carried out on the brains of nine cases of BSE-diagnosed cattle as part of the surveillance plan in Catalonia, Spain. The animals had no clinical symptoms reported and were thus at early stages of the disease. The first part of the study consisted of a characterization of PrP(BSE) deposits throughout the encephalon. The behaviour of the different immuno-labelling patterns was analysed and tropism of some patterns towards certain brain areas was described. This tropism is principally directed to the brain stem region; however, an association of the stellate pattern was found with areas where PrP(BSE) is deposited less abundantly, such as the cerebral cortex. Secondly, distinct pathogenesis mechanisms that take place in the early stages of BSE, which would include these cases were investigated. This study describes the glial response to the presence of PrP(BSE) (using antibodies against astrocytic glial fibrillary acidic protein and lectin from Griffonia simplicifolia to identify microglia), the presence of mild oxidative stress phenomena (antibodies against metallothioneins I and II and against nitrated aminoacidic residues: nitrotyrosine), the apparent absence of apoptotic cellular death (cleaved caspase 3) and the preservation of synaptic proteins synaptophysin and small synaptosome-associated 25 kDa protein immuno-labelling. Finally, no alteration of the extra-cellular matrix was detected with the use of Wisteria floribunda agglutinin, a marker for perineuronal nets.

The prion protein and lipid rafts

Prions are the causative agent of the transmissible spongiform encephalopathies, such as Creutzfeldt-Jakob disease in humans. In these prion diseases the normal cellular form of the prion protein (PrP(C)) undergoes a post-translational conformational conversion to the infectious form (PrP(Sc)). PrP(C) associates with cholesterol- and glycosphingolipid-rich lipid rafts through association of its glycosyl-phosphatidylinositol (GPI) anchor with saturated raft lipids and through interaction of its N-terminal region with an as yet unidentified raft associated molecule. PrP(C) resides in detergent-resistant domains that have different lipid and protein compositions to the domains occupied by another GPI-anchored protein, Thy-1. In some cells PrP(C) may endocytose through caveolae, but in neuronal cells, upon copper binding to the N-terminal octapeptide repeats, the protein translocates out of rafts into detergent-soluble regions of the plasma membrane prior to endocytosis through clathrin-coated pits. The current data suggest that the polybasic region at its N-terminus is required to engage PrP(C) with a transmembrane adaptor protein which in turn links with the clathrin endocytic machinery. PrP(C) associates in rafts with a variety of signalling molecules, including caveolin-1 and Fyn and Src tyrosine kinases. The clustering of PrP(C) triggers a range of signal transduction processes, including the recruitment of the neural cell adhesion molecule to rafts which in turn promotes neurite outgrowth. Lipid rafts appear to be involved in the conformational conversion of PrP(C) to PrP(Sc), possibly by providing a favourable environment for this process to occur and enabling disease progression.

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

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