Anti-PrPC monoclonal antibody infusion as a novel treatment for cognitive deficits in an Alzheimer's disease model mouse

Prion disease: a tale of folds and strains

Research on prions, the infectious agents of devastating neurological diseases in humans and animals, has been in the forefront of developing the concept of protein aggregation diseases. Prion diseases are distinguished from other neurodegenerative diseases by three peculiarities. First, prion diseases, in addition to being sporadic or genetic like all other neurodegenerative diseases, are infectious diseases. Animal models were developed early on (a long time before the advent of transgenic technology), and this has made possible the discovery of the prion protein as the infectious agent. Second, human prion diseases have true equivalents in animals, such as scrapie, which has been the subject of experimental research for many years. Variant Creutzfeldt-Jakob disease (vCJD) is a zoonosis caused by bovine spongiform encephalopathy (BSE) prions. Third, they show a wide variety of phenotypes in humans and animals, much wider than the variants of any other sporadic or genetic neurodegenerative disease. It has now become firmly established that particular PrP(Sc) isoforms are closely related to specific human prion strains. The variety of human prion diseases, still an enigma in its own right, is a focus of this article. Recently, a series of experiments has shown that the concept of aberrant protein folding and templating, first developed for prions, may apply to a variety of neurodegenerative diseases. In the wake of these discoveries, the term prion has come to be used for Aβ, α-synuclein, tau and possibly others. The self-propagation of alternative conformations seems to be the common denominator of these "prions," which in future, in order to avoid confusion, may have to be specified either as "neurodegenerative prions" or "infectious prions."

Alzheimer's disease and prion protein

Alzheimer's disease (AD) is a devastating neurodegenerative disease with progressive loss of memory and cognitive function, pathologically hallmarked by aggregates of the amyloid-beta (Aβ) peptide and hyperphosphorylated tau in the brain. Aggregation of Aβ under the form of amyloid fibrils has long been considered central to the pathogenesis of AD. However, recent evidence has indicated that soluble Aβ oligomers, rather than insoluble fibrils, are the main neurotoxic species in AD. The cellular prion protein (PrP(C)) has newly been identified as a cell surface receptor for Aβ oligomers. PrP(C) is a cell surface glycoprotein that plays a key role in the propagation of prions, proteinaceous infectious agents that replicate by imposing their abnormal conformation to PrP(C) molecules. In AD, PrP(C) acts to transduce the neurotoxic signals arising from Aβ oligomers, leading to synaptic failure and cognitive impairment. Interestingly, accumulating evidence has also shown that aggregated Aβ or tau possesses prion-like activity, a property that would allow them to spread throughout the brain. In this article, we review recent findings regarding the function of PrP(C) and its role in AD, and discuss potential therapeutic implications of PrP(C)-based approaches in the treatment of AD.

CSF prion protein concentration and cognition in patients with Alzheimer disease

BACKGROUND/OBJECTIVE

PrP (c) has been suggested to play a role in AD pathophysiology. CSF concentrations of PrP (c) have been shown to be reduced in AD compared with healthy controls. Furthermore, serum levels of PrP (c) have recently been reported to be associated with the cognitive status of healthy elderly subjects. Therefore, we hypothesized that CSF levels of PrP (c) could be associated with cognitive function of AD patients at the time of diagnosis.

METHODS

AD patients (n = 114) included into an observational study underwent CERAD testing and lumbar puncture at time of diagnosis / study inclusion. CSF PrP (c) was determined. Generalized linear models were fitted to assess the associations of PrP (c) plus a variety of possible confounding factors and CERAD subscale measures.

RESULTS

No association of CSF PrP (c) and cognitive status could be established, while other factors (i.e., use of antipsychotic drugs, use of anti-dementia drugs, female sex, pre-progression time) were related to worse cognitive function in some domains.

CONCLUSION

CSF PrP (c) appears not to be a useful biochemical surrogate of cognitive status in AD at the time of diagnosis. Follow-up analyses will examine possible associations with the speed of cognitive decline.

Prion protein-mediated toxicity of amyloid-β oligomers requires lipid rafts and the transmembrane LRP1

Soluble oligomers of the amyloid-β (Aβ) peptide cause neurotoxicity, synaptic dysfunction, and memory impairments that underlie Alzheimer disease (AD). The cellular prion protein (PrP^C) was recently identified as a high affinity neuronal receptor for Aβ oligomers. We report that fibrillar Aβ oligomers recognized by the OC antibody, which have been shown to correlate with the onset and severity of AD, bind preferentially to cells and neurons expressing PrP^C. The binding of Aβ oligomers to cell surface PrP^C, as well as their downstream activation of Fyn kinase, was dependent on the integrity of cholesterol-rich lipid rafts. In SH-SY5Y cells, fluorescence microscopy and co-localization with subcellular markers revealed that the Aβ oligomers co-internalized with PrP^C, accumulated in endosomes, and subsequently trafficked to lysosomes. The cell surface binding, internalization, and downstream toxicity of Aβ oligomers was dependent on the transmembrane low density lipoprotein receptor-related protein-1 (LRP1). The binding of Aβ oligomers to cell surface PrP^C impaired its ability to inhibit the activity of the β-secretase BACE1, which cleaves the amyloid precursor protein to produce Aβ. The green tea polyphenol (−)-epigallocatechin gallate and the red wine extract resveratrol both remodeled the fibrillar conformation of Aβ oligomers. The resulting nonfibrillar oligomers displayed significantly reduced binding to PrP^C-expressing cells and were no longer cytotoxic. These data indicate that soluble, fibrillar Aβ oligomers bind to PrP^C in a conformation-dependent manner and require the integrity of lipid rafts and the transmembrane LRP1 for their cytotoxicity, thus revealing potential targets to alleviate the neurotoxic properties of Aβ oligomers in AD.

The complex PrP(c)-Fyn couples human oligomeric Aβ with pathological tau changes in Alzheimer's disease

Amid controversy, the cellular form of the prion protein PrP(c) has been proposed to mediate oligomeric amyloid-β (Aβ)-induced deficits. In contrast, there is consistent evidence that the Src kinase Fyn is activated by Aβ oligomers and leads to synaptic and cognitive impairment in transgenic animals. However, the molecular mechanism by which soluble Aβ activates Fyn remains unknown. Combining the use of human and transgenic mouse brain tissue as well as primary cortical neurons, we demonstrate that soluble Aβ binds to PrP(c) at neuronal dendritic spines in vivo and in vitro where it forms a complex with Fyn, resulting in the activation of the kinase. Using the antibody 6D11 to prevent oligomeric Aβ from binding to PrP(c), we abolished Fyn activation and Fyn-dependent tau hyperphosphorylation induced by endogenous oligomeric Aβ in vitro. Finally, we showed that gene dosage of Prnp regulates Aβ-induced Fyn/tau alterations. Together, our findings identify a complete signaling cascade linking one specific endogenous Aβ oligomer, Fyn alteration, and tau hyperphosphorylation in cellular and animal models modeling aspects of the molecular pathogenesis of Alzheimer's disease.

An N-terminal fragment of the prion protein binds to amyloid-β oligomers and inhibits their neurotoxicity in vivo

A hallmark of Alzheimer disease (AD) is the accumulation of the amyloid-β (Aβ) peptide in the brain. Considerable evidence suggests that soluble Aβ oligomers are responsible for the synaptic dysfunction and cognitive deficit observed in AD. However, the mechanism by which these oligomers exert their neurotoxic effect remains unknown. Recently, it was reported that Aβ oligomers bind to the cellular prion protein with high affinity. Here, we show that N1, the main physiological cleavage fragment of the cellular prion protein, is necessary and sufficient for binding early oligomeric intermediates during Aβ polymerization into amyloid fibrils. The ability of N1 to bind Aβ oligomers is influenced by positively charged residues in two sites (positions 23–31 and 95–105) and is dependent on the length of the sequence between them. Importantly, we also show that N1 strongly suppresses Aβ oligomer toxicity in cultured murine hippocampal neurons, in a Caenorhabditis elegans-based assay, and in vivo in a mouse model of Aβ-induced memory dysfunction. These data suggest that N1, or small peptides derived from it, could be potent inhibitors of Aβ oligomer toxicity and represent an entirely new class of therapeutic agents for AD.

The neurodegeneration in Alzheimer disease and the prion protein

The concept of "prion-like" has been proposed to explain the pathogenic mechanism of the principal neurodegenerative disorders associated with protein misfolding, including Alzheimer disease (AD). Other evidence relates prion protein with AD: the cellular prion protein (PrP(C)) binds β amyloid oligomers, allegedly responsible for the neurodegeneration in AD, mediating their toxic effects. We and others have confirmed the high-affinity binding between β amyloid oligomers and PrP(C), but we were not able to assess the functional consequences of this interaction using behavioral investigations and in vitro tests. This discrepancy rather than being resolved with the classic explanations, differencies in methodological aspects, has been reinforced by new data from different sources. Here we present data obtained with PrP antibody that not interfere with the neurotoxic activity of β amyloid oligomers. Since the potential role of the PrP(C) in the neuronal dysfunction induced by β amyloid oligomers is an important issue, find reasonable explanation of the inconsistent results is needed. Even more important however is the relevance of this interaction in the context of the disease, so as to develop valid therapeutic strategies.

Drug pipeline in neurodegeneration based on transgenic mice models of Alzheimer's disease

Alzheimer's disease (AD) is one of the most important neurodegenerative disorders, bringing about huge medical and social burden in the elderly worldwide. Many aspects of its pathogenesis have remained unclear and no effective treatment exists for it. Within the past 20 years, various mice models harboring AD-related human mutations have been produced. These models imitate diverse AD-related pathologies and have been used for basic and therapeutic investigations in AD. In this regard, there are a wide variety of preclinical trials of potential therapeutic modalities using AD mice models which are of paramount importance for future clinical trials and applications. This review summarizes more than 140 substances and treatment modalities being used in transgenic AD mice models from 2001 to 2011. We also discuss advantages and disadvantages of each model to be used in therapeutic development for AD.

The heat shock response is modulated by and interferes with toxic effects of scrapie prion protein and amyloid β

The heat shock response (HSR) is an evolutionarily conserved pathway designed to maintain proteostasis and to ameliorate toxic effects of aberrant protein folding. We have studied the modulation of the HSR by the scrapie prion protein (PrP(Sc)) and amyloid β peptide (Aβ) and investigated whether an activated HSR or the ectopic expression of individual chaperones can interfere with PrP(Sc)- or Aβ-induced toxicity. First, we observed different effects on the HSR under acute or chronic exposure of cells to PrP(Sc) or Aβ. In chronically exposed cells the threshold to mount a stress response was significantly increased, evidenced by a decreased expression of Hsp72 after stress, whereas an acute exposure lowered the threshold for stress-induced expression of Hsp72. Next, we employed models of PrP(Sc)- and Aβ-induced toxicity to demonstrate that the induction of the HSR ameliorates the toxic effects of both PrP(Sc) and Aβ. Similarly, the ectopic expression of cytosolic Hsp72 or the extracellular chaperone clusterin protected against PrP(Sc)- or Aβ-induced toxicity. However, toxic signaling induced by a pathogenic PrP mutant located at the plasma membrane was prevented by an activated HSR or Hsp72 but not by clusterin, indicating a distinct mode of action of this extracellular chaperone. Our study supports the notion that different pathological protein conformers mediate toxic effects via similar cellular pathways and emphasizes the possibility to exploit the heat shock response therapeutically.

Amyloid beta precursor protein and prion protein have a conserved interaction affecting cell adhesion and CNS development

Genetic and biochemical mechanisms linking onset or progression of Alzheimer Disease and prion diseases have been lacking and/or controversial, and their etiologies are often considered independent. Here we document a novel, conserved and specific genetic interaction between the proteins that underlie these diseases, amyloid-β precursor protein and prion protein, APP and PRP, respectively. Knockdown of APP and/or PRNP homologs in the zebrafish (appa, appb, prp1, and prp2) produces a dose-dependent phenotype characterized by systemic morphological defects, reduced cell adhesion and CNS cell death. This genetic interaction is surprisingly exclusive in that prp1 genetically interacts with zebrafish appa, but not with appb, and the zebrafish paralog prp2 fails to interact with appa. Intriguingly, appa & appb are largely redundant in early zebrafish development yet their abilities to rescue CNS cell death are differentially contingent on prp1 abundance. Delivery of human APP or mouse Prnp mRNAs rescue the phenotypes observed in app-prp-depleted zebrafish, highlighting the conserved nature of this interaction. Immunoprecipitation revealed that human APP and PrP(C) proteins can have a physical interaction. Our study reports a unique in vivo interdependence between APP and PRP loss-of-function, detailing a biochemical interaction that considerably expands the hypothesized roles of PRP in Alzheimer Disease.

Cellular prion protein and Alzheimer disease: link to oligomeric amyloid-β and neuronal cell death

Soluble oligomeric amyloid-β (Aβ) has been suggested to impair synaptic and neuronal function, leading to neurodegeneration that is clinically observed as the memory and cognitive dysfunction characteristic of Alzheimer disease, while the precise mechanism(s) whereby oligomeric Aβ causes neurotoxicity remains unknown. Recently, the cellular prion protein (PrP (C) ) was reported to be an essential co-factor in mediating the neurotoxic effect of oligomeric Aβ. Our recent study showed that Prnp (-/-) mice are resistant to the neurotoxic effect of oligomeric Aβ in vivo and in vitro. Furthermore, application of an anti-PrP (C) antibody or PrP (C) peptide was able to block oligomeric Aβ-induced neurotoxicity. These findings demonstrate that PrP (C) may be involved in neuropathologic conditions other than conventional prion diseases, i.e., Creutzfeldt-Jakob disease.

Amyloid-β induced signaling by cellular prion protein and Fyn kinase in Alzheimer disease

Alzheimer disease (AD) is the most prevalent cause of dementia. Amyloid-β (Aβ) oligomers are potent synaptotoxins thought to mediate AD-related phenotypes. Cellular prion protein (PrP(C)) has been identified as a high-affinity receptor for Aβ oligomers. Herein, we review the functional consequences of Aβ oligomer binding to PrP(C) on the neuronal surface. We highlight recent evidence that Fyn kinase mediates signal transduction downstream of the PrP(C)-Aβ oligomer complex. These studies suggest that PrP(C) has a central role in AD pathogenesis and may provide a target for therapeutic intervention in AD.

The N-terminal, polybasic region of PrP(C) dictates the efficiency of prion propagation by binding to PrP(Sc)

Prion propagation involves a templating reaction in which the infectious form of the prion protein (PrP(Sc)) binds to the cellular form (PrP(C)), generating additional molecules of PrP(Sc). While several regions of the PrP(C) molecule have been suggested to play a role in PrP(Sc) formation based on in vitro studies, the contribution of these regions in vivo is unclear. Here, we report that mice expressing PrP deleted for a short, polybasic region at the N terminus (residues 23-31) display a dramatically reduced susceptibility to prion infection and accumulate greatly reduced levels of PrP(Sc). These results, in combination with biochemical data, demonstrate that residues 23-31 represent a critical site on PrP(C) that binds to PrP(Sc) and is essential for efficient prion propagation. It may be possible to specifically target this region for treatment of prion diseases as well as other neurodegenerative disorders due to β-sheet-rich oligomers that bind to PrP(C).

Targeting synaptic dysfunction in Alzheimer's disease therapy

In the past years, major efforts have been made to understand the genetics and molecular pathogenesis of Alzheimer's disease (AD), which has been translated into extensive experimental approaches aimed at slowing down or halting disease progression. Advances in transgenic (Tg) technologies allowed the engineering of different mouse models of AD recapitulating a range of AD-like features. These Tg models provided excellent opportunities to analyze the bases for the temporal evolution of the disease. Several lines of evidence point to synaptic dysfunction as a cause of AD and that synapse loss is a pathological correlate associated with cognitive decline. Therefore, the phenotypic characterization of these animals has included electrophysiological studies to analyze hippocampal synaptic transmission and long-term potentiation, a widely recognized cellular model for learning and memory. Transgenic mice, along with non-Tg models derived mainly from exogenous application of Aβ, have also been useful experimental tools to test the various therapeutic approaches. As a result, numerous pharmacological interventions have been reported to attenuate synaptic dysfunction and improve behavior in the different AD models. To date, however, very few of these findings have resulted in target validation or successful translation into disease-modifying compounds in humans. Here, we will briefly review the synaptic alterations across the different animal models and we will recapitulate the pharmacological strategies aimed at rescuing hippocampal plasticity phenotypes. Finally, we will highlight intrinsic limitations in the use of experimental systems and related challenges in translating preclinical studies into human clinical trials.

Association of prion protein with cognitive functioning in humans

OBJECTIVES

Recent animal studies have suggested a key role for cellular prion protein (PrPc) in the pathological consequences of amyloid plaque formation, the hallmark of Alzheimer's disease. This epidemiological study investigated whether serum concentrations of PrPc are associated with cognitive functioning in humans.

DESIGN, SETTING, PARTICIPANTS

Cross-sectional study of 1,322 participants from the elderly general population in Germany, aged 65+ years at baseline (2000-2002).

MEASUREMENTS

Cognitive functioning was assessed by the COGTEL phone interview 5years after baseline. Serum PrPc was determined by a commercial immunoassay.

RESULTS

In multiple linear regression adjusted for important confounders, subjects in higher PrPc quintiles appeared to have lower cognitive functioning scores than those in the lowest PrPc quintile. Spline regression suggested pronounced non-linearity with an inverse association between PrPc and cognitive functioning levelling off beyond median PrPc. Cognitive subdomain-specific models produced somewhat heterogeneous results.

CONCLUSION

The findings are suggestive of an independent association of PrPc with cognitive functioning in humans. Confirmatory and longitudinal studies are needed to elucidate the potential of PrPc for applications in early risk stratification for cognitive impairment.

Alzheimer amyloid-β oligomer bound to postsynaptic prion protein activates Fyn to impair neurons

Amyloid-beta (Aβ) oligomers are thought to trigger Alzheimer’s disease (AD) pathophysiology. Cellular Prion Protein (PrP^C) selectively binds oligomeric Aβ and can mediate AD-related phenotypes. Here, we examined the specificity, distribution and signaling from Aβ/PrP complexes, seeking to explain how they might alter the function of NMDA receptors in neurons. PrP^C is enriched in post-synaptic densities, and Aβ/PrP^C interaction leads to Fyn kinase activation. Soluble Aβ assemblies derived from human AD brain interact with PrP^C to activate Fyn. Aβ engagement of PrP^C/Fyn signaling yields phosphorylation of the NR2B subunit of NMDA-receptors, which is coupled to an initial increase and then loss of surface NMDA-receptors. Aβ-induced LDH release and dendritic spine loss require both PrP^C and Fyn, and human familial AD transgene-induced convulsive seizures do not occur in mice lacking PrP^C. These results delineate an Aβ oligomer signal transduction pathway requiring PrP^C and Fyn to alter synaptic function with relevance to AD.

Alzheimer's disease Aβ assemblies mediating rapid disruption of synaptic plasticity and memory

Alzheimer’s disease (AD) is characterized by episodic memory impairment that often precedes clinical diagnosis by many years. Probing the mechanisms of such impairment may provide much needed means of diagnosis and therapeutic intervention at an early, pre-dementia, stage. Prior to the onset of significant neurodegeneration, the structural and functional integrity of synapses in mnemonic circuitry is severely compromised in the presence of amyloidosis. This review examines recent evidence evaluating the role of amyloid-ß protein (Aβ) in causing rapid disruption of synaptic plasticity and memory impairment. We evaluate the relative importance of different sizes and conformations of Aβ, including monomer, oligomer, protofibril and fibril. We pay particular attention to recent controversies over the relevance to the pathophysiology of AD of different water soluble Aβ aggregates and the importance of cellular prion protein in mediating their effects. Current data are consistent with the view that both low-n oligomers and larger soluble assemblies present in AD brain, some of them via a direct interaction with cellular prion protein, cause synaptic memory failure. At the two extremes of aggregation, monomers and fibrils appear to act in vivo both as sources and sinks of certain metastable conformations of soluble aggregates that powerfully disrupt synaptic plasticity. The same principle appears to apply to other synaptotoxic amyloidogenic proteins including tau, α-synuclein and prion protein.

Regulation of amyloid-β production by the prion protein

Alzheimer disease (AD) is characterized by the amyloidogenic processing of the amyloid precursor protein (APP), culminating in the accumulation of amyloid-β peptides in the brain. The enzymatic action of the β-secretase, BACE1 is the rate-limiting step in this amyloidogenic processing of APP. BACE1 cleavage of wild-type APP (APPWT) is inhibited by the cellular prion protein (PrP (C) ). Our recent study has revealed the molecular and cellular mechanisms behind this observation by showing that PrP (C) directly interacts with the pro-domain of BACE1 in the trans-Golgi network (TGN), decreasing the amount of BACE1 at the cell surface and in endosomes where it cleaves APPWT, while increasing BACE1 in the TGN where it preferentially cleaves APP with the Swedish mutation (APPSwe). PrP (C) deletion in transgenic mice expressing the Swedish and Indiana familial mutations (APPSwe,Ind) failed to affect amyloid-β accumulation, which is explained by the differential subcellular sites of action of BACE1 toward APPWT and APPSwe. This, together with our observation that PrP (C) is reduced in sporadic but not familial AD brain, suggests that PrP (C) plays a key protective role against sporadic AD. It also highlights the need for an APPWT transgenic mouse model to understand the molecular and cellular mechanisms underlying sporadic AD.

The association between prion proteins and Aβ₁₋₄₂ oligomers in cytotoxicity and apoptosis

Misfolding of prion protein (PrP to PrPSc) can cause neurodegenerative prion diseases. As a glycosylphosphatidylinositol (GPI)-anchored membrane protein, the normal form of PrP (PrPC) can function as a receptor for ligands in the extracellular space. PrPC was suggested to be involved in memory, synaptic neuronal communication, and anti-oxidation as a neuroprotective agent. The recently identified interaction between PrPC and Aβ(1-42) oligomers suggested another role for PrP as a receptor for Aβ(1-42) oligomers, thereby influencing cytotoxicity and apoptosis. Here, the association between PrPC and Aβ(1-42) oligomers was investigated by visualizing protein localization in neuronal cells by immunocytochemistry. Aβ(1-42) oligomer-induced cytotoxicity was tested in respective expressions of PrPC by using mouse neuroblastoma-2a (N2a) cells, the prion protein overexpressed cells (L2-2B1), and a Prnp-null mouse hippocampal cell line (HpL 3-4). Moreover, apoptotic proteins such as caspase-8 were used to assess the effect of PrPC on Aβ(1-42) oligomer-mediated apoptosis. In L2-2B1 and HpL 3-4 cells, the difference in the cytotoxicity of Aβ(1-42) oligomers could be clearly distinguished. In addition, Aβ(1-42) oligomers induced mitochondria dysfunction, reactive oxygen species (ROS) generation, and calcium influx PrPC-dependently. Apoptosis, related to mitochondria dysfunction, was further investigated to determine the cytotoxic pathway; the results suggest that PrPC could be involved in both the intrinsic and extrinsic apoptotic pathways. Finally, cells with abundant PrPC expression seemed to be more susceptible to Aβ(1-42) oligomer toxicity, suggesting the importance of the level of PrPC expression in the induction of apoptosis.

Proteolytic processing of the prion protein in health and disease

A variety of physiological functions, not only restricted to the nervous system, are discussed for the cellular prion protein (PrP(C)). A prominent, non-physiological property of PrPC is the conversion into its pathogenic isoform (PrP(Sc)) during fatal, transmissible, and neurodegenerative prion diseases. The prion protein is subject to posttranslational proteolytic processing and these cleavage events have been shown i) to regulate its physiological functions, ii) to produce biologically active fragments, and iii) to potentially influence the course of prion disease. Here, we give an overview on the proteolytic processing under physiological and pathological conditions and critically review what is currently known about the three main cleavage events of the prion protein, namely α-cleavage, β-cleavage, and ectodomain shedding. The biological relevance of resulting fragments as well as controversies regarding candidate proteases, with special emphasis on members of the A-disintegrin-and-metalloproteinase (ADAM) family, will be discussed. In addition, we make suggestions aimed at facilitating clarity and progress in this important research field. The better understanding of this issue will not only answer basic questions in prion biology but will likely impact research on other neurodegenerative diseases as well.

Overexpression of cellular prion protein (PrP(C)) prevents cognitive dysfunction and apoptotic neuronal cell death induced by amyloid-β (Aβ₁₋₄₀) administration in mice

The cellular prion protein (PrP(C)) is a neuronal-anchored glycoprotein that has been associated with several functions in the CNS such as synaptic plasticity, learning and memory and neuroprotection. There is great interest in understanding the role of PrP(C) in the deleterious effects induced by the central accumulation of amyloid-β (Aβ) peptides, a pathological hallmark of Alzheimer's disease, but the existent results are still controversial. Here we compared the effects of a single intracerebroventricular (i.c.v.) injection of aggregated Aβ(1-40) peptide (400pmol/mouse) on the spatial learning and memory performance as well as hippocampal cell death biomarkers in adult wild type (Prnp(+/+)), PrP(C) knockout (Prnp(0/0)) and the PrP(C) overexpressing Tg-20 mice. Tg-20 mice, which present a fivefold increase in PrP(C) expression in comparison to wild type mice, were resistant to the Aβ(1-40)-induced spatial learning and memory impairments as indicated by reduced escape latencies to find the platform and higher percentage of time spent in the correct quadrant during training and probe test sessions of the water maze task. The protection against Aβ(1-40)-induced cognitive impairments observed in Tg-20 mice was accompanied by a significant decrease in the hippocampal expression of the activated caspase-3 protein and Bax/Bcl-2 ratio as well as reduced hippocampal cell damage assessed by MTT and propidium iodide incorporation assays. These findings indicate that the overexpression of PrP(C) prevents Aβ(1-40)-induced spatial learning and memory deficits in mice and that this response is mediated, at least in part, by the modulation of programed cell death pathways.

Prion protein at the crossroads of physiology and disease

The presence of the cellular prion protein (PrP(C)) on the cell surface is critical for the neurotoxicity of prions. Although several biological activities have been attributed to PrP(C), a definitive demonstration of its physiological function remains elusive. In this review, we discuss some of the proposed functions of PrP(C), focusing on recently suggested roles in cell adhesion, regulation of ionic currents at the cell membrane and neuroprotection. We also discuss recent evidence supporting the idea that PrP(C) may function as a receptor for soluble oligomers of the amyloid β peptide and possibly other toxic protein aggregates. These data suggest surprising new connections between the physiological function of PrP(C) and its role in neurodegenerative diseases beyond those caused by prions.

Cellular prion protein is essential for oligomeric amyloid-β-induced neuronal cell death

In Alzheimer disease (AD), amyloid-β (Aβ) oligomer is suggested to play a critical role in imitating neurodegeneration, although its pathogenic mechanism remains to be determined. Recently, the cellular prion protein (PrP(C)) has been reported to be an essential co-factor in mediating the neurotoxic effect of Aβ oligomer. However, these previous studies focused on the synaptic plasticity in either the presence or the absence of PrP(C) and no study to date has reported whether PrP(C) is required for the neuronal cell death, the most critical element of neurodegeneration in AD. Here, we show that Prnp(-/-) mice are resistant to the neurotoxic effect of Aβ oligomer in vivo and in vitro. Furthermore, application of an anti-PrP(C) antibody or PrP(C) peptide prevents Aβ oligomer-induced neurotoxicity. These findings are the first to demonstrate that PrP(C) is required for Aβ oligomer-induced neuronal cell death, the pathology essential to cognitive loss.

Styryl-based and tricyclic compounds as potential anti-prion agents

Prion diseases currently have no effective therapy. These illnesses affect both animal and human populations, and are characterized by the conformational change of a normal self protein PrP^C (C for cellular) to a pathological and infectious conformer, PrP^Sc (Sc for scrapie). We used a well characterized tissue culture model of prion infection, where mouse neuroblastoma cells (N2a) were infected with 22L PrP^Sc, to screen compounds for anti-prion activity. In a prior study we designed a library of styryl based, potential imaging compounds which were selected for high affinity binding to Alzheimer's disease β-amyloid plaques and good blood-brain barrier permeability. In the current study we screened this library for activity in the N2a/22L tissue culture system. We also tested the anti-prion activity of two clinically used drugs, trimipramine and fluphenazine, in the N2a/22L system. These were selected based on their structural similarity to quinacrine, which was previously reported to have anti-prion activity. All the compounds were also screened for toxicity in tissue culture and their ability to disaggregate amyloid fibrils composed of PrP and β-amyloid synthetic peptides in vitro. Two of the imaging agents, 23I and 59, were found to be both effective at inhibiting prion infection in N2a/22L tissue culture and to be non-toxic. These two compounds, as well as trimipramine and fluphenazine were evaluated in vivo using wild-type CD-1 mice infected peripherally with 139A PrP^Sc. All four agents significantly prolonged the asymptomatic incubation period of prion infection (p<0.0001 log-rank test), as well as significantly reducing the degree of spongiform change, astrocytosis and PrP^Sc levels in the brains of treated mice. These four compounds can be considered, with further development, as candidates for prion therapy.

Alzheimer's disease brain-derived amyloid-β-mediated inhibition of LTP in vivo is prevented by immunotargeting cellular prion protein

Synthetic amyloid-β protein (Aβ) oligomers bind with high affinity to cellular prion protein (PrP(C)), but the role of this interaction in mediating the disruption of synaptic plasticity by such soluble Aβ in vitro is controversial. Here we report that intracerebroventricular injection of Aβ-containing aqueous extracts of Alzheimer's disease (AD) brain robustly inhibits long-term potentiation (LTP) without significantly affecting baseline excitatory synaptic transmission in the rat hippocampus in vivo. Moreover, the disruption of LTP was abrogated by immunodepletion of Aβ. Importantly, intracerebroventricular administration of antigen-binding antibody fragment D13, directed to a putative Aβ-binding site on PrP(C), prevented the inhibition of LTP by AD brain-derived Aβ. In contrast, R1, a Fab directed to the C terminus of PrP(C), a region not implicated in binding of Aβ, did not significantly affect the Aβ-mediated inhibition of LTP. These data support the pathophysiological significance of SDS-stable Aβ dimer and the role of PrP(C) in mediating synaptic plasticity disruption by soluble Aβ.

Genetic cross-interaction between APOE and PRNP in sporadic Alzheimer's and Creutzfeldt-Jakob diseases

Alzheimer's disease (AD) and Creutzfeldt-Jakob disease (CJD) represent two distinct clinical entities belonging to a wider group, generically named as conformational disorders that share common pathophysiologic mechanisms. It is well-established that the APOE ε4 allele and homozygosity at polymorphic codon 129 in the PRNP gene are the major genetic risk factors for AD and human prion diseases, respectively. However, the roles of PRNP in AD, and APOE in CJD are controversial. In this work, we investigated for the first time, APOE and PRNP genotypes simultaneously in 474 AD and 175 sporadic CJD (sCJD) patients compared to a common control population of 335 subjects. Differences in genotype distribution between patients and control subjects were studied by logistic regression analysis using age and gender as covariates. The effect size of risk association and synergy factors were calculated using the logistic odds ratio estimates. Our data confirmed that the presence of APOE ε4 allele is associated with a higher risk of developing AD, while homozygosity at PRNP gene constitutes a risk for sCJD. Opposite, we found no association for PRNP with AD, nor for APOE with sCJD. Interestingly, when AD and sCJD patients were stratified according to their respective main risk genes (APOE for AD, and PRNP for sCJD), we found statistically significant associations for the other gene in those strata at higher previous risk. Synergy factor analysis showed a synergistic age-dependent interaction between APOE and PRNP in both AD (SF = 3.59, p = 0.027), and sCJD (SF = 7.26, p = 0.005). We propose that this statistical epistasis can partially explain divergent data from different association studies. Moreover, these results suggest that the genetic interaction between APOE and PRNP may have a biological correlate that is indicative of shared neurodegenerative pathways involved in AD and sCJD.

Insoluble cellular prion protein and its association with prion and Alzheimer diseases

The soluble cellular prion protein (PrP(C)) is best known for its association with prion disease (PrD) through its conversion to a pathogenic insoluble isoform (PrP(Sc)). However, its deleterious effects independent of PrP(Sc) have recently been observed not only in PrD but also in Alzheimer disease (AD), two diseases which mainly affect cognition. At the same time, PrP(C) itself seems to have broad physiologic functions including involvement in cognitive processes. The PrP(C) that is believed to be soluble and monomeric has so far been the only PrP conformer observed in the uninfected brain. In 2006, we identified an insoluble PrP(C) conformer (termed iPrP(C) ) in uninfected human and animal brains. Remarkably, the PrP(Sc) -like iPrPC shares the immunoreactivity behavior and fragmentation with a newly-identified PrP(Sc) species in a novel human PrD termed variably protease-sensitive prionopathy. Moreover, iPrP(C) has been observed as the major PrP species that interacts with amyloid β (Aβ) in AD. This article highlights evidence of PrP involvement in two putatively beneficial and deleterious PrP-implicated pathways in cognition, and hypothesizes first, that beneficial and deleterious effects of PrP(C) are attributable to the chameleon-like conformation of the protein and second, that the iPrP(C) conformer is associated with PrD and AD.

Interaction between prion protein and toxic amyloid β assemblies can be therapeutically targeted at multiple sites

A role for PrP in the toxic effect of oligomeric forms of Aβ, implicated in Alzheimer's disease (AD), has been suggested but remains controversial. Here we show that PrP is required for the plasticity-impairing effects of ex vivo material from human AD brain and that standardized Aβ-derived diffusible ligand (ADDL) preparations disrupt hippocampal synaptic plasticity in a PrP-dependent manner. We screened a panel of anti-PrP antibodies for their ability to disrupt the ADDL–PrP interaction. Antibodies directed to the principal PrP/Aβ-binding site and to PrP helix-1, were able to block Aβ binding to PrP suggesting that the toxic Aβ species are of relatively high molecular mass and/or may bind multiple PrP molecules. Two representative and extensively characterized monoclonal antibodies directed to these regions, ICSM-35 and ICSM-18, were shown to block the Aβ-mediated disruption of synaptic plasticity validating these antibodies as candidate therapeutics for AD either individually or in combination.

The ability of synthetic amyloid β-protein to bind to prion proteins and alter synaptic plasticity has been previously reported. Here the relevance of this binding is investigated in brains of Alzheimer's disease patients and the interaction is shown to be blocked by antibodies to two distinct regions of prion proteins.

Alzheimer's disease brain-derived amyloid-β-mediated inhibition of LTP in vivo is prevented by immunotargeting cellular prion protein

Synthetic amyloid-β protein (Aβ) oligomers bind with high affinity to cellular prion protein (PrP(C)), but the role of this interaction in mediating the disruption of synaptic plasticity by such soluble Aβ in vitro is controversial. Here we report that intracerebroventricular injection of Aβ-containing aqueous extracts of Alzheimer's disease (AD) brain robustly inhibits long-term potentiation (LTP) without significantly affecting baseline excitatory synaptic transmission in the rat hippocampus in vivo. Moreover, the disruption of LTP was abrogated by immunodepletion of Aβ. Importantly, intracerebroventricular administration of antigen-binding antibody fragment D13, directed to a putative Aβ-binding site on PrP(C), prevented the inhibition of LTP by AD brain-derived Aβ. In contrast, R1, a Fab directed to the C terminus of PrP(C), a region not implicated in binding of Aβ, did not significantly affect the Aβ-mediated inhibition of LTP. These data support the pathophysiological significance of SDS-stable Aβ dimer and the role of PrP(C) in mediating synaptic plasticity disruption by soluble Aβ.

Amyloid-beta42 interacts mainly with insoluble prion protein in the Alzheimer brain

The prion protein (PrP) is best known for its association with prion diseases. However, a controversial new role for PrP in Alzheimer disease (AD) has recently emerged. In vitro studies and mouse models of AD suggest that PrP may be involved in AD pathogenesis through a highly specific interaction with amyloid-β (Aβ42) oligomers. Immobilized recombinant human PrP (huPrP) also exhibited high affinity and specificity for Aβ42 oligomers. Here we report the novel finding that aggregated forms of huPrP and Aβ42 are co-purified from AD brain extracts. Moreover, an anti-PrP antibody and an agent that specifically binds to insoluble PrP (iPrP) co-precipitate insoluble Aβ from human AD brain. Finally, using peptide membrane arrays of 99 13-mer peptides that span the entire sequence of mature huPrP, two distinct types of Aβ binding sites on huPrP are identified in vitro. One specifically binds to Aβ42 and the other binds to both Aβ42 and Aβ40. Notably, Aβ42-specific binding sites are localized predominantly in the octapeptide repeat region, whereas sites that bind both Aβ40 and Aβ42 are mainly in the extreme N-terminal or C-terminal domains of PrP. Our study suggests that iPrP is the major PrP species that interacts with insoluble Aβ42 in vivo. Although this work indicated the interaction of Aβ42 with huPrP in the AD brain, the pathophysiological relevance of the iPrP/Aβ42 interaction remains to be established.

Immunomodulation for prion and prion-related diseases

Prion diseases are a unique category of illness, affecting both animals and humans, where the underlying pathogenesis is related to a conformational change of a normal self protein called cellular prion protein to a pathological and infectious conformer known as scrapie prion protein (PrP(Sc)). Currently, all prion diseases lack effective treatment and are universally fatal. Past experiences with bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease mainly in Europe, as well as the current epidemic of chronic wasting disease in North America, have highlighted the need to develop prophylactic and/or therapeutic approaches. In Alzheimer's disease that, like prion disease, is a conformational neurodegenerative disorder, both passive and active immunization has been shown to be highly effective in model animals at preventing disease and cognitive deficits, with emerging data from human trials suggesting that this approach is able to reduce amyloid-related pathology. However, any immunomodulatory approach aimed at a self-antigen has to finely balance an effective humoral immune response with potential autoimmune toxicity. The prion diseases most commonly acquired by infection typically have the alimentary tract as a portal of infectious agent entry. This makes mucosal immunization a potentially attractive method to produce a local immune response that partially or completely prevents prion entry across the gut barrier, while at the same time producing modulated systemic immunity that is unlikely to be associated with toxicity. Our results using an attenuated Salmonella vaccine strain expressing the prion protein showed that mucosal vaccination can protect against prion infection from a peripheral source, suggesting the feasibility of this approach. It is also possible to develop active and/or passive immunomodulatory approaches that more specifically target PrP(Sc) or target the shared pathological conformer found in numerous conformational disorders. Such approaches could have a significant impact on many of the common age-associated dementias.

Neuroprotective and neurotoxic signaling by the prion protein

Prion diseases in humans and animals are characterized by progressive neurodegeneration and the formation of infectious particles called prions. Both features are intimately linked to a conformational transition of the cellular prion protein (PrP(C)) into aberrantly folded conformers with neurotoxic and self-replicating activities. Interestingly, there is increasing evidence that the infectious and neurotoxic properties of PrP conformers are not necessarily coupled. Transgenic mouse models revealed that some PrP mutants interfere with neuronal function in the absence of infectious prions. Vice versa, propagation of prions can occur without causing neurotoxicity. Consequently, it appears plausible that two partially independent pathways exist, one pathway leading to the propagation of infectious prions and another one that mediates neurotoxic signaling. In this review we will summarize current knowledge of neurotoxic PrP conformers and discuss the role of PrP(C) as a mediator of both stress-protective and neurotoxic signaling cascades.

β-amyloid oligomers and prion protein: Fatal attraction?

The relationship between Alzheimer disease (AD) and prion-related encephalopathies (TSE) has been proposed by different points of view. Recently, the scientific attention has been attracted by the results proposing the possibility that PrPc, the protein whose pathologic form is responsible of TSE, can mediated the toxic effect of β amyloid (Aβ) oligomers. The oligomers are considered the culprit of the neurodegenerative process associated to AD, although the pathogenic mechanism activated by these small aggregates remain to be elucidated. In the initial study based on the binding screening PrPc was identified as ligand /receptor of Aβ oligomers, while long term potentiation (LTP) analysis in vitro and behavioural studies in vivo, demonstrated that the absence of PrPc abolished the damage induced by Aβ oligomers. The high affinity binding Aβ oligomers-PrPc has been confirmed, whereas a functional role of this association has been excluded by three different studies. We approached this issue by the direct application of Aβ oligomers in the brain followed by the behavioural examination of memory deficits. Our data using PrP knock-out mice suggest that Aβ 1-42 oligomers are responsible for cognitive impairment in AD but PrPc is not required for their effect. Similarly, in two other studies the LTP alterations induced by Aβ 1-42 oligomers was not influenced by the absence of PrP. Possible explanations of these contradictory results are discussed.