Highly infectious prions are not directly neurotoxic.

Prions are infectious agents which cause rapidly lethal neurodegenerative diseases in humans and animals following long, clinically silent incubation periods. They are composed of multichain assemblies of misfolded cellular prion protein. While it has long been assumed that prions are themselves neurotoxic, recent development of methods to obtain exceptionally pure prions from mouse brain with maintained strain characteristics, and in which defined structures-paired rod-like double helical fibers-can be definitively correlated with infectivity, allowed a direct test of this assertion. Here we report that while brain homogenates from symptomatic prion-infected mice are highly toxic to cultured neurons, exceptionally pure intact high-titer infectious prions are not directly neurotoxic. We further show that treatment of brain homogenates from prion-infected mice with sodium lauroylsarcosine destroys toxicity without diminishing infectivity. This is consistent with models in which prion propagation and toxicity can be mechanistically uncoupled.

Amyloid ß Oligomers Disrupt Blood-CSF Barrier Integrity by Activating Matrix Metalloproteinases.

The blood-CSF barrier (BCSFB) consists of a monolayer of choroid plexus epithelial (CPE) cells that maintain CNS homeostasis by producing CSF and restricting the passage of undesirable molecules and pathogens into the brain. Alzheimer's disease is the most common progressive neurodegenerative disorder and is characterized by the presence of amyloid ß (Aß) plaques and neurofibrillary tangles in the brain. Recent research shows that Alzheimer's disease is associated with morphological changes in CPE cells and compromised production of CSF. Here, we studied the direct effects of Aß on the functionality of the BCSFB. Intracerebroventricular injection of Aß1-42 oligomers into the cerebral ventricles of mice, a validated Alzheimer's disease model, caused induction of a cascade of detrimental events, including increased inflammatory gene expression in CPE cells and increased levels of proinflammatory cytokines and chemokines in the CSF. It also rapidly affected CPE cell morphology and tight junction protein levels. These changes were associated with loss of BCSFB integrity, as shown by an increase in BCSFB leakage. Aß1-42 oligomers also increased matrix metalloproteinase (MMP) gene expression in the CPE and its activity in CSF. Interestingly, BCSFB disruption induced by Aß1-42 oligomers did not occur in the presence of a broad-spectrum MMP inhibitor or in MMP3-deficient mice. These data provide evidence that MMPs are essential for the BCSFB leakage induced by Aß1-42 oligomers. Our results reveal that Alzheimer's disease-associated soluble Aß1-42 oligomers induce BCSFB dysfunction and suggest MMPs as a possible therapeutic target. SIGNIFICANCE STATEMENT: No treatments are yet available to cure Alzheimer's disease; however, soluble Aß oligomers are believed to play a crucial role in the neuroinflammation that is observed in this disease. Here, we studied the effect of Aß oligomers on the often neglected barrier between blood and brain, called the blood-CSF barrier (BCSFB). This BCSFB is formed by the choroid plexus epithelial cells and is important in maintaining brain homeostasis. We observed Aß oligomer-induced changes in morphology and loss of BCSFB integrity that might play a role in Alzheimer's disease progression. Strikingly, both inhibition of matrix metalloproteinase (MMP) activity and MMP3 deficiency could protect against the detrimental effects of Aß oligomer. Clearly, our results suggest that MMP inhibition might have therapeutic potential.

The mechanism of γ-Secretase dysfunction in familial Alzheimer disease.

The mechanisms by which mutations in the presenilins (PSEN) or the amyloid precursor protein (APP) genes cause familial Alzheimer disease (FAD) are controversial. FAD mutations increase the release of amyloid ß (Aß)42 relative to Aß40 by an unknown, possibly gain-of-toxic-function, mechanism. However, many PSEN mutations paradoxically impair γ-secretase and 'loss-of-function' mechanisms have also been postulated. Here, we use kinetic studies to demonstrate that FAD mutations affect Aß generation via three different mechanisms, resulting in qualitative changes in the Aß profiles, which are not limited to Aß42. Loss of ɛ-cleavage function is not generally observed among FAD mutants. On the other hand, γ-secretase inhibitors used in the clinic appear to block the initial ɛ-cleavage step, but unexpectedly affect more selectively Notch than APP processing, while modulators act as activators of the carboxypeptidase-like (γ) activity. Overall, we provide a coherent explanation for the effect of different FAD mutations, demonstrating the importance of qualitative rather than quantitative changes in the Aß products, and suggest fundamental improvements for current drug development efforts.

The Alzheimer disease protective mutation A2T modulates kinetic and thermodynamic properties of amyloid-ß (Aß) aggregation.

Missense mutations in alanine 673 of the amyloid precursor protein (APP), which corresponds to the second alanine of the amyloid ß (Aß) sequence, have dramatic impact on the risk for Alzheimer disease; A2V is causative, and A2T is protective. Assuming a crucial role of amyloid-Aß in neurodegeneration, we hypothesized that both A2V and A2T mutations cause distinct changes in Aß properties that may at least partially explain these completely different phenotypes. Using human APP-overexpressing primary neurons, we observed significantly decreased Aß production in the A2T mutant along with an enhanced Aß generation in the A2V mutant confirming earlier data from non-neuronal cell lines. More importantly, thioflavin T fluorescence assays revealed that the mutations, while having little effect on Aß42 peptide aggregation, dramatically change the properties of the Aß40 pool with A2V accelerating and A2T delaying aggregation of the Aß peptides. In line with the kinetic data, Aß A2T demonstrated an increase in the solubility at equilibrium, an effect that was also observed in all mixtures of the A2T mutant with the wild type Aß40. We propose that in addition to the reduced ß-secretase cleavage of APP, the impaired propensity to aggregate may be part of the protective effect conferred by A2T substitution. The interpretation of the protective effect of this mutation is thus much more complicated than proposed previously.

Neurotoxicity of Alzheimer's disease Aß peptides is induced by small changes in the Aß42 to Aß40 ratio.

The amyloid peptides Aß(40) and Aß(42) of Alzheimer's disease are thought to contribute differentially to the disease process. Although Aß(42) seems more pathogenic than Aß(40), the reason for this is not well understood. We show here that small alterations in the Aß(42):Aß(40) ratio dramatically affect the biophysical and biological properties of the Aß mixtures reflected in their aggregation kinetics, the morphology of the resulting amyloid fibrils and synaptic function tested in vitro and in vivo. A minor increase in the Aß(42):Aß(40) ratio stabilizes toxic oligomeric species with intermediate conformations. The initial toxic impact of these Aß species is synaptic in nature, but this can spread into the cells leading to neuronal cell death. The fact that the relative ratio of Aß peptides is more crucial than the absolute amounts of peptides for the induction of neurotoxic conformations has important implications for anti-amyloid therapy. Our work also suggests the dynamic nature of the equilibrium between toxic and non-toxic intermediates.

Neurotoxicity and memory deficits induced by soluble low-molecular-weight amyloid-ß1-42 oligomers are revealed in vivo by using a novel animal model.

Neuronal and synaptic degeneration are the best pathological correlates for memory decline in Alzheimer's disease (AD). Although the accumulation of soluble low-molecular-weight amyloid-ß (Aß) oligomers has been suggested to trigger neurodegeneration in AD, animal models overexpressing or infused with Aß lack neuronal loss at the onset of memory deficits. Using a novel in vivo approach, we found that repeated hippocampal injections of small soluble Aß(1-42) oligomers in awake, freely moving mice were able to induce marked neuronal loss, tau hyperphosphorylation, and deficits in hippocampus-dependent memory. The neurotoxicity of small Aß(1-42) species was observed in vivo as well as in vitro in association with increased caspase-3 activity and reduced levels of the NMDA receptor subunit NR2B. We found that the sequestering agent transthyretin is able to bind the toxic Aß(1-42) species and attenuated the loss of neurons and memory deficits. Our novel mouse model provides evidence that small, soluble Aß(1-42) oligomers are able to induce extensive neuronal loss in vivo and initiate a cascade of events that mimic the key neuropathological hallmarks of AD.

Molecular plasticity regulates oligomerization and cytotoxicity of the multipeptide-length amyloid-ß peptide pool.

Current therapeutic approaches under development for Alzheimer disease, including γ-secretase modulating therapy, aim at increasing the production of Aß(1-38) and Aß(1-40) at the cost of longer Aß peptides. Here, we consider the aggregation of Aß(1-38) and Aß(1-43) in addition to Aß(1-40) and Aß(1-42), in particular their behavior in mixtures representing the complex in vivo Aß pool. We demonstrate that Aß(1-38) and Aß(1-43) aggregate similar to Aß(1-40) and Aß(1-42), respectively, but display a variation in the kinetics of assembly and toxicity due to differences in short timescale conformational plasticity. In biologically relevant mixtures of Aß, Aß(1-38) and Aß(1-43) significantly affect the behaviors of Aß(1-40) and Aß(1-42). The short timescale conformational flexibility of Aß(1-38) is suggested to be responsible for enhancing toxicity of Aß(1-40) while exerting a cyto-protective effect on Aß(1-42). Our results indicate that the complex in vivo Aß peptide array and variations thereof is critical in Alzheimer disease, which can influence the selection of current and new therapeutic strategies.

A high-content neuron imaging assay demonstrates inhibition of prion disease-associated neurotoxicity by an anti-prion protein antibody.

There is an urgent need to develop disease-modifying therapies to treat neurodegenerative diseases which pose increasing challenges to global healthcare systems. Prion diseases, although rare, provide a paradigm to study neurodegenerative dementias as similar disease mechanisms involving propagation and spread of multichain assemblies of misfolded protein ("prion-like" mechanisms) are increasingly recognised in the commoner conditions such as Alzheimer's disease. However, studies of prion disease pathogenesis in mouse models showed that prion propagation and neurotoxicity can be mechanistically uncoupled and in vitro assays confirmed that highly purified prions are indeed not directly neurotoxic. To aid development of prion disease therapeutics we have therefore developed a cell-based assay for the specific neurotoxicity seen in prion diseases rather than to simply assess inhibition of prion propagation. We applied this assay to examine an anti-prion protein mouse monoclonal antibody (ICSM18) known to potently cure prion-infected cells and to delay onset of prion disease in prion-infected mice. We demonstrate that whilst ICSM18 itself lacks inherent neurotoxicity in this assay, it potently blocks prion disease-associated neurotoxicity.

Synaptic dysfunction in hippocampus of transgenic mouse models of Alzheimer's disease: a multi-electrode array study.

APP.V717I and Tau.P301L transgenic mice develop Alzheimer's disease pathology comprising important aspects of human disease including increased levels of amyloid peptides, cognitive and motor impairment, amyloid plaques and neurofibrillary tangles. The combined model, APP.V717I×Tau.P301L bigenic mice (biAT mice) exhibit aggravated amyloid and tau pathology with severe cognitive and behavioral defects. In the present study, we investigated early changes in synaptic function in the CA1 and CA3 regions of acute hippocampal slices of young APP.V717I, Tau.P301L and biAT transgenic animals. We have used planar multi-electrode arrays (MEA) and improved methods for simultaneous multi-site recordings from two hippocampal sub-regions. In the CA1 region, long-term potentiation (LTP) was severely impaired in all transgenic animals when compared with age-matched wild-type controls, while basal synaptic transmission and paired-pulse facilitation were minimally affected. In the CA3 region, LTP was normal in Tau.P301L and APP.V717I but clearly impaired in biAT mice. Surprisingly, frequency facilitation in CA3 was significantly enhanced in Tau.P301L mice, while not affected in APP.V717I mice and depressed in biAT mice. The findings demonstrate important synaptic changes that differ considerably in the hippocampal sub-regions already at young age, well before the typical amyloid or tau pathology is evident.

Brazilin Removes Toxic Alpha-Synuclein and Seeding Competent Assemblies from Parkinson Brain by Altering Conformational Equilibrium.

Alpha-synuclein (α-syn) fibrils, a major constituent of the neurotoxic Lewy Bodies in Parkinson's disease, form via nucleation dependent polymerization and can replicate by a seeding mechanism. Brazilin, a small molecule derived from red cedarwood trees in Brazil, has been shown to inhibit the fibrillogenesis of amyloid-beta (Aß) and α-syn as well as remodel mature fibrils and reduce cytotoxicity. Here we test the effects of Brazilin on both seeded and unseeded α-syn fibril formation and show that the natural polyphenol inhibits fibrillogenesis of α-syn by a unique mechanism that alters conformational equilibria in two separate points of the assembly mechanism: Brazilin preserves the natively unfolded state of α-syn by specifically binding to the compact conformation of the α-syn monomer. Brazilin also eliminates seeding competence of α-syn assemblies from Parkinson's disease patient brain tissue, and reduces toxicity of pre-formed assemblies in primary neurons by inducing the formation of large fibril clusters. Molecular docking of Brazilin shows the molecule to interact both with unfolded α-syn monomers and with the cross-ß sheet structure of α-syn fibrils. Our findings suggest that Brazilin has substantial potential as a neuroprotective and therapeutic agent for Parkinson's disease.

Mixing Aß(1-40) and Aß(1-42) peptides generates unique amyloid fibrils.

Recent structural studies show distinct morphologies for the fibrils of Aß(1-42) and Aß(1-40), which are believed not to co-fibrillize. We describe here a novel, structurally-uniform 1 : 1 mixed fibrillar species, which differs from both pure fibrils. It forms preferentially even when Aß(1-42) : Aß(1-40) peptides are mixed in a non-stoichiometric ratio.

A New Cell Model for Investigating Prion Strain Selection and Adaptation.

Prion diseases are fatal neurodegenerative diseases that affect humans and animals. Prion strains, conformational variants of misfolded prion proteins, are associated with distinct clinical and pathological phenotypes. Host-strain interactions result in the selective damage of distinct brain areas and they are responsible for strain selection and/or adaptation, but the underlying molecular mechanisms are unknown. Prion strains can be distinguished by their cell tropism in vivo and in vitro, which suggests that susceptibility to distinct prion strains is determined by cellular factors. The neuroblastoma cell line PK1 is refractory to the prion strain Me7, but highly susceptible to RML. We challenged a large number of clonal PK1 lines with Me7 and successfully selected highly Me7-susceptible subclones (PME) to investigate whether the prion strain repertoire of PK1 can be expanded. Notably, the Me7-infected PME clones were more protease-resistant when compared to RML-infected PME clones, which suggested that cell-adapted Me7 and RML are distinct prion strains. Strikingly, Me7-refractory cells, including PK1 and astrocytes in cortico-hippocampal cultures, are highly susceptible to prions, being derived from homogenates of Me7-infected PME cells, suggesting that the passage of Me7 in PME cells leads to an extended host range. Thus, PME clones represent a compelling cell model for strain selection and adaptation.

Amyloid beta oligomers induce neuronal elasticity changes in age-dependent manner: a force spectroscopy study on living hippocampal neurons.

Small soluble species of amyloid-beta (Aß) formed during early peptide aggregation stages are responsible for several neurotoxic mechanisms relevant to the pathology of Alzheimer's disease (AD), although their interaction with the neuronal membrane is not completely understood. This study quantifies the changes in the neuronal membrane elasticity induced by treatment with the two most common Aß isoforms found in AD brains: Aß40 and Aß42. Using quantitative atomic force microscopy (AFM), we measured for the first time the static elastic modulus of living primary hippocampal neurons treated with pre-aggregated Aß40 and Aß42 soluble species. Our AFM results demonstrate changes in the elasticity of young, mature and aged neurons treated for a short time with the two Aß species pre-aggregated for 2 hours. Neurons aging under stress conditions, showing aging hallmarks, are the most susceptible to amyloid binding and show the largest decrease in membrane stiffness upon Aß treatment. Membrane stiffness defines the way in which cells respond to mechanical forces in their environment and has been shown to be important for processes such as gene expression, ion-channel gating and neurotransmitter vesicle transport. Thus, one can expect that changes in neuronal membrane elasticity might directly induce functional changes related to neurodegeneration.

De novo design of a biologically active amyloid.

Most human proteins possess amyloidogenic segments, but only about 30 are associated with amyloid-associated pathologies, and it remains unclear what determines amyloid toxicity. We designed vascin, a synthetic amyloid peptide, based on an amyloidogenic fragment of vascular endothelial growth factor receptor 2 (VEGFR2), a protein that is not associated to amyloidosis. Vascin recapitulates key biophysical and biochemical characteristics of natural amyloids, penetrates cells, and seeds the aggregation of VEGFR2 through direct interaction. We found that amyloid toxicity is observed only in cells that both express VEGFR2 and are dependent on VEGFR2 activity for survival. Thus, amyloid toxicity here appears to be both protein-specific and conditional-determined by VEGFR2 loss of function in a biological context in which target protein function is essential.

Amyloid-ß protofibrils: size, morphology and synaptotoxicity of an engineered mimic.

Structural and biochemical studies of the aggregation of the amyloid-ß peptide (Aß) are important to understand the mechanisms of Alzheimer's disease, but research is complicated by aggregate inhomogeneity and instability. We previously engineered a hairpin form of Aß called Aßcc, which forms stable protofibrils that do not convert into amyloid fibrils. Here we provide a detailed characterization of Aß42cc protofibrils. Like wild type Aß they appear as smooth rod-like particles with a diameter of 3.1 (±0.2) nm and typical lengths in the range 60 to 220 nm when observed by atomic force microscopy. Non-perturbing analytical ultracentrifugation and nanoparticle tracking analyses are consistent with such rod-like protofibrils. Aß42cc protofibrils bind the ANS dye indicating that they, like other toxic protein aggregates, expose hydrophobic surface. Assays with the OC/A11 pair of oligomer specific antibodies put Aß42cc protofibrils into the same class of species as fibrillar oligomers of wild type Aß. Aß42cc protofibrils may be used to extract binding proteins in biological fluids and apolipoprotein E is readily detected as a binder in human serum. Finally, Aß42cc protofibrils act to attenuate spontaneous synaptic activity in mouse hippocampal neurons. The experiments indicate considerable structural and chemical similarities between protofibrils formed by Aß42cc and aggregates of wild type Aß42. We suggest that Aß42cc protofibrils may be used in research and applications that require stable preparations of protofibrillar Aß.

The toxic Aß oligomer and Alzheimer's disease: an emperor in need of clothes.

The 'toxic Aß oligomer' hypothesis has attracted considerable attention among Alzheimer's disease researchers as a way of resolving the lack of correlation between deposited amyloid-ß (Aß) in amyloid plaques-in terms of both amount and location-and cognitive impairment or neurodegeneration. However, the lack of a common, agreed-upon experimental description of the toxic Aß oligomer makes interpretation and direct comparison of data between different research groups impossible. Here we critically review the evidence supporting toxic Aß oligomers as drivers of neurodegeneration and make some suggestions that might facilitate progress in this complex field.

Amyloid precursor protein mutation E682K at the alternative ß-secretase cleavage ß'-site increases Aß generation.

BACE1 cleaves the amyloid precursor protein (APP) at the ß-cleavage site (Met(671) -Asp(672) ) to initiate the generation of amyloid peptide Aß. BACE1 is also known to cleave APP at a much less well-characterized ß'-cleavage site (Tyr(681) -Glu(682) ). We describe here the identification of a novel APP mutation E682K located at this ß'-site in an early onset Alzheimer's disease (AD) case. Functional analysis revealed that this E682K mutation blocked the ß'-site and shifted cleavage of APP to the ß-site, causing increased Aß production. This work demonstrates the functional importance of APP processing at the ß'-site and shows how disruption of the balance between ß- and ß'-site cleavage may enhance the amyloidogenic processing and consequentially risk for AD. Increasing exon- and exome-based sequencing efforts will identify many more putative pathogenic mutations without conclusive segregation-based evidence in a single family. Our study shows how functional analysis of such mutations allows to determine the potential pathogenic nature of these mutations. We propose to classify the E682K mutation as probable pathogenic awaiting further independent confirmation of its association with AD in other patients.

Prion protein in Alzheimer's pathogenesis: a hot and controversial issue.

The role for cellular prion protein PrP(c) in beta-amyloid (Abeta) oligomer-induced synaptic impairment is a topic of great interest and some controversy. In this issue of EMBO Molecular Medicine Aguzzi and co-workers explore the contribution of PrP(c) to deficient long term potentiation (LTP) and soluble Abeta levels in an Alzheimer's disease mouse model and show that the role of prions in Abeta related toxicity is far from 'black and white' suggesting complex interpretations of the data available thus far.