Prion-like seeding and nucleation of intracellular amyloid-β

"Prion-like" seeding and propagation of oligomeric protein assemblies in neurodegenerative disorders

Intra- or extracellular aggregates of proteins are central pathogenic features in most neurodegenerative disorders. The accumulation of such proteins in diseased brains is believed to be the end-stage of a stepwise aggregation of misfolded monomers to insoluble cross-β fibrils via a series of differently sized soluble oligomers/protofibrils. Several studies have shown how α-synuclein, amyloid-β, tau and other amyloidogenic proteins can act as nucleating particles and thereby share properties with misfolded forms, or strains, of the prion protein. Although the roles of different protein assemblies in the respective aggregation cascades remain unclear, oligomers/protofibrils are considered key pathogenic species. Numerous observations have demonstrated their neurotoxic effects and a growing number of studies have indicated that they also possess seeding properties, enabling their propagation within cellular networks in the nervous system. The seeding behavior of oligomers differs between the proteins and is also affected by various factors, such as size, shape and epitope presentation. Here, we are providing an overview of the current state of knowledge with respect to the "prion-like" behavior of soluble oligomers for several of the amyloidogenic proteins involved in neurodegenerative diseases. In addition to providing new insight into pathogenic mechanisms, research in this field is leading to novel diagnostic and therapeutic opportunities for neurodegenerative diseases.

Differential accumulation of human β-amyloid and tau from enriched extracts in neuronal and endothelial cells

While Aβ and Tau cellular distribution has been largely studied, the comparative internalization and subcellular accumulation of Tau and Aβ isolated from human brain extracts in endothelial and neuronal cells has not yet been unveiled. We have previously demonstrated that controlled enrichment of Aβ from human brain extracts constitutes a valuable tool to monitor cellular internalization in vitro and in vivo. Herein, we establish an alternative method to strongly enrich Aβ and Tau aggregates from human AD brains, which has allowed us to study and compare the cellular internalization, distribution and toxicity of both proteins within brain barrier endothelial (bEnd.3) and neuronal (Neuro2A) cells. Our findings demonstrate the suitability of human enriched brain extracts to monitor the intracellular distribution of human Aβ and Tau, which, once internalized, show dissimilar sorting to different organelles within the cell and differential toxicity, exhibiting higher toxic effects on neuronal cells than on endothelial cells. While tau is strongly concentrated preferentially in mitochondria, Aβ is distributed predominantly within the endolysosomal system in endothelial cells, whereas the endoplasmic reticulum was its preferential location in neurons. Altogether, our findings display a picture of the interactions that human Aβ and Tau might establish in these cells.

Neuropathogenesis-on-chips for neurodegenerative diseases

Developing diagnostics and treatments for neurodegenerative diseases (NDs) is challenging due to multifactorial pathogenesis that progresses gradually. Advanced in vitro systems that recapitulate patient-like pathophysiology are emerging as alternatives to conventional animal-based models. In this review, we explore the interconnected pathogenic features of different types of ND, discuss the general strategy to modelling NDs using a microfluidic chip, and introduce the organoid-on-a-chip as the next advanced relevant model. Lastly, we overview how these models are being applied in academic and industrial drug development. The integration of microfluidic chips, stem cells, and biotechnological devices promises to provide valuable insights for biomedical research and developing diagnostic and therapeutic solutions for NDs.

Angiotensin type 1 receptor activation promotes neuronal and glial alpha-synuclein aggregation and transmission

The brain renin-angiotensin system (RAS) has been related to dopaminergic degeneration, and high expression of the angiotensin II (AngII) type 1 receptor (AT1) gene is a marker of the most vulnerable neurons in humans. However, it is unknown whether AngII/AT1 overactivation affects α-synuclein aggregation and transmission. In vitro, AngII/AT1 activation increased α-synuclein aggregation in dopaminergic neurons and microglial cells, which was related to AngII-induced NADPH-oxidase activation and intracellular calcium raising. In mice, AngII/AT1 activation was involved in MPTP-induced increase in α-synuclein expression and aggregation, as they significantly decreased in mice treated with the AT1 blocker telmisartan and AT1 knockout mice. Cell co-cultures (transwells) revealed strong transmission of α-synuclein from dopaminergic neurons to astrocytes and microglia. AngII induced a higher α-synuclein uptake by microglial cells and an increase in the transfer of α-synuclein among astroglial cells. However, AngII did not increase the release of α-synuclein by neurons. The results further support brain RAS dysregulation as a major mechanism for the progression of Parkinson's disease, and AT1 inhibition and RAS modulation as therapeutic targets.

Aβ and Tau Prions Causing Alzheimer's Disease

Studies show that patients with Alzheimer's disease (AD) have both Aβ and tau prions, and thus, AD is a double-prion disease. AD patients with the greatest longevity exhibited low levels of both Aβ and tau prions; tau prions were nearly absent in the brains of almost half of the patients who lived beyond 80 years of age. Using cellular bioassays for prions in postmortem samples, we found that both Aβ and tau proteins misfold into prions leading to AD, which is either a sporadic or familial dementing disorder. Although AD is transmissible experimentally, there is no evidence that AD is either communicable or contagious. Since the progression of AD correlates poorly with insoluble Aβ in the central nervous system (CNS), it was difficult to distinguish between inert amyloids and Aβ prions. To measure the progression of AD, we devised rapid bioassays to measure the abundance of isoform-specific Aβ prions in the brains of transgenic (Tg) mice and in postmortem human CNS samples from AD victims and people who died of other neurodegenerative diseases (NDs). We found significant correlations between the longevity of individuals with AD, sex, and genetic background, despite the fact that all postmortem brain tissue had essentially the same confirmed neuropathology.Although brains from all AD patients had measurable levels of Aβ prions at death, the oldest individuals had lower Aβ prion levels than the younger ones. Additionally, the long-lived individuals had low tau prion levels that correlated with the extent of phosphorylated tau (p-tau). Unexpectedly, a longevity-dependent decrease in tau prions was found in spite of increasing amounts of total insoluble tau. When corrected for the abundance of insoluble tau, the tau prion levels decreased exponentially with respect to the age at death with a half-time of approximately one decade, and this correlated with the abundance of phosphorylated tau.Even though our findings with tau prions were not unexpected, they were counterintuitive; thus, tau phosphorylation and tau prion activity decreased exponentially with longevity in patients with AD ranging from ages 37 to 99 years. Our findings demonstrated an inverse correlation between longevity in AD patients and the abundance of neurotoxic tau prions. Moreover, our discovery may have profound implications for the selection of phenotypically distinct patient populations and the development of diagnostics and effective therapeutics for AD.

The 3-O sulfation of heparan sulfate proteoglycans contributes to the cellular internalization of tau aggregates

BACKGROUND

Considering the high correlation between the functional decline in Alzheimer's disease (AD) and the propagation of aggregated tau protein, many research efforts are focused on determining the underlying molecular mechanisms of tau spreading. Heparan sulfate proteoglycans (HSPGs) were reported to mediate cellular uptake of tau aggregates. Specifically, the heparan sulfates (HS) sulfation plays a critical role in the interaction of HSPGs with aggregated tau. HS can be N-/2-O/6-O- or 3-O-sulfated, some of which have been reported to take part in the interaction with tau aggregates. However, the role of the 3-O sulfation remains enigmatic.

RESULTS

Here, we studied the contribution of HS 3-O sulfation in the binding and cellular uptake of tau aggregates. We observed reduced tau aggregates uptake in absence of 3-O sulfation or when outcompeting available cellular 3-O sulfated HS (3S-HS) with antithrombin III. The lack of HS3ST1-generated HS products in the HS3ST1-/- cells was further corroborated with an LC-MS/MS using 13C-labeled HS calibrants. Here, we showed that these functional changes can be explained by a higher affinity of aggregated tau to 3S-HS. When targeting tau aggregates with 3-O sulfation-containing HS, we observed an increase in inhibition of tau aggregates uptake.

CONCLUSIONS

These data indicate that HS 3-O sulfation plays a role in the binding of tau aggregates and, thus, contributes to their cellular uptake, highlighting a potential target value to modulate tau pathogenesis.

Destruction of Lysozyme Amyloid Fibrils Induced by Magnetoferritin and Reconstructed Ferritin

Neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), or systemic amyloidosis, are characterized by the specific protein transformation from the native state to stable insoluble deposits, e.g., amyloid plaques. The design of potential therapeutic agents and drugs focuses on the destabilization of the bonds in their beta-rich structures. Surprisingly, ferritin derivatives have recently been proposed to destabilize fibril structures. Using atomic force microscopy (AFM) and fluorescence spectrophotometry, we confirmed the destructive effect of reconstructed ferritin (RF) and magnetoferritin (MF) on lysosome amyloid fibrils (LAF). The presence of iron was shown to be the main factor responsible for the destruction of LAF. Moreover, we found that the interaction of RF and MF with LAF caused a significant increase in the release of potentially harmful ferrous ions. Zeta potential and UV spectroscopic measurements of LAF and ferritin derivative mixtures revealed a considerable difference in RF compared to MF. Our results contribute to a better understanding of the mechanism of fibril destabilization by ferritin-like proteins. From this point of view, ferritin derivatives seem to have a dual effect: therapeutic (fibril destruction) and adverse (oxidative stress initiated by increased Fe²⁺ release). Thus, ferritins may play a significant role in various future biomedical applications.

The effect of Aβ seeding is dependent on the presence of knock-in genes in the App NL-G-F mice

Alzheimer's disease (AD) is characterized by the prion-like propagation of amyloid-β (Aβ). However, the role of Aβ in cognitive impairment is still unclear. To determine the causal role of Aβ in AD, we intracerebrally seeded the entorhinal cortex of a 2-month-old App NL-G-F mouse model with an Aβ peptide derived from patients who died from rapidly progressing AD. When the mice were 3 months of age or 1 month following seeding, spatial learning and memory were tested using the Morris water task. Immunohistochemical labeling showed seeding with the Aβ was found accelerate Aβ plaque deposition and microgliosis in the App NL-G-F mice, but this was dependent on the presence of the knocked-in genes. However, we found no correlation between pathology and spatial performance. The results of the present study show the seeding effects in the App NL-G-F knock-in model, and how these are dependent on the presence of a humanized App gene. But these pathological changes were not initially causal in memory impairment.

Role of Intracellular Amyloid β as Pathway Modulator, Biomarker, and Therapy Target

The β- and γ-secretase-driven cleavage of the amyloid precursor protein (APP) gives rise to the amyloid β peptide, which is believed to be the main driver of neurodegeneration in Alzheimer’s disease (AD). As it is prominently detectable in extracellular plaques in post-mortem AD brain samples, research in recent decades focused on the pathological role of extracellular amyloid β aggregation, widely neglecting the potential meaning of very early generation of amyloid β inside the cell. In the last few years, the importance of intracellular amyloid β (iAβ) as a strong player in neurodegeneration has been indicated by a rising number of studies. In this review, iAβ is highlighted as a crucial APP cleavage fragment, able to manipulate intracellular pathways and foster neurodegeneration. We demonstrate its relevance as a pathological marker and shed light on initial studies aiming to modulate iAβ through pharmacological treatment, which has been shown to have beneficial effects on cognitive properties in animal models. Finally, we display the relevance of viral infections on iAβ generation and point out future directions urgently needed to manifest the potential relevance of iAβ in Alzheimer’s disease.

The role of astrocytes in prion-like mechanisms of neurodegeneration

Accumulating evidence suggests that neurodegenerative diseases are not merely neuronal in nature but comprise multicellular involvement, with astrocytes emerging as key players. The pathomechanisms of several neurodegenerative diseases involve the deposition of misfolded protein aggregates in neurons that have characteristic prion-like behaviours such as template-directed seeding, intercellular propagation, distinct conformational strains and protein-mediated toxicity. The role of astrocytes in dealing with these pathological prion-like protein aggregates and whether their responses either protect from or conspire with the disease process is currently unclear. Here we review the existing literature implicating astrocytes in multiple neurodegenerative proteinopathies with a focus on prion-like behaviour in this context.

Fas Apoptosis Inhibitory Molecule Blocks and Dissolves Pathological Amyloid-β Species

A number of neurodegenerative diseases are associated with the accumulation of misfolded proteins, including Alzheimer’s disease (AD). In AD, misfolded proteins such as tau and amyloid-β (Aβ) form pathological insoluble deposits. It is hypothesized that molecules capable of dissolving such protein aggregates might reverse disease progression and improve the lives of afflicted AD patients. Here we report new functions of the highly conserved mammalian protein, Fas Apoptosis Inhibitory Molecule (FAIM). We found that FAIM-deficient Neuro 2A cells accumulate Aβ oligomers/fibrils. We further found that recombinant human FAIM prevents the generation of pathologic Aβ oligomers and fibrils in a cell-free system, suggesting that FAIM functions without any additional cellular components. More importantly, recombinant human FAIM disaggregates and solubilizes established Aβ fibrils. Our results identify a previously unknown, completely novel candidate for understanding and treating irremediable, irreversible, and unrelenting neurodegenerative diseases.

Neuronal spreading and plaque induction of intracellular Aβ and its disruption of Aβ homeostasis

The amyloid-beta peptide (Aβ) is thought to have prion-like properties promoting its spread throughout the brain in Alzheimer’s disease (AD). However, the cellular mechanism(s) of this spread remains unclear. Here, we show an important role of intracellular Aβ in its prion-like spread. We demonstrate that an intracellular source of Aβ can induce amyloid plaques in vivo via hippocampal injection. We show that hippocampal injection of mouse AD brain homogenate not only induces plaques, but also damages interneurons and affects intracellular Aβ levels in synaptically connected brain areas, paralleling cellular changes seen in AD. Furthermore, in a primary neuron AD model, exposure of picomolar amounts of brain-derived Aβ leads to an apparent redistribution of Aβ from soma to processes and dystrophic neurites. We also observe that such neuritic dystrophies associate with plaque formation in AD-transgenic mice. Finally, using cellular models, we propose a mechanism for how intracellular accumulation of Aβ disturbs homeostatic control of Aβ levels and can contribute to the up to 10,000-fold increase of Aβ in the AD brain. Our data indicate an essential role for intracellular prion-like Aβ and its synaptic spread in the pathogenesis of AD.

Is Multiple System Atrophy a Prion-like Disorder?

Multiple system atrophy (MSA) is a rapidly progressive, fatal neurodegenerative disease of uncertain aetiology that belongs to the family of α-synucleinopathies. It clinically presents with parkinsonism, cerebellar, autonomic, and motor impairment in variable combinations. Pathological hallmarks are fibrillary α-synuclein (αSyn)-rich glial cytoplasmic inclusions (GCIs) mainly involving oligodendroglia and to a lesser extent neurons, inducing a multisystem neurodegeneration, glial activation, and widespread demyelinization. The neuronal αSyn pathology of MSA has molecular properties different from Lewy bodies in Parkinson's disease (PD), both of which could serve as a pool of αSyn (prion) seeds that could initiate and drive the pathogenesis of synucleinopathies. The molecular cascade leading to the "prion-like" transfer of "strains" of aggregated αSyn contributing to the progression of the disease is poorly understood, while some presented evidence that MSA is a prion disease. However, this hypothesis is difficult to reconcile with postmortem analysis of human brains and the fact that MSA-like pathology was induced by intracerebral inoculation of human MSA brain homogenates only in homozygous mutant 53T mice, without production of disease-specific GCIs, or with replication of MSA prions in primary astrocyte cultures from transgenic mice expressing human αSyn. Whereas recent intrastriatal injection of Lewy body-derived or synthetic human αSyn fibrils induced PD-like pathology including neuronal αSyn aggregates in macaques, no such transmission of αSyn pathology in non-human primates by MSA brain lysate has been reported until now. Given the similarities between αSyn and prions, there is a considerable debate whether they should be referred to as "prions", "prion-like", "prionoids", or something else. Here, the findings supporting the proposed nature of αSyn as a prion and its self-propagation through seeding as well as the transmissibility of neurodegenerative disorders are discussed. The proof of disease causation rests on the concordance of scientific evidence, none of which has provided convincing evidence for the classification of MSA as a prion disease or its human transmission until now.

Age-Related Intraneuronal Aggregation of Amyloid-β in Endosomes, Mitochondria, Autophagosomes, and Lysosomes

 This work provides new insight into the age-related basis of Alzheimer’s disease (AD), the composition of intraneuronal amyloid (iAβ), and the mechanism of an age-related increase in iAβ in adult AD-model mouse neurons. A new end-specific antibody for Aβ₄₅ and another for aggregated forms of Aβ provide new insight into the composition of iAβ and the mechanism of accumulation in old adult neurons from the 3xTg-AD model mouse. iAβ levels containing aggregates of Aβ₄₅ increased 30-50-fold in neurons from young to old age and were further stimulated upon glutamate treatment. iAβ was 8 times more abundant in 3xTg-AD than non-transgenic neurons with imaged particle sizes following the same log-log distribution, suggesting a similar snow-ball mechanism of intracellular biogenesis. Pathologically misfolded and mislocalized Alz50 tau colocalized with iAβ and rapidly increased following a brief metabolic stress with glutamate. AβPP-CTF, Aβ₄₅, and aggregated Aβ colocalized most strongly with mitochondria and endosomes and less with lysosomes and autophagosomes. Differences in iAβ by sex were minor. These results suggest that incomplete carboxyl-terminal trimming of long Aβs by gamma-secretase produced large intracellular deposits which limited completion of autophagy in aged neurons. Understanding the mechanism of age-related changes in iAβ processing may lead to application of countermeasures to prolong dementia-free health span.

Amyloid Structural Changes Studied by Infrared Microspectroscopy in Bigenic Cellular Models of Alzheimer's Disease

Alzheimer's disease affects millions of lives worldwide. This terminal disease is characterized by the formation of amyloid aggregates, so-called amyloid oligomers. These oligomers are composed of β-sheet structures, which are believed to be neurotoxic. However, the actual secondary structure that contributes most to neurotoxicity remains unknown. This lack of knowledge is due to the challenging nature of characterizing the secondary structure of amyloids in cells. To overcome this and investigate the molecular changes in proteins directly in cells, we used synchrotron-based infrared microspectroscopy, a label-free and non-destructive technique available for in situ molecular imaging, to detect structural changes in proteins and lipids. Specifically, we evaluated the formation of β-sheet structures in different monogenic and bigenic cellular models of Alzheimer's disease that we generated for this study. We report on the possibility to discern different amyloid signatures directly in cells using infrared microspectroscopy and demonstrate that bigenic (amyloid-β, α-synuclein) and (amyloid-β, Tau) neuron-like cells display changes in β-sheet load. Altogether, our findings support the notion that different molecular mechanisms of amyloid aggregation, as opposed to a common mechanism, are triggered by the specific cellular environment and, therefore, that various mechanisms lead to the development of Alzheimer's disease.

A reaction-diffusion model of spatial propagation of A[Formula: see text] oligomers in early stage Alzheimer's disease

The misconformation and aggregation of the protein Amyloid-Beta (A[Formula: see text]) is a key event in the propagation of Alzheimer's Disease (AD). Different types of assemblies are identified, with long fibrils and plaques deposing during the late stages of AD. In the earlier stages, the disease spread is driven by the formation and the spatial propagation of small amorphous assemblies called oligomers. We propose a model dedicated to studying those early stages, in the vicinity of a few neurons and after a polymer seed has been formed. We build a reaction-diffusion model, with a Becker-Döring-like system that includes fragmentation and size-dependent diffusion. We hereby establish the theoretical framework necessary for the proper use of this model, by proving the existence of solutions using a fixed point method.

Spreading of Beta-Amyloid in Organotypic Mouse Brain Slices and Microglial Elimination and Effects on Cholinergic Neurons

The extracellular deposition of β-amyloid (Aβ) is one of the major characteristics in Alzheimer´s disease (AD). The "spreading hypothesis" suggests that a pathological protein (similar to prions) spreads over the entire brain. The aim of the present study was to use organotypic brain slices of postnatal day 8-10 mice. Using collagen hydrogels, we applied different Aβ peptides onto brain slices and analyzed spreading as well as glial reactions after eight weeks of incubation. Our data showed that from all tested Aβ peptides, human Aβ42 had the most potent activity to spread over into adjacent "target" areas. This effect was potentiated when brain slices from transgenic AD mice (APP_SweDI) were cultured. When different brain areas were connected to the "target slice" the spreading activity was more intense, originating from ventral striatum and brain stem. Reactive glial-fibrillary acidic protein (GFAP) astrogliosis increased over time, but Aβ depositions co-localized only with Iba1+ microglia but not with astrocytes. Application of human Aβ42 did not cause a degeneration of cholinergic neurons. We concluded that human Aβ42 spreads over into other "target areas", causing activation of glial cells. Most of the spread Aβ42 was taken up by microglia, and thus toxic free Aβ could not damage cholinergic neurons.

Multifaceted Role of Matrix Metalloproteinases in Neurodegenerative Diseases: Pathophysiological and Therapeutic Perspectives

Neurodegeneration is the pathological condition, in which the nervous system or neuron loses its structure, function, or both, leading to progressive degeneration or the death of neurons, and well-defined associations of tissue system, resulting in clinical manifestations. Neuroinflammation has been shown to precede neurodegeneration in several neurodegenerative diseases (NDs). No drug is yet known to delay or treat neurodegeneration. Although the etiology and potential causes of NDs remain widely indefinable, matrix metalloproteinases (MMPs) evidently have a crucial role in the progression of NDs. MMPs, a protein family of zinc (Zn²⁺)-containing endopeptidases, are pivotal agents that are involved in various biological and pathological processes in the central nervous system (CNS). The current review delineates the several emerging evidence demonstrating the effects of MMPs in the progression of NDs, wherein they regulate several processes, such as (neuro)inflammation, microglial activation, amyloid peptide degradation, blood brain barrier (BBB) disruption, dopaminergic apoptosis, and α-synuclein modulation, leading to neurotoxicity and neuron death. Published papers to date were searched via PubMed, MEDLINE, etc., while using selective keywords highlighted in our manuscript. We also aim to shed a light on pathophysiological effect of MMPs in the CNS and focus our attention on its detrimental and beneficial effects in NDs, with a special focus on Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), multiple sclerosis (MS), and Huntington's disease (HD), and discussed various therapeutic strategies targeting MMPs, which could serve as potential modulators in NDs. Over time, several agents have been developed in order to overcome challenges and open up the possibilities for making selective modulators of MMPs to decipher the multifaceted functions of MMPs in NDs. There is still a greater need to explore them in clinics.

Doxycycline inhibits α-synuclein-associated pathologies in vitro and in vivo

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are neurodegenerative disorders characterized by the misfolding and aggregation of alpha-synuclein (aSyn). Doxycycline, a tetracyclic antibiotic shows neuroprotective effects, initially proposed to be due to its anti-inflammatory properties. More recently, an additional mechanism by which doxycycline may exert its neuroprotective effects has been proposed as it has been shown that it inhibits amyloid aggregation. Here, we studied the effects of doxycycline on aSyn aggregation in vivo, in vitro and in a cell free system using real-time quaking induced conversion (RT-QuiC). Using H4, SH-SY5Y and HEK293 cells, we found that doxycycline decreases the number and size of aSyn aggregates in cells. In addition, doxycycline inhibits the aggregation and seeding of recombinant aSyn, and attenuates the production of mitochondrial-derived reactive oxygen species. Finally, we found that doxycycline induces a cellular redistribution of aggregates in a C.elegans animal model of PD, an effect that is associated with a recovery of dopaminergic function. In summary, we provide strong evidence that doxycycline treatment may be an effective strategy against synucleinopathies.

Amyloid-beta peptide and phosphorylated tau in the frontopolar cerebral cortex and in the cerebellum of toothed whales: aging versus hypoxia

Hypoxia could be a possible risk factor for neurodegenerative alterations in cetaceans' brain. Among toothed whales, the beaked whales are particularly cryptic and routinely dive deeper than 1000 m for about 1 h in order to hunt squids and fishes. Samples of frontal cerebral and cerebellar cortex were collected from nine animals, representing six different species of the suborder Odontoceti. Immunohistochemical analysis employed anti-β-amyloid (Aβ) and anti-neurofibrillary tangle (NFT) antibodies. Six of nine (67%) animals showed positive immunolabeling for Aβ and/or NFT. The most striking findings were intranuclear Aβ immunopositivity in cerebral cortical neurons and NFT immunopositivity in cerebellar Purkinje neurons with granulovacuolar degeneration. Aβ plaques were also observed in one elderly animal. Herein, we present immunohistopathological findings classic of Alzheimer's and other neurodegenerative diseases in humans. Our findings could be linked to hypoxic phenomena, as they were more extensive in beaked whales. Despite their adaptations, cetaceans could be vulnerable to sustained and repetitive brain hypoxia.

Evidence for aggregation-independent, PrPC-mediated Aβ cellular internalization

Evidence linking amyloid beta (Aβ) cellular uptake and toxicity has burgeoned, and mechanisms underlying this association are subjects of active research. Two major, interconnected questions are whether Aβ uptake is aggregation-dependent and whether it is sequence-specific. We recently reported that the neuronal uptake of Aβ depends significantly on peptide chirality, suggesting that the process is predominantly receptor-mediated. Over the past decade, the cellular prion protein (PrP^C) has emerged as an important mediator of Aβ-induced toxicity and of neuronal Aβ internalization. Here, we report that the soluble, nonfibrillizing Aβ (1-30) peptide recapitulates full-length Aβ stereoselective cellular uptake, allowing us to decouple aggregation from cellular, receptor-mediated internalization. Moreover, we found that Aβ (1-30) uptake is also dependent on PrP^C expression. NMR-based molecular-level characterization identified the docking site on PrP^C that underlies the stereoselective binding of Aβ (1-30). Our findings therefore identify a specific sequence within Aβ that is responsible for the recognition of the peptide by PrP^C, as well as PrP^C-dependent cellular uptake. Further uptake stereodifferentiation in PrP^C-free cells points toward additional receptor-mediated interactions as likely contributors for Aβ cellular internalization. Taken together, our results highlight the potential of targeting cellular surface receptors to inhibit Aβ cellular uptake as an alternative route for future therapeutic development for Alzheimer's disease.

Exploring the Potential of Therapeutic Agents Targeted towards Mitigating the Events Associated with Amyloid-β Cascade in Alzheimer's Disease

One of the most commonly occurring neurodegenerative disorders, Alzheimer's disease (AD), encompasses the loss of cognitive and memory potential, impaired learning, dementia and behavioral defects, and has been prevalent since the 1900s. The accelerating occurrence of AD is expected to reach 65.7 million by 2030. The disease results in neural atrophy and disrupted inter-neuronal connections. Amongst multiple AD pathogenesis hypotheses, the amyloid beta (Aβ) cascade is the most relevant and accepted form of the hypothesis, which suggests that Aβ monomers are formed as a result of the cleavage of amyloid precursor protein (APP), followed by the conversion of these monomers to toxic oligomers, which in turn develop β-sheets, fibrils and plaques. The review targets the events in the amyloid hypothesis and elaborates suitable therapeutic agents that function by hindering the steps of plaque formation and lowering Aβ levels in the brain. The authors discuss treatment possibilities, including the inhibition of β- and γ-secretase-mediated enzymatic cleavage of APP, the immune response generating active immunotherapy and passive immunotherapeutic approaches targeting monoclonal antibodies towards Aβ aggregates, the removal of amyloid aggregates by the activation of enzymatic pathways or the regulation of Aβ circulation, glucagon-like peptide-1 (GLP-1)-mediated curbed accumulation and the neurotoxic potential of Aβ aggregates, bapineuzumab-mediated vascular permeability alterations, statin-mediated Aβ peptide degradation, the potential role of ibuprofen and the significance of natural drugs and dyes in hindering the amyloid cascade events. Thus, the authors aim to highlight the treatment perspective, targeting the amyloid hypothesis, while simultaneously emphasizing the need to conduct further investigations, in order to provide an opportunity to neurologists to develop novel and reliable treatment therapies for the retardation of AD progression.

Potential Role of Natural Polyphenols against Protein Aggregation Toxicity: In Vitro, In Vivo, and Clinical Studies

One of the main features of neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease is the amyloidogenic behavior of disease-specific proteins including amyloid β, tau, α-synuclein, and mutant Huntingtin which participate in the formation, accumulation, and deposition of toxic misfolded aggregates. Consequently, these proteins not only associated with the progress of their respective neurodegenerative pathologies but also qualify as disease-specific biomarkers. The aim of using natural polyphenols is to target amyloid-dependent proteopathies by decreasing free radical damage and inhibiting and dissolving amyloid fibrils. We explore the effectiveness of the polyphenols epigallocatechin-3-gallate, oleuropein aglycone, and quercetin on their ability to inhibit aggregation of amyloid β, tau, and α-synuclein and mitigate other pathological features for Alzheimer's disease and Parkinson's disease. The analysis was carried from in vitro and cell line studies to animal models and clinical trials. This Review describes the use of phytochemical compounds as prophylactic agents for Alzheimer's disease, Parkinson's disease, and other proteopathies.

Protein-protein interactions in neurodegenerative diseases: A conspiracy theory

Neurodegenerative diseases such as Alzheimer's or Parkinson's are associated with the prion-like propagation and aggregation of toxic proteins. A long standing hypothesis that amyloid-beta drives Alzheimer's disease has proven the subject of contemporary controversy; leading to new research in both the role of tau protein and its interaction with amyloid-beta. Conversely, recent work in mathematical modeling has demonstrated the relevance of nonlinear reaction-diffusion type equations to capture essential features of the disease. Such approaches have been further simplified, to network-based models, and offer researchers a powerful set of computationally tractable tools with which to investigate neurodegenerative disease dynamics. Here, we propose a novel, coupled network-based model for a two-protein system that includes an enzymatic interaction term alongside a simple model of aggregate transneuronal damage. We apply this theoretical model to test the possible interactions between tau proteins and amyloid-beta and study the resulting coupled behavior between toxic protein clearance and proteopathic phenomenology. Our analysis reveals ways in which amyloid-beta and tau proteins may conspire with each other to enhance the nucleation and propagation of different diseases, thus shedding new light on the importance of protein clearance and protein interaction mechanisms in prion-like models of neurodegenerative disease.

Passive immunotherapies targeting Aβ and tau in Alzheimer's disease

Amyloid-β (Aβ) and tau proteins currently represent the two most promising targets to treat Alzheimer's disease. The most extensively developed method to treat the pathologic forms of these proteins is through the administration of exogenous antibodies, or passive immunotherapy. In this review, we discuss the molecular-level strategies that researchers are using to design an effective therapeutic antibody, given the challenges in treating this disease. These challenges include selectively targeting a protein that has misfolded or is pathological rather than the more abundant, healthy protein, designing strategic constructs for immunizing an animal to raise an antibody that has the appropriate conformational selectivity to achieve this end, and clearing the pathological protein species before prion-like cell-to-cell spread of misfolded protein has irreparably damaged neurons, without invoking damaging inflammatory responses in the brain that naturally arise when the innate immune system is clearing foreign agents. The various solutions to these problems in current clinical trials will be discussed.

Invited Review: The role of prion-like mechanisms in neurodegenerative diseases

The prototype of transmissible neurodegenerative proteinopathies is prion diseases, characterized by aggregation of abnormally folded conformers of the native prion protein. A wealth of mechanisms has been proposed to explain the conformational conversion from physiological protein into misfolded, pathological form, mode of toxicity, propagation from cell-to-cell and regional spread. There is increasing evidence that other neurodegenerative diseases, most notably Alzheimer's disease (Aβ and tau), Parkinson's disease (α-synuclein), frontotemporal dementia (TDP43, tau or FUS) and motor neurone disease (TDP43), exhibit at least some of the misfolded prion protein properties. In this review, we will discuss to what extent each of the properties of misfolded prion protein is known to occur for Aβ, tau, α-synuclein and TDP43, with particular focus on self-propagation through seeding, conformational strains, selective cellular and regional vulnerability, stability and resistance to inactivation, oligomers, toxicity and summarize the most recent literature on transmissibility of neurodegenerative disorders.

Structure and function of the perivascular fluid compartment and vertebral venous plexus: Illumining a novel theory on mechanisms underlying the pathogenesis of Alzheimer's, cerebral small vessel, and neurodegenerative diseases

Blood dynamically and richly supplies the cerebral tissue via microvessels invested in pia matter perforating the cerebral substance. Arteries penetrating the cerebral substance derive an investment from one or two successive layers of pia mater, luminally apposed to the pial-glial basal lamina of the microvasculature and abluminally apposed to a series of aquaporin IV-studded astrocytic end feet constituting the soi-disant glia limitans. The full investment of successive layers forms the variably continuous walls of the periarteriolar, pericapillary, and perivenular divisions of the perivascular fluid compartment. The pia matter disappears at the distal periarteriolar division of the perivascular fluid compartment. Plasma from arteriolar blood sequentially transudates into the periarteriolar division of the perivascular fluid compartment and subarachnoid cisterns in precession to trickling into the neural interstitium. Fluid from the neural interstitium successively propagates into the venules through the subarachnoid cisterns and perivenular division of the perivascular fluid compartment. Fluid fluent within the perivascular fluid compartment flows gegen the net direction of arteriovenular flow. Microvessel oscillations at the central tendency of the cerebral vasomotion generate corresponding oscillations of within the surrounding perivascular fluid compartment, interposed betwixt the abluminal surface of the vessels and internal surface of the pia mater. The precise microanatomy of this most fascinating among designable spaces has eluded the efforts of various investigators to interrogate its structure, though most authors non-consensusly concur the investing layers effectively and functionally segregate the perivascular and subarachnoid fluid compartments. Enlargement of the perivascular fluid compartment in a variety of neurological disorders, including senile dementia of the Alzheimer's type and cerebral small vessel disease, may alternately or coordinately constitute a correlative marker of disease severity and a possible cause implicated in the mechanistic pathogenesis of these conditions. Venular pressures modulating oscillatory dynamic flow within the perivascular fluid compartment may similarly contribute to the development of a variety among neurological disorders. An intimate understanding of subtle features typifying microanatomy and microphysiology of the investing structures and spaces of the cerebral microvasculature may powerfully inform mechanistic pathophysiology mediating a variety of neurovascular ischemic, neuroinfectious, neuroautoimmune, and neurodegenerative diseases.

Role of Membrane Tension Sensitive Endocytosis and Rho GTPases in the Uptake of the Alzheimer's Disease Peptide Aβ(1-42)

Intraneuronal accumulation of amyloid-β (Aβ) is an early pathological signum of Alzheimer's disease, and compartments of the endolysosomal system have been implicated in both seeding and cell-cell propagation of Aβ aggregation. We have studied how clathrin-independent mechanisms contribute to Aβ endocytosis, exploring pathways that are sensitive to changes in membrane tension and the regulation of Rho GTPases. Using live cell confocal microscopy and flow cytometry, we show the uptake of monomeric Aβ(1-42) into endocytic vesicles and vacuole-like dilations, following relaxation of osmotic pressure-induced cell membrane tension. This indicates Aβ(1-42) uptake via clathrin independent carriers (CLICs), although overexpression of the bar-domain protein GRAF1, a key regulator of CLICs, had no apparent effect. We furthermore report reduced Aβ(1-42) uptake following overexpression of constitutively active forms of the Rho GTPases Cdc42 and RhoA, whereas modulation of Rac1, which is linked to macropinosome formation, had no effect. Our results confirm that uptake of Aβ(1-42) is clathrin- and dynamin-independent and point to the involvement of a new and distinct clathrin-independent endocytic mechanism which is similar to uptake via CLICs or macropinocytosis but that also appear to involve yet uncharacterized molecular players.

Testing Amyloid Cross-Toxicity in the Vertebrate Brain

While amyloid proteins such as amyloid β (Aβ),α-synuclein, tau, and lysozyme are known to be prion-like; emerging data have revealed that they are also able to seed the misfolding of prion-like proteins differing in sequence. In the present study, we have developed a tool designed to test neurohistochemical outcomes associated with the entry of an amyloid protein into heterotypic neurons, i.e., neurons that do not express the invading amyloid and, instead, endogenously express amyloids differing in sequence. The stereotaxic introduction of Aβ into the rodent tegmental area of the mid-brain revealed that the foreign amyloid had infiltrated into nigral neurons. Furthermore, Aβ was found colocalized with α-synuclein, an amyloid endogenous to the substantia nigra and differing in sequence relative to Aβ. Disruption of α-synuclein status in the substantia nigra is associated with Parkinson's disease onset and progress. In addition to the study findings, a significant inroad to future neurodegenerative research was made via the stereotaxic introduction of the foreign amyloid. This technique limits the presence of confounding neurometabolic variables that may be prevalent in transgenic animal models of cross-toxicity and, thereby, better addresses the role of individual neuronal factors in cross-toxicity. Finally, the data from this work may help reconcile the high frequency of clinical comorbidity seen in neurodegenerative diseases.

Neuroprotective Effect of Brazilin on Amyloid β (25-35)-Induced Pathology in a Human Neuroblastoma Model

Until the recent past, the sole exemplar of proteins as infectious agents leading to neurodegenerative disorders remained the prion protein. Since then, the self-seeding mechanism characteristic of the prion protein has also been attributed to other neurodegenerative-disease-associated proteins, including amyloid-β (Aβ), tau, and α-synuclein (α-Syn). In model cell line studies, truncated Aβ, viz. amyloid beta (25-35), has been found to influence cellular homeostasis through its interactions with, and via, the disruption of key housekeeping machinery. Here, we demonstrate that the incubation of human neuroblastoma (SH-SY5Y) cell line with Brazilin ((6aS,11bR)-7,11b-dihydro-6H-indeno[2,1-c]chromene-3,6a,9,10-tetrol) prior to Aβ (25-35)-insult protected the cells from oxidative stress and apoptotic cell death. Furthermore, Brazilin mitigated Aβ-induced alterations in protein disulfide isomerase (PDI) and α-synuclein status, both of which are important biomarkers that report on Parkinson's pathogenesis. The results obtained in this study suggest that the tetrol is neuroprotective and helps resist Aβ-induced cross-pathology and amyloidogenic onset.

Amyloid-β oligomers in cellular models of Alzheimer's disease

Amyloid-β (Aβ) dysmetabolism is tightly associated with pathological processes in Alzheimer's disease (AD). Currently, it is thought that, in addition to Aβ fibrils that give rise to plaque formation, Aβ aggregates into non-fibrillar soluble oligomers (AβOs). Soluble AβOs have been extensively studied for their synaptotoxic and neurotoxic properties. In this review, we discuss physicochemical properties of AβOs and their impact on different brain cell types in AD. Additionally, we summarize three decades of studies with AβOs, providing a compelling bulk of evidence regarding cell-specific mechanisms of toxicity. Cellular models may lead us to a deeper understanding of the detrimental effects of AβOs in neurons and glial cells, putatively shedding light on the development of innovative therapies for AD.

Mosaic Somatic Gene Recombination as a Potentially Unifying Hypothesis for Alzheimer's Disease

The recent identification of somatic gene recombination(SGR) in human neurons affecting the well-known Alzheimer's disease (AD) pathogenic gene, amyloid precursor protein (APP), has implications for the normal and the diseased human brain. The amyloid hypothesis has been the prevailing theory for sporadic AD (SAD) pathogenesis since the discovery of APP gene involvement in familial AD and Down syndrome. Yet, despite enormous scientific and clinical effort, no disease-modifying therapy has emerged. SGR offers a novel mechanism to explain AD pathogenesis and the failures of amyloid-related clinical trials, while maintaining consistency with most aspects of the amyloid hypothesis and additionally supporting possible roles for tau, oxidative stress, inflammation, infection, and prions. SGR retro-inserts novel "genomic complementary DNAs" (gencDNAs) into neuronal genomes and becomes dysregulated in SAD, producing numerous mosaic APP variants, including DNA mutations observed in familial AD. Notably, SGR requires gene transcription, DNA strand-breaks, and reverse transcriptase (RT) activity, all of which may be promoted by well-known AD risk factors and provide a framework for the pursuit of new SGR-based therapeutics. In this perspective, we review evidence for APP SGR in AD pathogenesis and discuss its possible relevance to other AD-related dementias. Further, SGR's requirement for RT activity and the relative absence of AD in aged HIV -infected patients exposed to RT inhibitors suggest that these Food and Drug Administration (FDA)-approved drugs may represent a near-term disease-modifying therapy for AD.

Oligomerization and Conformational Change Turn Monomeric β-Amyloid and Tau Proteins Toxic: Their Role in Alzheimer's Pathogenesis

The structural polymorphism and the physiological and pathophysiological roles of two important proteins, β-amyloid (Aβ) and tau, that play a key role in Alzheimer's disease (AD) are reviewed. Recent results demonstrate that monomeric Aβ has important physiological functions. Toxic oligomeric Aβ assemblies (AβOs) may play a decisive role in AD pathogenesis. The polymorph fibrillar Aβ (fAβ) form has a very ordered cross-β structure and is assumed to be non-toxic. Tau monomers also have several important physiological actions; however, their oligomerization leads to toxic oligomers (TauOs). Further polymerization results in probably non-toxic fibrillar structures, among others neurofibrillary tangles (NFTs). Their structure was determined by cryo-electron microscopy at atomic level. Both AβOs and TauOs may initiate neurodegenerative processes, and their interactions and crosstalk determine the pathophysiological changes in AD. TauOs (perhaps also AβO) have prionoid character, and they may be responsible for cell-to-cell spreading of the disease. Both extra- and intracellular AβOs and TauOs (and not the previously hypothesized amyloid plaques and NFTs) may represent the novel targets of AD drug research.

Increased transcription of transglutaminase 1 mediates neuronal death in in vitro models of neuronal stress and Aβ1-42-mediated toxicity

Alzheimer's disease (AD) is the most common cause of dementia. At the pre-symptomatic phase of the disease, the processing of the amyloid precursor protein (APP) produces toxic peptides, called amyloid-β 1-42 (Aβ 1-42). The downstream effects of Aβ 1-42 production are not completely uncovered. Here, we report the involvement of transglutaminase 1 (TG1) in in vitro AD models of neuronal toxicity. TG1 was increased at late stages of the disease in the hippocampus of a mouse model of AD and in primary cortical neurons undergoing stress. Silencing of TGM1 gene was sufficient to prevent Aβ-mediated neuronal death. Conversely, its overexpression enhanced cell death. TGM1 upregulation was mediated at the transcriptional level by an activator protein 1 (AP1) binding site that when mutated halted TGM1 promoter activation. These results indicate that TG1 acts downstream of Aβ-toxicity, and that its stress-dependent increase makes it suitable for pharmacological intervention.

Safety and pharmacokinetics of the orally available antiprionic compound PRI-002: A single and multiple ascending dose phase I study

INTRODUCTION

PRI-002 is an orally available anti-amyloid beta (Aβ) prionic compound developed for direct disassembly of toxic Aβ oligomers relevant to Alzheimer's disease.

METHODS

Two placebo-controlled clinical phase I trials with oral dosing of PRI-002 were conducted in healthy young subjects: A single ascending dose trial (4, 12, 36, 108, or 320 mg PRI-002 or placebo) in 40 participants followed by a multiple ascending dose study with daily 160 mg PRI-002 for 14 days or 320 mg for 28 days in 24 participants. The main objectives were safety, tolerability, and evaluation of pharmacokinetic (PK) parameters.

RESULTS

PRI-002 was safe and well tolerated after single and multiple oral administration up to the highest doses. PRI-002 was absorbed rapidly and drug exposure increased proportional to dose. During repeated daily administration, the drug accumulated by a factor of about three. Steady-state conditions were reached after 1 to 2 weeks.

CONCLUSIONS

The safety and PK results encourage further clinical development of PRI-002.

The PERK-Dependent Molecular Mechanisms as a Novel Therapeutic Target for Neurodegenerative Diseases

Higher prevalence of neurodegenerative diseases is strictly connected with progressive aging of the world population. Interestingly, a broad range of age-related, neurodegenerative diseases is characterized by a common pathological mechanism-accumulation of misfolded and unfolded proteins within the cells. Under certain circumstances, such protein aggregates may evoke endoplasmic reticulum (ER) stress conditions and subsequent activation of the unfolded protein response (UPR) signaling pathways via the protein kinase RNA-like endoplasmic reticulum kinase (PERK)-dependent manner. Under mild to moderate ER stress, UPR has a pro-adaptive role. However, severe or long-termed ER stress conditions directly evoke shift of the UPR toward its pro-apoptotic branch, which is considered to be a possible cause of neurodegeneration. To this day, there is no effective cure for Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), or prion disease. Currently available treatment approaches for these diseases are only symptomatic and cannot affect the disease progression. Treatment strategies, currently under detailed research, include inhibition of the PERK-dependent UPR signaling branches. The newest data have reported that the use of small-molecule inhibitors of the PERK-mediated signaling branches may contribute to the development of a novel, ground-breaking therapeutic approach for neurodegeneration. In this review, we critically describe all the aspects associated with such targeted therapy against neurodegenerative proteopathies.

Spatially-extended nucleation-aggregation-fragmentation models for the dynamics of prion-like neurodegenerative protein-spreading in the brain and its connectome

The prion-like hypothesis of neurodegenerative diseases states that the accumulation of misfolded proteins in the form of aggregates is responsible for tissue death and its associated neurodegenerative pathology and cognitive decline. Some disease-specific misfolded proteins can interact with healthy proteins to form long chains that are transported through the brain along axonal pathways. Since aggregates of different sizes have different transport properties and toxicity, it is important to follow independently their evolution in space and time. Here, we model the spreading and propagation of aggregates of misfolded proteins in the brain using the general Smoluchowski theory of nucleation, aggregation, and fragmentation. The transport processes considered here are either anisotropic diffusion along axonal bundles or discrete Laplacian transport along a network. In particular, we model the spreading and aggregation of both amyloid-β and τ molecules in the brain connectome. We show that these two models lead to different size distributions and different propagation along the network. A detailed analysis of these two models also reveals the existence of four different stages with different dynamics and invasive properties.

Intra- and extracellular β-amyloid overexpression via adeno-associated virus-mediated gene transfer impairs memory and synaptic plasticity in the hippocampus

Alzheimer's disease (AD), the most common age-related neurodegenerative disorder, is currently conceptualized as a disease of synaptic failure. Synaptic impairments are robust within the AD brain and better correlate with dementia severity when compared with other pathological features of the disease. Nevertheless, the series of events that promote synaptic failure still remain under debate, as potential triggers such as β-amyloid (Aβ) can vary in size, configuration and cellular location, challenging data interpretation in causation studies. Here we present data obtained using adeno-associated viral (AAV) constructs that drive the expression of oligomeric Aβ either intra or extracellularly. We observed that expression of Aβ in both cellular compartments affect learning and memory, reduce the number of synapses and the expression of synaptic-related proteins, and disrupt chemical long-term potentiation (cLTP). Together, these findings indicate that during the progression AD the early accumulation of Aβ inside neurons is sufficient to promote morphological and functional cellular toxicity, a phenomenon that can be exacerbated by the buildup of Aβ in the brain parenchyma. Moreover, our AAV constructs represent a valuable tool in the investigation of the pathological properties of Aβ oligomers both in vivo and in vitro.

Alzheimer Disease Pathogenesis: Insights From Molecular and Cellular Biology Studies of Oligomeric Aβ and Tau Species

Alzheimer disease (AD) represents an oncoming epidemic that without an effective treatment promises to exact extraordinary human and financial burdens. Studies of pathogenesis are essential for defining targets for discovering disease-modifying treatments. Past studies of AD neuropathology provided valuable, albeit limited, insights. Nevertheless, building on these findings, recent studies have provided an increasingly rich harvest of genetic, molecular and cellular data that are creating unprecedented opportunities to both understand and treat AD. Among the most significant are those documenting the presence within the AD brain of toxic oligomeric species of Aβ and tau. Existing data support the view that such species can propagate and spread within neural circuits. To place these findings in context we first review the genetics and neuropathology of AD, including AD in Down syndrome (AD-DS). We detail studies that support the existence of toxic oligomeric species while noting the significant unanswered questions concerning their precise structures, the means by which they spread and undergo amplification and how they induce neuronal dysfunction and degeneration. We conclude by offering a speculative synthesis for how oligomers of Aβ and tau initiate and drive pathogenesis. While 100 years after Alzheimer's first report there is much still to learn about pathogenesis and the discovery of disease-modifying treatments, the application of new concepts and sophisticated new tools are poised to deliver important advances for combatting AD.

Do We Need Anti-Prion Compounds to Treat Alzheimer's Disease?

Background: While phase III clinical trials for the treatment of Alzheimer’s disease (AD) keep failing regardless of the target, more and more data suggest that the toxic protein assemblies of amyloid-beta protein (Aβ) and tubulin binding protein (TAU) behave like prions. Irrespective of the question of whether AD is theoretically or practically contagious, the presence of a self-replicating toxic etiologic agent in the brains of AD patients must have decisive consequences for drug development programs and clinical trial designs. Objectives: We intend to challenge the hypothesis that the underlying etiologic agent of AD is behaving prion-like. We want to discuss whether the outcome of clinical trials could have been predicted based on this hypothesis, and whether compounds that directly disassemble the toxic prion could be more beneficial for AD treatment. Method: We collected publicly accessible pre-clinical efficacy data of Aβ targeting compounds that failed or still are in phase III clinical trials. We describe the desired properties of an anti-prionic compound and compare it the properties of past and current phase III drug candidates. Results: We could not find convincing and reproducible pre-clinical efficacy data of past and current phase III drug candidates on cognition other than in preventive treatment settings. The desired properties of an anti-Aβ-prionic compound are fulfilled by the drug candidate RD2, which has been developed to directly disassemble toxic Aβ oligomers. Conclusion: RD2 is the first anti-prionic drug candidate. It is able to enhance cognition and impede neurodegeneration in three different transgenic AD mouse models, even under truly non-preventive conditions and even when applied orally. In addition, it is safe in humans.

Recent therapeutic strategies targeting beta amyloid and tauopathies in Alzheimer's disease

Alzheimer's disease (AD) has been a global concern for years due to its severe implications that affects the quality of life of the patients. The available line of therapy for treating Alzheimer's includes acetylcholinesterase inhibitors, NMDA(N-methyl-D-aspartate) antagonists and their combination which gives only symptomatic relief rather than treating the root cause of AD. Senile plaques and neurofibrillary tangles are the characteristic features underlying Alzheimer's pathology. Several attempts have been made towards exploring the niceties of these hallmarks and targeting various aspects of amyloid and tau pathology at different stages to eliminate the ultimate cause. Approaches targeting cleavage and formation of toxic amyloid fragments by secretases, aggregation of amyloid monofilaments, and immunotherapy against amyloid deposits has been extensively studied to treat amyloid pathology. Similarly, for tau pathology, tau hyperphosphorylation, microtubule stabilization, anti-tau immunotherapy has been explored. This article focuses on AD pathology and current pharmacotherapy, precisely for amyloid and tau. Furthermore, preclinical and clinical studies along with potential leads discovered under these approaches have also been included in this article. However, despite extensive research in drug development, overcoming clinical barrier still remain a major challenge for Alzheimer's pharmacotherapy.

Yeast Models of Prion-Like Proteins That Cause Amyotrophic Lateral Sclerosis Reveal Pathogenic Mechanisms

Many proteins involved in the pathogenic mechanisms of amyotrophic lateral sclerosis (ALS) are remarkably similar to proteins that form prions in the yeast Saccharomyces cerevisiae. These ALS-associated proteins are not orthologs of yeast prion proteins, but are similar in having long, intrinsically disordered domains that are rich in hydrophilic amino acids. These so-called prion-like domains are particularly aggregation-prone and are hypothesized to participate in the mislocalization and misfolding processes that occur in the motor neurons of ALS patients. Methods developed for characterizing yeast prions have been adapted to studying ALS-linked proteins containing prion-like domains. These yeast models have yielded major discoveries, including identification of new ALS genetic risk factors, new ALS-causing gene mutations and insights into how disease mutations enhance protein aggregation.

Redox nanoparticles: synthesis, properties and perspectives of use for treatment of neurodegenerative diseases

Oxidative stress (OS) and nitrative stress (NS) accompany many diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD). Antioxidants have been proposed to counteract OS/NS in these diseases. Nevertheless, the effects of antioxidants are limited and new, more efficient antioxidants are searched for. Redox-active nanoparticles (RNPs), containing antioxidants create a new therapeutical perspective. This review examines the recent literature describing synthesis and potential applications of cerium oxide RNPs, boron cluster-containing and silica containing RNPs, Gd₃N_@C₈₀ encapsulated RNPs, and concentrates on nitroxide-containing RNPs. Nitroxides are promising antioxidants, preventing inter alia glycation and nitration, but their application poses several problems. It can be expected that application of RNPs containing covalently bound nitroxides, showing low toxicity and able to penetrate the blood-brain barrier will be more efficient in the treatment of neurodegenerative disease, in particular AD and PD basing on their effects in cellular and animal models of neurodegenerative diseases.

Alzheimer's disease (AD) therapeutics - 2: Beyond amyloid - Re-defining AD and its causality to discover effective therapeutics

Compounds targeted for the treatment of Alzheimer's Disease (AD) have consistently failed in clinical trials despite evidence for target engagement and pharmacodynamic activity. This questions the relevance of compounds acting at current AD drug targets - the majority of which reflect the seminal amyloid and, to a far lesser extent, tau hypotheses - and limitations in understanding AD causality as distinct from general dementia. The preeminence of amyloid and tau led to many alternative approaches to AD therapeutics being ignored or underfunded to the extent that their causal versus contributory role in AD remains unknown. These include: neuronal network dysfunction; cerebrovascular disease; chronic, local or systemic inflammation involving the innate immune system; infectious agents including herpes virus and prion proteins; neurotoxic protein accumulation associated with sleep deprivation, circadian rhythm and glymphatic/meningeal lymphatic system and blood-brain-barrier dysfunction; metabolic related diseases including diabetes, obesity hypertension and hypocholesterolemia; mitochondrial dysfunction and environmental factors. As AD has become increasingly recognized as a multifactorial syndrome, a single treatment paradigm is unlikely to work in all patients. However, the biomarkers required to diagnose patients and parse them into mechanism/disease-based sub-groups remain rudimentary and unvalidated as do non-amyloid, non-tau translational animal models. The social and economic impact of AD is also discussed in the context of new FDA regulatory draft guidance and a proposed biomarker-based Framework (re)-defining AD and its stages as part of the larger landscape of treating dementia via the 2013 G8 initiative to identify a disease-modifying therapy for dementia/AD by 2025.

Reduced retromer function results in the accumulation of amyloid-beta oligomers

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive loss of multiple cognitive functions. Accumulation of amyloid beta oligomers (oAβ) play a major role in the neurotoxicity associated with the disease process. One of the early affected brain regions is the hippocampus, wherein a reduction of the vacuolar protein sorting-associated protein 35 (VPS35), the core protein comprising the retromer complex involved in cellular cargo sorting, has been identified. To investigate the role of the retromer function on the accumulation and clearance of oAβ, we reduced retromer function by selectively inhibiting VPS35 gene expression using siRNA in differentiated neuronal SH-SY5Y cells. As cell-to-cell transfer of oAβ to new brain regions is believed to be important for disease progression we investigated the effect of VPS35 reduction both in cells with direct uptake of oAβ and in cells receiving oAβ from donor cells. We demonstrate that reduced retromer function increases oAβ accumulation in both cell systems, both the number of cells containing intracellular oAβ and the amount within them. This effect was shown at different time points and regardless if the oAβ originated from the extracellular milieu or via a direct neuronal cell-to-cell transfer. Interestingly, not only did reduced VPS35 cause oAβ accumulation, but oAβ treatment alone also lead to a reduction of VPS35 protein content. The accumulated oAβ seems to co-localize with VPS35 and early endosome markers. Together, these findings provide evidence that reduced retromer function decreases the ability for neurons to transport and clear neurotoxic oAβ received through different routes resulting in the accumulation of oAβ. Thus, enhancing retromer function may be a potential therapeutic strategy to slow down the pathophysiology associated with the progression of AD.

Recent Insights on Alzheimer's Disease Originating from Yeast Models

In this review article, yeast model-based research advances regarding the role of Amyloid-β (Aβ), Tau and frameshift Ubiquitin UBB+1 in Alzheimer’s disease (AD) are discussed. Despite having limitations with regard to intercellular and cognitive AD aspects, these models have clearly shown their added value as complementary models for the study of the molecular aspects of these proteins, including their interplay with AD-related cellular processes such as mitochondrial dysfunction and altered proteostasis. Moreover, these yeast models have also shown their importance in translational research, e.g., in compound screenings and for AD diagnostics development. In addition to well-established Saccharomyces cerevisiae models, new upcoming Schizosaccharomyces pombe, Candida glabrata and Kluyveromyces lactis yeast models for Aβ and Tau are briefly described. Finally, traditional and more innovative research methodologies, e.g., for studying protein oligomerization/aggregation, are highlighted.

Unconventional Secretion and Intercellular Transfer of Mutant Huntingtin

The mechanism of intercellular transmission of pathological agents in neurodegenerative diseases has received much recent attention. Huntington’s disease (HD) is caused by a monogenic mutation in the gene encoding Huntingtin (HTT). Mutant HTT (mHTT) harbors a CAG repeat extension which encodes an abnormally long polyglutamine (polyQ) repeat at HTT’s N-terminus. Neuronal pathology in HD is largely due to the toxic gain-of-function by mHTT and its proteolytic products, which forms both nuclear and cytoplasmic aggregates that perturb nuclear gene transcription, RNA splicing and transport as well cellular membrane dynamics. The neuropathological effects of mHTT have been conventionally thought to be cell-autonomous in nature. Recent findings have, however, indicated that mHTT could be secreted by neurons, or transmitted from one neuronal cell to another via different modes of unconventional secretion, as well as via tunneling nanotubes (TNTs). These modes of transmission allow the intercellular spread of mHTT and its aggregates, thus plausibly promoting neuropathology within proximal neuronal populations and between neurons that are connected within neural circuits. Here, the various possible modes for mHTT’s neuronal cell exit and intercellular transmission are discussed.