Structure of amyloid oligomers and their mechanisms of toxicities: Targeting amyloid oligomers using novel therapeutic approaches

Morphological and Molecular Profiling of Amyloid-β Species in Alzheimer's Pathogenesis

Alzheimer's disease (AD) is the most common form of dementia around the world (~ 65%). Here, we portray the neuropathology of AD, biomarkers, and classification of amyloid plaques (diffuse, non-cored, dense core, compact). Tau pathology and its involvement with Aβ plaques and cell death are discussed. Amyloid cascade hypotheses, aggregation mechanisms, and molecular species formed in vitro and in vivo (on- and off-pathways) are described. Aβ42/Aβ40 monomers, dimers, trimers, Aβ-derived diffusible ligands, globulomers, dodecamers, amylospheroids, amorphous aggregates, protofibrils, fibrils, and plaques are characterized (structure, size, morphology, solubility, toxicity, mechanistic steps). An update on AD-approved drugs by regulatory agencies, along with new Aβ-based therapies, is presented. Beyond prescribing Aβ plaque disruptors, cholinergic agonists, or NMDA receptor antagonists, other therapeutic strategies (RNAi, glutaminyl cyclase inhibitors, monoclonal antibodies, secretase modulators, Aβ aggregation inhibitors, and anti-amyloid vaccines) are already under clinical trials. New drug discovery approaches based on "designed multiple ligands", "hybrid molecules", or "multitarget-directed ligands" are also being put forward and may contribute to tackling this highly debilitating and fatal form of human dementia.

Design, Synthesis, and Biological Evaluation of Ferulic Acid-Piperazine Derivatives Targeting Pathological Hallmarks of Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent cause of dementia and is characterized by low levels of acetyl and butyrylcholine, increased oxidative stress, inflammation, accumulation of metals, and aggregations of Aβ and tau proteins. Current treatments for AD provide only symptomatic relief without impacting the pathological hallmarks of the disease. In our ongoing efforts to develop naturally inspired novel multitarget molecules for AD, through extensive medicinal chemistry efforts, we have developed 13a, harboring the key functional groups to provide not only symptomatic relief but also targeting oxidative stress, able to chelate iron, inhibiting NLRP3, and Aβ1-42 aggregation in various AD models. 13a exhibited promising anticholinesterase activity against AChE (IC50 = 0.59 ± 0.19 μM) and BChE (IC50 = 5.02 ± 0.14 μM) with excellent antioxidant properties in DPPH assay (IC50 = 5.88 ± 0.21 μM) over ferulic acid (56.49 ± 0.62 μM). The molecular docking and dynamic simulations further corroborated the enzyme inhibition studies and confirmed the stability of these complexes. Importantly, in the PAMPA-BBB assay, 13a turned out to be a promising molecule that can efficiently cross the blood-brain barrier. Notably, 13a also exhibited iron-chelating properties. Furthermore, 13a effectively inhibited self- and metal-induced Aβ1-42 aggregation. It is worth mentioning that 13a demonstrated no symptom of cytotoxicity up to 30 μM concentration in PC-12 cells. Additionally, 13a inhibited the NLRP3 inflammasome and mitigated mitochondrial-induced reactive oxygen species and mitochondrial membrane potential damage triggered by LPS and ATP in HMC-3 cells. 13a could effectively reduce mitochondrial and cellular reactive oxygen species (ROS) in the Drosophila model of AD. Finally, 13a was found to be efficacious in reversing memory impairment in a scopolamine-induced AD mouse model in the in vivo studies. In ex vivo assessments, 13a notably modulates the levels of superoxide, catalase, and malondialdehyde along with AChE and BChE. These findings revealed that 13a holds promise as a potential candidate for further development in AD management.

The mechanistic interaction, aggregation and neurotoxicity of α-synuclein after interaction with glycyrrhizic acid: Modulation of synucleinopathies

This article reveals the binding mechanism between glycyrrhizic acid (GA) and α-synuclein to may provide further information for the modulation of synucleinopathies using bioactive compounds. Therefore, the inhibitory activities of GA against α-synuclein aggregation and induced neurotoxicity were evaluated using different assays. Results showed that α-synuclein-GA binding was mediated by intermolecular hydrogen bonds leading to the formation of a slightly folded complex. Theoretical studies revealed that GA binds to the N-terminal domain of α-synuclein and triggers a compact structure around a major part of the N-terminal and the NAC regions along with fluctuations in the C-terminal domain, which are prerequisites for the inhibition of α-synuclein aggregation. Then, the cellular assays showed that GA as a potential small molecule can inhibit the oligomerization of α-synuclein and relevant neurotoxicity through modulation of neural viability, membrane leakage, and ROS formation in a concentration-dependent manner. As a result, the primary mechanism of GA's anti-aggregation and neuroprotective activities is the reorganized α-synuclein structure and fluctuating C-terminal domain, which promotes long-range transient intramolecular contacts between the N-terminal and the C-terminal domain.

Each big journey starts with a first step: Importance of oligomerization

Protein oligomers, widely found in nature, have significant physiological and pathological functions. They are classified into three groups based on their function and toxicity. Significant advancements are being achieved in the development of functional oligomers, with a focus on various applications and their engineering. The antimicrobial peptides oligomers play roles in death of bacterial and cancer cells. The predominant pathogenic species in neurodegenerative disorders, as shown by recent results, are amyloid oligomers, which are the main subject of this chapter. They are generated throughout the aggregation process, serving as both intermediates in the subsequent aggregation pathways and ultimate products. Some of them may possess potent cytotoxic properties and through diverse mechanisms cause cellular impairment, and ultimately, the death of cells and disease progression. Information regarding their structure, formation mechanism, and toxicity is limited due to their inherent instability and structural variability. This chapter aims to provide a concise overview of the current knowledge regarding amyloid oligomers.

The Oligomeric Form of Amyloid Beta Triggers Astrocyte Activation, Independent of Neurons

The most common aging-related neurodegenerative disorder is Alzheimer's disease (AD), of which the main symptom is memory disturbance. Though the mechanism of AD pathogenesis is not fully defined, abnormal aggregation of amyloid beta (Aβ) plaques and tau have been considered as key factors and main histological hallmarks of the disease. Astrocyte is responsible for the control of cells and the environment around brain and spinal cord cells. Astrocytes have been implicated with AD. However, the exact function of astrocytes in AD has not been established. In this study, we investigated the regulation of astrocytes in the AD model using primary cultures. We have demonstrated that oligomerized Aβ is toxic to neurons and can induce cell death in primary cultures. In the primary cultures containing neurons and astrocytes, amyloid beta uptake was observed in both neurons and astrocytes. To verify if the uptake of amyloid beta in astrocytes is dependent on neurons, we separated and cultured primary astrocytes with no neurons. Amyloid uptake was still observed in this pure astrocyte culture, suggesting that the uptake of amyloid beta is a neuron-independent function of astrocytes. Astrocyte activation was observed in both pure and mixed cultures. Taken together, our data suggest that astrocyte is activated by oligomerized Aβ and uptakes it, which is independent of neurons.

Carbamate as a potential anti-Alzheimer's pharmacophore: A review

Alzheimer's disease (AD) is a progressive age-related neurodegenerative brain disorder, which leads to loss of memory and other cognitive dysfunction. The underlying mechanisms of AD pathogenesis are very complex and still not fully explored. Cholinergic neuronal loss, accumulation of amyloid plaque, metal ions dyshomeostasis, tau hyperphosphorylation, oxidative stress, neuroinflammation, and mitochondrial dysfunction are major hallmarks of AD. The current treatment options for AD are acetylcholinesterase inhibitors (donepezil, rivastigmine, and galantamine) and NMDA receptor antagonists (memantine). These FDA-approved drugs mainly provide symptomatic relief without addressing the pathological aspects of disease progression. So, there is an urgent need for novel drug development that not only addresses the basic mechanisms of the disease but also shows the neuroprotective property. Various research groups across the globe are working on the development of multifunctional agents for AD amelioration using different core scaffolds for their design, and carbamate is among them. Rivastigmine was the first carbamate drug investigated for AD management. The carbamate fragment, a core scaffold of rivastigmine, act as a potential inhibitor of acetylcholinesterase. In this review, we summarize the last 10 years of research conducted on the modification of carbamate with different substituents which primarily target ChE inhibition, reduce oxidative stress, and modulate Aβ aggregation.

Probing the interactions between amyloidogenic proteins and bio-membranes

Protein misfolding diseases (PMDs) in humans are characterized by the deposition of protein aggregates in tissues, including Alzheimer's disease, Parkinson's disease, type 2 diabetes, and amyotrophic lateral sclerosis. Misfolding and aggregation of amyloidogenic proteins play a central role in the onset and progression of PMDs, and these processes are regulated by multiple factors, especially the interaction between proteins and bio-membranes. Bio-membranes induce conformational changes in amyloidogenic proteins and affect their aggregation; on the other hand, the aggregates of amyloidogenic proteins may cause membrane damage or dysfunction leading to cytotoxicity. In this review, we summarize the factors that affect the binding of amyloidogenic proteins and membranes, the effects of bio-membranes on the aggregation of amyloidogenic proteins, mechanisms of membrane disruption by amyloidogenic aggregates, technical approaches for detecting these interactions, and finally therapeutic strategies targeting membrane damage caused by amyloidogenic proteins.

Action of the Purinergic and Cholinergic Anti-inflammatory Pathways on Oxidative Stress in Patients with Alzheimer's Disease in the Context of the COVID-19 Pandemic

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the etiologic agent of the 2019 coronavirus disease (COVID-19), has affected more than 20 million people in Brazil and caused a global health emergency. This virus has the potential to affect various parts of the body and compromise metabolic functions. The virus-mediated neural inflammation of the nervous system is due to a storm of cytokines and oxidative stress, which are the clinical features of Alzheimer's disease (AD). This neurodegenerative disease is aggravated in cases involving SARS-CoV-2 and its inflammatory biomarkers, accelerating accumulation of β-amyloid peptide, hyperphosphorylation of tau protein, and production of reactive oxygen species, which lead to homeostasis imbalance. The cholinergic system, through neurons and the neurotransmitter acetylcholine (ACh), modulates various physiological pathways, such as the response to stress, sleep and wakefulness, sensory information, and the cognitive system. Patients with AD have low concentrations of ACh; hence, therapeutic methods are aimed at adjusting the ACh titers available to the body for maintaining functionality. Herein, we focused on acetylcholinesterase inhibitors, responsible for the degradation of ACh in the synaptic cleft, and muscarinic and nicotinic receptor agonists of the cholinergic system owing to the therapeutic potential of the cholinergic anti-inflammatory pathway in AD associated with SARS-CoV-2 infection.

π-System Functionalization Transforms Amyloidogenic Peptide Fragment of Human Islet Amyloid Polypeptide into a Super Hydrogelator

While a considerable number of ultra-short/short amyloid peptides have been reported to form 3D supramolecular hydrogels, they all possess high minimum gelation concentration (MGC) (≥1 wt%), which preclude their applications. In this context, we demonstrate that functionalisation of a well-known amyloidogenic ultra-short peptide fragment NFGAIL (IAPf) of human Islet amyloid polypeptide with a π-system (Fluorenyl, Fm) at the N-terminus of the peptide (Fm-IAPf) yield not only highly thermostable hydrogel at physiological pH but also exhibited super gelator nature as the MGC (0.08 wt%) falls below 0.1 wt%. Various experimental results confirmed that aromatic π-π interactions from fluorenyl moieties and hydrogen bonding interactions between the IAPf drive the self-assembly/fibril formation. Fm-IAPf is the first super hydrogelator derived from amyloid-based ultra-short peptides, to the best of our knowledge. We strongly believe that this report, i. e., functionalization of an amyloid peptide with π-system, provides a lead to develop super hydrogelators from other amyloid-forming peptide fragments for their potential applications.

Prediction of amyloid PET positivity via machine learning algorithms trained with EDTA-based blood amyloid-β oligomerization data

Background

The tendency of amyloid-β to form oligomers in the blood as measured with Multimer Detection System-Oligomeric Amyloid-β (MDS-OAβ) is a valuable biomarker for Alzheimer’s disease and has been verified with heparin-based plasma. The objective of this study was to evaluate the performance of ethylenediaminetetraacetic acid (EDTA)-based MDS-OAβ and to develop machine learning algorithms to predict amyloid positron emission tomography (PET) positivity.

Methods

The performance of EDTA-based MDS-OAβ in predicting PET positivity was evaluated in 312 individuals with various machine learning models. The models with various combinations of features (i.e., MDS-OAβ level, age, apolipoprotein E4 alleles, and Mini-Mental Status Examination [MMSE] score) were tested 50 times on each dataset.

Results

The random forest model best-predicted amyloid PET positivity based on MDS-OAβ combined with other features with an accuracy of 77.14 ± 4.21% and an F1 of 85.44 ± 3.10%. The order of significance of predictive features was MDS-OAβ, MMSE, Age, and APOE. The Support Vector Machine using the MDS-OAβ value only showed an accuracy of 71.09 ± 3.27% and F−1 value of 80.18 ± 2.70%.

Conclusions

The Random Forest model using EDTA-based MDS-OAβ combined with the MMSE and apolipoprotein E status can be used to prescreen for amyloid PET positivity.

Curcumin-loaded protein imprinted mesoporous nanosphere for inhibiting amyloid aggregation

Some natural variants of human lysozyme are associated with systemic non-neurological amyloidosis that leads to amyloid protein fibril deposition in different tissues. Inhibition of amyloid fibrillation by nanomaterials is considered to be an effective approach to treating amyloidosis. Here, we prepared a targeted, highly loaded curcumin lysozyme-imprinted nanosphere (CUR-MIMS) that could effectively inhibit the aggregation of lysozyme with lysozyme adsorption capacity of 193.57 mg g^-1 and the imprinting factor (IF) of 3.72. CUR-MIMS could bind to lysozyme through hydrophobic interactions and effectively reduce the hydrophobicity of the total solvent-exposed surface in lysozyme fibrillation, thus reducing the self-assembly process triggered by hydrophobic interactions. Thioflavin T (ThT) analysis demonstrated that CUR-MIMS inhibited the aggregation of amyloid fibrils in a dose-dependent manner (inhibition efficiency of 56.07 %). Circular dichroism (CD) spectrum further illustrated that CUR-MIMS could significantly inhibit the transition of lysozyme from α-helix structure to β-sheet. More importantly, biological experiments proved the good biocompatibility of CUR-MIMS, which indicated the potential of our system as a future therapeutic platform for amyloidosis.

Continuous Theta-Burst Stimulation Promotes Paravascular CSF-Interstitial Fluid Exchange through Regulation of Aquaporin-4 Polarization in APP/PS1 Mice

Amyloid-β (Aβ) deposition plays a crucial role in the occurrence and development of Alzheimer's disease (AD), and impaired Aβ clearance is the leading cause of Aβ deposition. Recently, studies have found that the glymphatic system performs similar functions to the peripheral lymphatic system. Glymphatic fluid transport mainly consists of cerebrospinal fluid (CSF) entering the brain from the paravascular space (PVS) by penetrating arteries and CSF and interstitial fluid exchanging mediated by aquaporin-4 (AQP4). This system promotes the drainage of interstitial fluid (ISF) in the parenchyma and removes metabolic waste, including Aβ, in the brain. Glymphatic system dysfunction plays an essential role in the occurrence and progression of AD. Regulation of glymphatic fluid transport may be a critical target for AD therapy. This study explored the regulatory effects of continuous theta-burst stimulation (CTBS) on the glymphatic system in APPswe/PS1dE9 (APP/PS1) mice with two-photon imaging. The results demonstrated that CTBS could increase glymphatic fluid transport, especially CSF and ISF exchange, mediated by improved AQP4 polarization. In addition, the accelerated glymphatic pathway reduced Aβ deposition and enhanced spatial memory cognition. It provided new insight into the clinical prevention and treatment of Aβ deposition-related diseases.

Rates of β-amyloid deposition indicate widespread simultaneous accumulation throughout the brain

Amyloid plaque aggregation is a pathologic hallmark of Alzheimer's disease (AD) that occurs early in the disease. However, little is known about its progression throughout the brain. Using Pittsburgh Compound B (PIB)-PET imaging, we investigated the progression of regional amyloid accumulation in cognitively normal older adults. We found that all examined regions reached their peak accumulation rates 24-28 years after an estimated initiation corresponding to the mean baseline PIB-PET signal in amyloid-negative older adults. We also investigated the effect of increased genetic risk conferred by the apolipoprotein-E ɛ4 allele on rates of amyloid accumulation, as well as the relationship between regional amyloid accumulation and regional tau pathology, another hallmark of AD, measured with Flortaucipir-PET. Carriers of the ɛ4 allele had faster amyloid accumulation in all brain regions. Furthermore, in all regions excluding the temporal lobe, faster amyloid accumulation was associated with greater tau burden. These results indicate that amyloid accumulates near-simultaneously throughout the brain and is associated with higher AD pathology, and that genetic risk of AD is associated with faster amyloid accumulation.

TAR DNA-binding protein 43 oligomers in physiology and pathology

TAR DNA-binding protein 43 (TDP-43) is an RNA/DNA-binding protein involved in RNA regulation and diseases. In 2006, TDP-43 inclusions were found in the disease lesions of several neurodegenerative diseases. It is the pathological hallmark in both amyotrophic lateral sclerosis and frontotemporal lobar dementia. It also presents in a large portion of patients with Alzheimer's disease. TDP-43 is prone to aggregate; however, the role of TDP-43 oligomers remains poorly understood in both physiological and pathological conditions. In this review, we emphasize the role of oligomeric TDP-43 in both physiological and pathological conditions and discuss the potential mechanisms of oligomer formation. Finally, we suggest therapeutic strategies against the TDP-43 oligomers in neurodegenerative diseases.

Tin oxide nanoparticles trigger the formation of amyloid β oligomers/protofibrils and underlying neurotoxicity as a marker of Alzheimer's diseases

Alzheimer's disease (AD) is known as one of the most common forms of dementia, and oligomerization of amyloid β (Aβ₄₂) peptides can result in the onset of AD. Tin oxide nanoparticles (SnO₂ NPs) showed several applications in biomedical fields can trigger unwanted interaction with proteins and inducing protein aggregation. Herein, we synthesized SnO₂ NPs via the hydrothermal method and characterized by UV-visible, XRD, FTIR, TEM, and DLS techniques. Afterward, the formation of Aβ₄₂ amyloid oligomers/protofibrils treated alone and with SnO₂ NPs was explored by ThT and Nile red fluorescence and CD spectroscopic methods along with TEM imaging. The neurotoxicity of different spices of Aβ₄₂ samples against PC-12 cells was then explored by MTT and caspase-3 activity assays. The characterization of SnO₂ NPs confirmed the successful synthesis of crystalline NPs (20-30 nm). Different biophysical and cellular analyses indicated that SnO₂ NPs accelerated Aβ₄₂ fibrillogenesis and promoted amyloid oligomers/protofibrils cytotoxicity. As compared to the Aβ₄₂ samples grown alone, the ThT and ANS fluorescence intensity along with ellipticity results indicated the promotory effect of SnO₂ NPs on the formation of oligomers/protofibrils. Also, the cellular results showed that the treated Aβ₄₂ samples with SnO₂ NPs further reduced cell viability through activation of caspase-3. In conclusion, SnO₂ NPs greatly accelerate the fibrillation of Aβ₄₂ peptides and lead to the formation of more toxic species. The present data may offer further warrants into nano-based systems for biomedical applications in the central nervous system.

Imidazolium-based ionic liquids with increasing alkyl chain length of cations decrease the stability and fibrillation propensity of lysozyme

Imidazolium ionic liquids with longer alkyl side chains show a larger destabilization effect on lysozyme. Increased hydrophobicity of the IL increases its binding affinity and inhibits the fibril formation of lysozyme.

The role of amyloids in Alzheimer's and Parkinson's diseases

With varying clinical symptoms, most neurodegenerative diseases are associated with abnormal loss of neurons. They share the same common pathogenic mechanisms involving misfolding and aggregation, and these visible aggregates of proteins are deposited in the central nervous system. Amyloid formation is thought to arise from partial unfolding of misfolded proteins leading to the exposure of hydrophobic surfaces, which interact with other similar structures and give rise to form dimers, oligomers, protofibrils, and eventually mature fibril aggregates. Accumulating evidence indicates that amyloid oligomers, not amyloid fibrils, are the most toxic species that causes Alzheimer's disease (AD) and Parkinson's disease (PD). AD has recently been recognized as the 'twenty-first century plague', with an incident rate of 1% at 60 years of age, which then doubles every fifth year. Currently, 5.3 million people in the US are afflicted with this disease, and the number of cases is expected to rise to 13.5 million by 2050. PD, a disorder of the brain, is the second most common form of dementia, characterized by difficulty in walking and movement. Keeping the above views in mind, in this review we have focused on the roles of amyloid in neurodegenerative diseases including AD and PD, the involvement of amyloid in mitochondrial dysfunction leading to neurodegeneration, are also considered in the review.

Gene Therapy Strategy for Alzheimer's and Parkinson's Diseases Aimed at Preventing the Formation of Neurotoxic Oligomers in SH-SY5Y Cells

We present here a gene therapy approach aimed at preventing the formation of Ca2+-permeable amyloid pore oligomers that are considered as the most neurotoxic structures in both Alzheimer's and Parkinson's diseases. Our study is based on the design of a small peptide inhibitor (AmyP53) that combines the ganglioside recognition properties of the β-amyloid peptide (Aβ, Alzheimer) and α-synuclein (α-syn, Parkinson). As gangliosides mediate the initial binding step of these amyloid proteins to lipid rafts of the brain cell membranes, AmyP53 blocks, at the earliest step, the Ca2+ cascade that leads to neurodegeneration. Using a lentivirus vector, we genetically modified brain cells to express the therapeutic coding sequence of AmyP53 in a secreted form, rendering these cells totally resistant to oligomer formation by either Aβ or α-syn. This protection was specific, as control mCherry-transfected cells remained fully sensitive to these oligomers. AmyP53 was secreted at therapeutic concentrations in the supernatant of cultured cells, so that the therapy was effective for both transfected cells and their neighbors. This study is the first to demonstrate that a unique gene therapy approach aimed at preventing the formation of neurotoxic oligomers by targeting brain gangliosides may be considered for the treatment of two major neurodegenerative disorders, Alzheimer's and Parkinson's diseases.

Mechanisms of amyloid proteins aggregation and their inhibition by antibodies, small molecule inhibitors, nano-particles and nano-bodies

Protein misfolding and aggregation can be induced by a wide variety of factors, such as dominant disease-associated mutations, changes in the environmental conditions (pH, temperature, ionic strength, protein concentration, exposure to transition metal ions, exposure to toxins, posttranslational modifications including glycation, phosphorylation, and sulfation). Misfolded intermediates interact with similar intermediates and progressively form dimers, oligomers, protofibrils, and fibrils. In amyloidoses, fibrillar aggregates are deposited in the tissues either as intracellular inclusion or extracellular plaques (amyloid). When such proteinaceous deposit occurs in the neuronal cells, it initiates degeneration of neurons and consequently resulting in the manifestation of various neurodegenerative diseases. Several different types of molecules have been designed and tested both in vitro and in vivo to evaluate their anti-amyloidogenic efficacies. For instance, the native structure of a protein associated with amyloidosis could be stabilized by ligands, antibodies could be used to remove plaques, oligomer-specific antibody A11 could be used to remove oligomers, or prefibrillar aggregates could be removed by affibodies. Keeping the above views in mind, in this review we have discussed protein misfolding and aggregation, mechanisms of protein aggregation, factors responsible for aggregations, and strategies for aggregation inhibition.

Molecular insight into the early stage of amyloid-β(1-42) Homodimers aggregation influenced by histidine tautomerism

Aggregated amyloid β-peptide (Aβ) in small oligomeric forms inside the brain causes synaptic function disruption and the development of Alzheimer's disease (AD). Histidine is an important amino acid that may lead to structural changes. Aβ42 monomer chain includes 3 histidine residues that considering two ε and δ tautomers 8 isomers, including (εεε) and (εδδ) could be formed. Molecular dynamics simulation on homodimerization of (εεε) (the most common type of tautomers) and (εδδ) tautomers with different initial configurations using monomer chains from our previous work were performed to uncover the tautomeric behavior of histidine on Aβ42 aggregation in a physiological pH which is still largely unknown and impossible to observe experimentally. We found a higher propensity of forming β-sheet in (εδδ) homodimers and specifically in a greater amount from Aβ42 than from Aβ40. A smaller amount of β-sheet formation was observed for (εεε) homodimers compared with (εδδ). Additionally, interactions in (εδδ) homodimers may indicate the importance of the hydrophobic core and C-/N-terminals during oligomerization. Our findings indicate the important role of the tautomeric effect of histidine and (εδδ) homodimers at the early stage of Aβ aggregation.

Oral Treponema denticola Infection Induces Aβ1-40 and Aβ1-42 Accumulation in the Hippocampus of C57BL/6 Mice

Accumulation of amyloid-β (Aβ) in the brain is a central component of pathology in Alzheimer's disease. A growing volume of evidence demonstrates close associations between periodontal pathogens including Porphyromonas gingivalis (P. gingivalis) and Treponema denticola (T. denticola) and AD. However, the effect and mechanisms of T. denticola on accumulation of Aβ remain to be unclear. In this study, we demonstrated that T. denticola was able to enter the brain and act directly on nerve cells resulting in intra- and extracellular Aβ_1-40 and Aβ_1-42 accumulation in the hippocampus of C57BL/6 mice by selectively activating both β-secretase and γ-secretase. Furthermore, both KMI1303, an inhibitor of β-secretase, as well as DAPT, an inhibitor of γ- secretase, were found to be able to inhibit the effect of T. denticola on Aβ accumulation in N2a neuronal cells. Overall, it is concluded that T. denticola increases the expression of Aβ_1-42 and Aβ_1-40 by its regulation on beta-site amyloid precursor protein cleaving enzyme-1 and presenilin 1.

The interplay between lipid and Aβ amyloid homeostasis in Alzheimer's Disease: risk factors and therapeutic opportunities

Alzheimer's Diseases (AD) is characterized by the accumulation of amyloid deposits of Aβ peptide in the brain. Besides genetic background, the presence of other diseases and an unhealthy lifestyle are known risk factors for AD development. Albeit accumulating clinical evidence suggests that an impaired lipid metabolism is related to Aβ deposition, mechanistic insights on the link between amyloid fibril formation/clearance and aberrant lipid interactions are still unavailable. Recently, many studies have described the key role played by membrane bound Aβ assemblies in neurotoxicity. Moreover, it has been suggested that a derangement of the ubiquitin proteasome pathway and autophagy is significantly correlated with toxic Aβ aggregation and dysregulation of lipid levels. Thus, studies focusing on the role played by lipids in Aβ aggregation and proteostasis could represent a promising area of investigation for the design of valuable treatments. In this review we examine current knowledge concerning the effects of lipids in Aβ aggregation and degradation processes, focusing on the therapeutic opportunities that a comprehensive understanding of all biophysical, biochemical, and biological processes involved may disclose.

Degradation Products of Amyloid Protein: Are They The Culprits?

Objectives:

Beta-amyloid (Aβ) peptides are most toxic to cells in oligomeric form. It is commonly accepted that oligomers can form ion channels in cell membranes and allow calcium and other ions to enter cells. The activation of other mechanisms, such as apoptosis or lipid peroxidation, aggravates the toxicity, but it itself can result from the same initial point, that is, ion disturbance due to an increased permeability of membranes. However, experimental studies of membrane channels created by Aβ are surprisingly limited. Methods: Here, we report a novel flow cytometry technique which can be used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in membranes made of the neutral phosphatidylcholine.

Methods:

Here, we report a novel flow cytometry technique which can be used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in membranes made of the neutral phosphatidylcholine.

Results:

In the Discussion section, we have analyzed several issues which could be critical for understanding the pathogenesis of Alzheimer’s disease, specifically 1) the need for a negatively charged membrane to produce the ion channel; 2) the potential role of the aggregated form in cellular toxicity of Ab peptides; 3) channel-forming ability of multiple degradation products of amyloid; 4) non-specificity of ion channels formed by amyloid peptides. Potential targets of channel-forming oligomers appear to be intracellular and are organelles well-known for dysfunction in Alzheimer’s disease (mitochondria and lysosomes). In fact, lysosomes can also be the producers of degraded amyloid. Provided speculations support the hypothesis that neuronal toxicity can be caused by the degradation products of beta-amyloid.

Understanding and exploiting interactions between cellular proteostasis pathways and infectious prion proteins for therapeutic benefit

Several neurodegenerative diseases of humans and animals are caused by the misfolded prion protein (PrP^Sc), a self-propagating protein infectious agent that aggregates into oligomeric, fibrillar structures and leads to cell death by incompletely understood mechanisms. Work in multiple biological model systems, from simple baker's yeast to transgenic mouse lines, as well as in vitro studies, has illuminated molecular and cellular modifiers of prion disease. In this review, we focus on intersections between PrP and the proteostasis network, including unfolded protein stress response pathways and roles played by the powerful regulators of protein folding known as protein chaperones. We close with analysis of promising therapeutic avenues for treatment enabled by these studies.

An Isoquinolinium Dual Inhibitor of Cholinesterases and Amyloid β Aggregation Mitigates Neuropathological Changes in a Triple-Transgenic Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder affecting millions of people worldwide. The underlying pathologic mechanisms of AD are unclear. Over the decades, the development of single target agent did not lead to any successful treatment for AD. A multitarget agent that could tackle more than one AD phenotype may be helpful as a treatment strategy. Cholinesterases (ChEs) including acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), are currently the drug targets with approved treatments. Moreover, amyloid beta (Aβ) deposition is a hallmark of AD that receives considerable attention. Herein, 9Q, a previously reported dual target inhibitor dealing with cholinergic dysfunction and amyloid deposition for AD treatment, has undergone thorough investigations. In vitro studies revealed that 9Q exhibited over 80% inhibition of ChE activity at 100 μM and more than 30% inhibition of Aβ aggregation at 1 mM concentration. Moreover 9Q was able to penetrate the blood-brain barrier (BBB) and enhance the cerebral acetylcholine level in triple transgenic AD (3xTg-AD) mice. Following one month treatment with 9Q, the amyloid burden and the cognitive deficits in 3xTg-AD mice were significantly ameliorated. It was observed that 9Q treatment mitigated synapse dysfunction, decreased amyloidogenic APP processing, and reduced the tau pathology in 3xTg-AD mice. Taken together, our results suggested that dual inhibition of cholinesterases and Aβ aggregation could be a promising approach in AD treatment.

A Mechanistic Evaluation of Antioxidant Nutraceuticals on Their Potential against Age-Associated Neurodegenerative Diseases

Nutraceuticals have been extensively studied worldwide due to its neuroprotective effects in in vivo and in vitro studies, attributed by the antioxidative properties. Alzheimer (AD) and Parkinson disease (PD) are the two main neurodegenerative disorders that are discussed in this review. Both AD and PD share the similar involvement of oxidative stress in their pathophysiology. Nutraceuticals exert their antioxidative effects via direct scavenging of free radicals, prevent damage to biomolecules, indirectly stimulate the endogenous antioxidative enzymes and gene expressions, inhibit activation of pro-oxidant enzymes, and chelate metals. In addition, nutraceuticals can act as modulators of pro-survival, pro-apoptotic, and inflammatory signaling pathways. They have been shown to be effective particularly in preclinical stages, due to their multiple mechanisms of action in attenuating oxidative stress underlying AD and PD. Natural antioxidants from food sources and natural products such as resveratrol, curcumin, green tea polyphenols, and vitamin E are promising therapeutic agents in oxidative stress-mediated neurodegenerative disease as they have fewer adverse effects, more tolerable, cheaper, and sustainable for long term consumption.

Ribosylation induced structural changes in Bovine Serum Albumin: understanding high dietary sugar induced protein aggregation and amyloid formation

Non-enzymatic glycation of proteins is believed to be the root cause of high dietary sugar associated pathophysiological maladies. We investigated the structural changes in protein during progression of glycation using ribosylated Bovine Serum Albumin (BSA). Non enzymatic attachment of about 45 ribose molecules to BSA resulted in gradual reduction of hydrophobicity and aggregation as indicated by red-shifted tryptophan fluorescence, reduced ANS binding and lower anisotropy of FITC-conjugated protein. Parallely, there was a significant decrease of alpha helicity as revealed by Circular Dichroism (CD) and Fourier transformed-Infra Red (FT-IR) spectra. The glycated proteins assumed compact globular structures with enhanced Thioflavin-T binding resembling amyloids. The gross structural transition affected by ribosylation led to enhanced thermostability as indicated by melting temperature and Transmission Electron Microscopy. At a later stage of glycation, the glycated proteins developed non-specific aggregates with increase in size and loss of amyloidogenic behaviour. A parallel non-glycated control incubated under similar conditions indicated that amyloid formation and associated changes were specific for ribosylation and not driven by thermal denaturation due to incubation at 37 °C. Functionality of the glycated protein was significantly altered as probed by Isothermal Titration Calorimetry using polyphenols as substrates. The studies demonstrated that glycation driven globular amyloids form and persist as transient intermediates during formation of misfolded glycated adducts. To the best of our knowledge, the present study is the first systematic attempt to understand glycation associated changes in a protein and provides important insights towards designing therapeutics for arresting dietary sugar induced amyloid formation.

Progress toward Alzheimer's disease treatment: Leveraging the Achilles' heel of Aβ oligomers?

After three decades of false hopes and failures, a pipeline of therapeutic drugs that target the actual root cause of Alzheimer's disease (AD) is now available. Challenging the old paradigm that focused on β-amyloid peptide (Aβ) aggregation in amyloid plaques, these compounds are designed to prevent the neurotoxicity of Aβ oligomers that form Ca²⁺ permeable pores in the membranes of brain cells. By triggering an intracellular Ca²⁺ overdose, Aβ oligomers induce a cascade of neurotoxic events including oxidative stress, tau hyperphosphorylation, and neuronal loss. Targeting any post-Ca²⁺ entry steps (e.g., tau) will not address the root cause of the disease. Thus, preventing Aβ oligomers formation and/or blocking their toxicity is by essence the best approach to stop any progression of AD. Three categories of anti-oligomer compounds are already available: antibodies, synthetic peptides, and small drugs. Independent in silico-based designs of a peptide (AmyP53) and a monoclonal antibody (PMN310) converged to identify a histidine motif (H13/H14) that is critical for oligomer neutralization. This "histidine trick" can be viewed as the Achilles' heel of Aβ in the fight against AD. Moreover, lipid rafts and especially gangliosides play a critical role in the formation and toxicity of Aβ oligomers. Recognizing AD as a membrane disorder and gangliosides as the key anti-oligomer targets will provide innovative opportunities to find an efficient cure. A "full efficient" solution would also need to be affordable to anyone, as the number of patients has been following an exponential increase, affecting every part of the globe.

Amyloid-β oligomerization monitored by single-molecule stepwise photobleaching

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Method enables investigation of amyloid-β oligomer stoichiometry without requiring extrinsic fluorescent probes.

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Uses single-molecule stepwise photobleaching in vitro.

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Unveils heterogeneity within populations of oligomers.

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Assays oligomer-induced dysregulation of intracellular Ca²⁺ homeostasis in living cells.

A major hallmark of Alzheimer’s disease is the misfolding and aggregation of the amyloid- β peptide (Aβ). While early research pointed towards large fibrillar- and plaque-like aggregates as being the most toxic species, recent evidence now implicates small soluble Aβ oligomers as being orders of magnitude more harmful. Techniques capable of characterizing oligomer stoichiometry and assembly are thus critical for a deeper understanding of the earliest stages of neurodegeneration and for rationally testing next-generation oligomer inhibitors. While the fluorescence response of extrinsic fluorescent probes such as Thioflavin-T have become workhorse tools for characterizing large Aβ aggregates in solution, it is widely accepted that these methods suffer from many important drawbacks, including an insensitivity to oligomeric species. Here, we integrate several biophysics techniques to gain new insight into oligomer formation at the single-molecule level. We showcase single-molecule stepwise photobleaching of fluorescent dye molecules as a powerful method to bypass many of the traditional limitations, and provide a step-by-step guide to implementing the technique in vitro. By collecting fluorescence emission from single Aβ(1–42) peptides labelled at the N-terminal position with HiLyte Fluor 555 via wide-field total internal reflection fluorescence (TIRF) imaging, we demonstrate how to characterize the number of peptides per single immobile oligomer and reveal heterogeneity within sample populations. Importantly, fluorescence emerging from Aβ oligomers cannot be easily investigated using diffraction-limited optical microscopy tools. To assay oligomer activity, we also demonstrate the implementation of another biophysical method involving the ratiometric imaging of Fura-2-AM loaded cells which quantifies the rate of oligomer-induced dysregulation of intracellular Ca²⁺ homeostasis. We anticipate that the integrated single-molecule biophysics approaches highlighted here will develop further and in principle may be extended to the investigation of other protein aggregation systems under controlled experimental conditions.

NMR insights into the pre-amyloid ensemble and secretion targeting of the curli subunit CsgA

The biofilms of Enterobacteriaceae are fortified by assembly of curli amyloid fibres on the cell surface. Curli not only provides structural reinforcement, but also facilitates surface adhesion. To prevent toxic intracellular accumulation of amyloid precipitate, secretion of the major curli subunit, CsgA, is tightly regulated. In this work, we have employed solution state NMR spectroscopy to characterise the structural ensemble of the pre-fibrillar state of CsgA within the bacterial periplasm, and upon recruitment to the curli pore, CsgG, and the secretion chaperone, CsgE. We show that the N-terminal targeting sequence (N) of CsgA binds specifically to CsgG and that its subsequent sequestration induces a marked transition in the conformational ensemble, which is coupled to a preference for CsgE binding. These observations lead us to suggest a sequential model for binding and structural rearrangement of CsgA at the periplasmic face of the secretion machinery.

Design, synthesis and biological evaluation of novel naturally-inspired multifunctional molecules for the management of Alzheimer's disease

In our overall goal to overcome the limitations associated with natural products for the management of Alzheimer's disease and to develop in-vivo active multifunctional cholinergic inhibitors, we embarked on the development of ferulic acid analogs. A systematic SAR study to improve upon the cholinesterase inhibition of ferulic acid with analogs that also had lower logP was carried out. Enzyme inhibition and kinetic studies identified compound 7a as a lead molecule with preferential acetylcholinesterase inhibition (AChE IC₅₀ = 5.74 ± 0.13 μM; BChE IC₅₀ = 14.05 ± 0.10 μM) compared to the parent molecule ferulic acid (% inhibition of AChE and BChE at 20 μM, 15.19 ± 0.59 and 19.73 ± 0.91, respectively). Molecular docking and dynamics studies revealed that 7a fits well into the active sites of AChE and BChE, forming stable and strong interactions with key residues Asp74, Trp286, and Tyr337 in AChE and with Tyr128, Trp231, Leu286, Ala328, Phe329, and Tyr341 in BChE. Compound 7a was found to be an efficacious antioxidant in a DPPH assay (IC₅₀ = 57.35 ± 0.27 μM), and it also was able to chelate iron. Data from atomic force microscopy images demonstrated that 7a was able to modulate aggregation of amyloid β_1-42. Upon oral administration, 7a exhibited promising in-vivo activity in the scopolamine-induced AD animal model and was able to improve spatial memory in cognitive deficit mice in the Y-maze model. Analog 7a could effectively reverse the increased levels of AChE and BChE in scopolamine-treated animals and exhibited potent ex-vivo antioxidant properties. These findings suggest that 7a can act as a lead molecule for the development of naturally-inspired multifunctional molecules for the management of Alzheimer's and other neurodegenerative disorders.

The presence of cross-β-structure as a key determinant of carbonic anhydrase amyloid fibrils cytotoxicity

In most cases high cytotoxicity is characteristic of aggregates formed during lag phase of amyloid formation, whereas mature fibrils represent the depot of protein molecules incapable of damaging cell membranes. However, new experimental data show that in cases of some proteins the fibrils are the most toxic type of aggregates. Meanwhile, structural characteristics of cytotoxic fibrils and mechanisms of their cell damaging action are insufficiently explored. This work is dedicated to studying amyloid aggregation of bovine carbonic anhydrase (BCA) and effect of aggregates formed at different stages of amyloid formation on viability of the cells. Here we demonstrate that oligomers formed during lag phase do not decrease cell viability, whereas protofibrils and amyloids of BCA are cytotoxic. Obtained results allow concluding that toxicity of BCA aggregates is associated with the presence of amyloid cross-β-structure, which signature is absorbance peak at low wavenumbers at FTIR spectra (1615-1630 cm^-1). Our data suppose that cross-β-core of ВСА amyloid fibrils is responsible for their cytotoxicity.

Rubrofusarin inhibits Aβ aggregation and ameliorates memory loss in an Aβ-induced Alzheimer's disease-like mouse model

The misfolding and aggregation of amyloid β (Aβ) peptide is a common histopathologic characteristic in patients with Alzheimer's disease, so is considered to play an critical role. In the present study, we examined the effect of rubrofusarin, an ingredient of Cassiae semen, on Aβ aggregation and memory loss in an AD mouse model. Rubrofusarin inhibited Aβ aggregation in a concentration-dependent manner. Moreover, rubrofusarin dis-aggregated preformed Aβ fibrils in a concentration-dependent manner. Although aggregated Aβ induced memory loss, Aβ pre-incubated with rubrofusarin failed to induce memory loss. Moreover, rubrofusarin administration ameliorated Aβ aggregates-induced memory loss. Finally, rubrofusarin reduced glial fibrillary acidic protein or Iba-1-positive area, markers of neuroinflammation, in the hippocampus of Aβ-treated mice. These results suggest that rubrofusarin can decrease Aβ fibril formation and ameliorate memory loss in the AD mouse model.

Memory Loss and the Onset of Alzheimer's Disease Could Be Under the Control of Extracellular Heat Shock Proteins

Alzheimer's disease (AD) is a major contemporary and escalating malady in which amyloid-β (Aβ) peptides are the most likely causative agent. Aβ peptides spontaneously tend to aggregate in extracellular fluids following a progression from a monomeric state, through intermediate forms, ending in amyloid fibers and plaques. It is generally accepted now that the neurotoxic agents leading to cellular death, memory loss, and other AD characteristics are the Aβ intermediate aggregated states. However, Aβ peptides are continuously produced, released into the extracellular space, and rapidly cleared from healthy brains. Coincidentally, members of the heat shock proteins (hsp) family are present in the extracellular medium of healthy cells and body fluids, opening the possibility that hsps and Aβ could meet and interact in the extracellular milieu of the brain. In this perspective and reflection article, we place our investigation showing that the presence of Hsp70s mitigate the formation of low molecular weight Aβ peptide oligomers resulting in a reduction of cellular toxicity, in context of the current understanding of the disease. We propose that it may be an inverse relationship between the presence of Hsp70, the stage of Aβ oligomers, neurotoxicity, and the incidence of AD, particularly since the expression and circulating levels of hsp decrease with aging. Combining these observations, we propose that changes in the dynamics of Hsp70s and Aβ concentrations in the circulating brain fluids during aging defines the control of the formation of Aβ toxic aggregates, thus determining the conditions for neuron degeneration and the incidence of AD.

Label-free distribution of anti-amyloid D-AIP in Drosophila melanogaster: prevention of Aβ42-induced toxicity without side effects in transgenic flies

Soluble oligomers of the 42-amino acid amyloid beta (Aβ42) peptide are highly toxic and suspected as the causative agent of synaptic dysfunction and neuronal loss in Alzheimer's disease (AD). Previously, we have shown that a small, D-amino acid Aβ42-oligomer interacting peptide (D-AIP) can neutralize human Aβ42-mediated toxicity using in vitro and cell-based assays. In the present longitudinal study using a transgenic Drosophila melanogaster model, advanced live confocal imaging and mass spectrometry imaging (MALDI-MSI) showed that the eight amino acid D-AIP can attenuate Aβ42-induced toxicity in vivo. By separating male and female flies into distinct groups, the resultant distribution of ingested D-AIP was different between the sexes. The Aβ42-induced 'rough eye' phenotype could be rescued in the female transgenics, likely because of the co-localization of D-AIP with human Aβ42 in the female fly heads. Interestingly, the phenotype could not be rescued in the male transgenics, likely because of the co-localization of D-AIP with a confounding male-specific sex peptide (Acp70A candidate in MSI spectra) in the gut of the male flies. As a novel, more cost-effective strategy to prevent toxic amyloid formation during the early stages of AD (i.e. neutralization of toxic low-order Aβ42 oligomers without creating larger aggregates in the process), our longitudinal study establishes that D-AIP is a stable and highly effective neutralizer of toxic Aβ42 peptides in vivo. Cover Image for this issue: doi: 10.1111/jnc.14512.

Galectin-3 promotes Aβ oligomerization and Aβ toxicity in a mouse model of Alzheimer's disease

Amyloid-β (Aβ) oligomers largely initiate the cascade underlying the pathology of Alzheimer's disease (AD). Galectin-3 (Gal-3), which is a member of the galectin protein family, promotes inflammatory responses and enhances the homotypic aggregation of cancer cells. Here, we examined the role and action mechanism of Gal-3 in Aβ oligomerization and Aβ toxicities. Wild-type (WT) and Gal-3-knockout (KO) mice, APP/PS1;WT mice, APP/PS1;Gal-3^+/- mice and brain tissues from normal subjects and AD patients were used. We found that Aβ oligomerization is reduced in Gal-3 KO mice injected with Aβ, whereas overexpression of Gal-3 enhances Aβ oligomerization in the hippocampi of Aβ-injected mice. Gal-3 expression shows an age-dependent increase that parallels endogenous Aβ oligomerization in APP/PS1 mice. Moreover, Aβ oligomerization, Iba1 expression, GFAP expression and amyloid plaque accumulation are reduced in APP/PS1;Gal-3^+/- mice compared with APP/PS1;WT mice. APP/PS1;Gal-3^+/- mice also show better acquisition and retention performance compared to APP/PS1;WT mice. In studying the mechanism underlying Gal-3-promoted Aβ oligomerization, we found that Gal-3 primarily co-localizes with Iba1, and that microglia-secreted Gal-3 directly interacts with Aβ. Gal-3 also interacts with triggering receptor expressed on myeloid cells-2, which then mediates the ability of Gal-3 to activate microglia for further Gal-3 expression. Immunohistochemical analyses show that the distribution of Gal-3 overlaps with that of endogenous Aβ in APP/PS1 mice and partially overlaps with that of amyloid plaque. Moreover, the expression of the Aβ-degrading enzyme, neprilysin, is increased in Gal-3 KO mice and this is associated with enhanced integrin-mediated signaling. Consistently, Gal-3 expression is also increased in the frontal lobe of AD patients, in parallel with Aβ oligomerization. Because Gal-3 expression is dramatically increased as early as 3 months of age in APP/PS1 mice and anti-Aβ oligomerization is believed to protect against Aβ toxicity, Gal-3 could be considered a novel therapeutic target in efforts to combat AD.

Free Heme and Amyloid-β: A Fatal Liaison in Alzheimer's Disease

While the etiology of Alzheimer's disease (AD) is still unknown, an increased formation of amyloid-β (Aβ) peptide and oxidative processes are major pathological mechanism of the disease. The interaction of Aβ with free heme leads to the formation of peroxidase-active Aβ-heme complexes. However, enzyme-kinetic data and systematic mutational studies are still missing. These aspects were addressed in this study to evaluate the role of Aβ-heme complexes in AD. The enzyme-kinetic measurements showed peroxidase-specific pH- and H2O2-dependencies. In addition, the enzymatic activity of Aβ-heme complexes constantly increased at higher peptide excess. Moreover, the role of the Aβ sequence for the named enzymatic activity was tested, depicting human-specific R5, Y10, and H13 as essential amino acids. Also by studying Y10 as an endogenous peroxidase substrate for Aβ-heme complexes, ratio-specific effects were observed, showing an optimal dityrosine formation at an about 40-fold peptide excess. As dityrosine formation promotes Aβ fibrillation while free heme disturbs protein aggregation, we also investigated the effect of Aβ-heme complex-derived peroxidase activity on the formation of Aβ fibrils. The fluorescence measurements showed a different fibrillation behavior at strong peroxidase activity, leading also to altered fibril morphologies. The latter was detected by electron microscopy. As illustrated by selected in vivo measurements on a mouse model of AD, the disease is also characterized by Aβ-derived microvessel destructions and hemolytic processes. Thus, thrombo-hemorrhagic events are discussed as a source for free heme in brain tissue. In summary, we suggest the formation and enzymatic activity of Aβ-heme complexes as pathological key features of AD.

A Synthetic Pro-Drug Peptide Reverses Amyloid-β-Induced Toxicity in the Rat Model of Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD), the most prevalent neurodegenerative disorder, involves the formation of the extracellular amyloid-β (Aβ) plaques and intracellular neurofibrillary tangles. The current therapies against AD are symptomatic with limited benefits but associated with major side effects. Inhibition of self-aggregation of Aβ peptides into higher order cross-β structure is one of the potential therapeutic approach which may counter oligomerization of Aβ peptide.

OBJECTIVE

The present study aimed to evaluate the neuroprotective and anti-inflammatory potential of a synthetic Pro-Drug type peptide (PDp) against Aβ-induced toxicity in rat model of AD.

METHODS

Intra-hippocampal microinjection of toxic Aβ40 (IHAβ40) by stereotaxic surgery was performed in the male Sprague-Dawley rats to generate an Aβ-induced AD model. Sub-chronic toxicity of synthetic PDp using hematological, biochemical, and histopathological parameters was investigated. Evaluation of PDp on Aβ-induced neurodegeneration and neuroinflammation was performed.

RESULTS

PDp inhibits plaque formation with increase in Nissl granule staining in the rat hippocampus. Aβ-induced toxicity associated imbalance in reactive oxygen species and antioxidant enzymes activity such as superoxide dismutase and catalase in the rat brain was overcome by PDp treatment. Tau protein hyperphosphorylation was normalized with PDp treatment. Also, the neuroinflammatory response was suppressed with PDp treatment.

CONCLUSION

The present study depicts the potential neuroprotective role of PDp against Aβ-induced toxicity in rat. PDp inhibits plaque formation thereby normalizing oxidative stress, inhibiting tau protein hyperphosphorylation, and suppressing neuroinflammatory responses. Future studies done in this direction will pave way for new therapeutic strategies.

Deceleration of the neurodegenerative phenotype in pyroglutamate-Aβ accumulating transgenic mice by oral treatment with the Aβ oligomer eliminating compound RD2

Alzheimer's disease, a multifactorial incurable disorder, is mainly characterised by progressive neurodegeneration, extracellular accumulation of amyloid-β protein (Aβ), and intracellular aggregation of hyperphosphorylated tau protein. During the last years, Aβ oligomers have been claimed to be the disease causing agent. Consequently, development of compounds that are able to disrupt already existing Aβ oligomers is highly desirable. We developed d-enantiomeric peptides, consisting solely of d-enantiomeric amino acid residues, for the direct and specific elimination of toxic Aβ oligomers. The drug candidate RD2 did show high oligomer elimination efficacy in vitro and the in vivo efficacy of RD2 was demonstrated in treatment studies by enhanced cognition in transgenic mouse models of amyloidosis. Here, we report on the in vitro and in vivo efficacy of the compound towards pyroglutamate-Aβ, a particular aggressive Aβ species. Using the transgenic TBA2.1 mouse model, which develops pyroglutamate-Aβ_(3-42) induced neurodegeneration, we are able to show that oral RD2 treatment resulted in a significant deceleration of the progression of the phenotype. The in vivo efficacy against this highly toxic Aβ species further validates RD2 as a drug candidate for the therapeutic use in humans.

Interaction between amyloidogenic proteins and biomembranes in protein misfolding diseases: Mechanisms, contributors, and therapy

The toxic deposition of misfolded amyloidogenic proteins is associated with more than fifty protein misfolding diseases (PMDs), including Alzheimer's disease, Parkinson's disease and type 2 diabetes mellitus. Protein deposition is a multi-step process modulated by a variety of factors, in particular by membrane-protein interaction. The interaction results in permeabilization of biomembranes contributing to the cytotoxicity that leads to PMDs. Different biological and physiochemical factors, such as protein sequence, lipid composition, and chaperones, are known to affect the membrane-protein interaction. Here, we provide a comprehensive review of the mechanisms and contributing factors of the interaction between biomembranes and amyloidogenic proteins, and a summary of the therapeutic approaches to PMDs that target this interaction. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Toxicity and infectivity: insights from de novo prion formation

Prions are infectious misfolded proteins that assemble into oligomers and large aggregates, and are associated with neurodegeneration. It is believed that the oligomers contribute to cytotoxicity, although genetic and environmental factors have also been shown to have additional roles. The study of the yeast prion [PSI +] has provided valuable insights into how prions form and why they are toxic. Our recent work suggests that SDS-resistant oligomers arise and remodel early during the prion formation process, and lysates containing these newly formed oligomers are infectious. Previous work shows that toxicity is associated with prion formation and this toxicity is exacerbated by deletion of the VPS5 gene. Here, we show that newly made oligomer formation and infectivity of vps5∆ lysates are similar to wild-type strains. However using green fluorescent protein fusions, we observe that the assembly of fluorescent cytoplasmic aggregates during prion formation is different in vps5∆ strains. Instead of large immobile aggregates, vps5∆ strains have an additional population of small mobile foci. We speculate that changes in the cellular milieu in vps5∆ strains may reduce the cell's ability to efficiently recruit and sequester newly formed prion particles into central deposition sites, resulting in toxicity.