Different Fates of Alzheimer’s Disease Amyloid-β Fibrils Remodeled by Biocompatible Small Molecules

Structure of cytotoxic amyloid oligomers generated during disaggregation

Amyloidosis is characterized by the abnormal accumulation of amyloid proteins. The causative proteins aggregate from monomers to oligomers and fibrils, among which some intermediate oligomers are considered as major toxins. Cytotoxic oligomers are generated not only by aggregation but also via fibril disaggregation. However, little is known about the structural characteristics and generation conditions of cytotoxic oligomers produced during disaggregation. Herein, we summarized the structural commonalities of cytotoxic oligomers formed under various disaggregation conditions, including the addition of heat shock proteins or small compounds. In vitro experimental data demonstrated the presence of high-molecular-weight oligomers (protofibrils or protofilaments) that exhibited a fibrous morphology and β-sheet structure. Molecular dynamics simulations indicated that the distorted β-sheet structure contributed to their metastability. The tendency of these cytotoxic oligomers to appear under mild disaggregation conditions, implied formation during the early stages of disaggregation. This review will aid researchers in exploring the characteristics of highly cytotoxic oligomers and developing drugs that target amyloid aggregates.

Origin of stronger binding of ionic pair (IP) inhibitor to Aβ42 than the equimolar neutral counterparts: synergy mechanism of IP in disrupting Aβ42 protofibril and inhibiting Aβ42 aggregation under two pH conditions

Fibrous aggregates of beta-amyloid (Aβ) is a hallmark of Alzheimer's disease (AD). Several major strategies of drugs or inhibitors, including neutral molecules, positive or negative ions, and dual-inhibitor, are used to inhibit the misfolding or aggregation of Aβ42, among which a kind of dual-inhibitor composed of a pair of positive and negative ions is emerging as the most powerful candidate. This knowledge lacks the origin of the strong inhibitory effect and synergy mechanisms blocking the development and application of such inhibitors. To this end, we employed 1 : 1 ionic pairs (IP) of oppositely charged benzothiazole molecules (+)BAM1-EG6 (Pos) and (-)BAM1-EG6 (Neg) as well as equimolar neutral BAM1-EG6 (Neu) counterpart at two pH conditions (5.5 and 7.0) to bind Aβ42 targets, Aβ42 monomer (AβM), soluble pentamer (AβP), and pentameric protofibril (AβF) models, respectively, corresponding to the products of three toxic Aβ42 development pathways, lag, exponential and fibrillation phases. Simulated results illustrated the details of the inhibitory mechanisms of IP and Neu for the AβY (Y = M, P, or F) in the three different phases, characterizing the roles of Pos and Neg of IP as well as their charged, hydrophobic groups and linker playing in the synergistic interaction, and elucidated a previously unknown molecular mechanism governing the IP-Aβ42 interaction. Most importantly, we first revealed the origin of the stronger binding of IP inhibitors to Aβ42 than that of the equimolar neutral counterparts, observing a perplexing phenomenon that the physiological condition (pH = 7.0) than the acidic one (pH = 5.5) is more favorable to the enhancement of IP binding, and finally disclosed that solvation is responsible to the enhancement because at pH 7.0, AβP and AβF act as anionic membranes, where solvation plays a critical role in the chemoelectromechanics. The result not only provides a new dimension in dual-inhibitor/drug design and development but also a new perspective for uncovering charged protein disaggregation under IP-like inhibitors.

Subcutaneous Implantable Microneedle System for the Treatment of Alzheimer's Disease by Delivering Donepezil

To alleviate the dilemma of drug administration in Alzheimer's disease (AD) patients, it is of great significance to develop a new drug delivery system. In this study, a subcutaneously implanted microneedle (MN) device with a swellable gelatin methacryloyl (GelMA) needle body and a dissolvable polyvinyl alcohol (PVA) backing layer was designed. The backing layer quickly dissolved once the MN was introduced into the subcutaneous, and the hydrogel needles were implanted in the subcutaneous to enable prolonged drug release. Compared with oral administration, the MN system offers the benefits of a high administration rate, a fast onset of effect, and a longer duration of action. By detecting the concentration of acetylcholine (ACH) and Aβ 1-42, it was found that MN administration exhibited a stronger therapeutic effect. The biological safety of the MN system was also assessed, and no obvious signs of hemolysis, cytotoxicity, and inflammatory reaction were observed. Together, these findings suggested that the MN system is a convenient, efficient, and safe method of delivering donepezil hydrochloride (DPH) and may provide AD patients with a novel medicine administration option.

Small-Molecule Targeted Aβ42 Aggregate Degradation: Negatively Charged Small Molecules Are More Promising than the Neutral Ones

Heavy evidence has confirmed that Aβ42 oligomers are the most neurotoxic aggregates and play a critical role in the occurrence and development of Alzheimer's disease by causing functional neuron death, cognitive damage, and dementia. Disordered Aβ42 oligomers are challenging therapeutic targets, and no drug is currently in clinical use that modifies the properties of their monomeric states. Here, a negatively charged molecule (ER), rather than the neutral TS1 one, is identified by a molecular dynamics simulation method to be more capable of binding and sequestering the intrinsically disordered amyloid-β peptide Aβ42 in its soluble pentameric state as well as its monomeric components. Results reveal that the ERs interact with Aβ and inhibit the primary nucleation pathways in its aggregation process in entropic expansion mechanism for both Aβ42 and Aβ40 oligomers but with opposite characteristics of hydrophobic surface area (HSA). The interaction between Aβ42 oligomer and either charged ER or neutral TS1/TS0 characterizes decreased HSA, and the decrease in ER-involved case is highly visible, consistent with the observations from in silico and in vitro studies. By contrast, the presence of these inhibitors causes the HSA of Aβ40 oligomer to change undetectably and there is even a bit of increase in the histidine isomerized Aβ40 oligomer. The HSA distinction between Aβ42 and Aβ40 oligomer is possibly derived from the different effects of M35-inhibitor interaction, which is analogous to the effect of M35 oxidation. In comparison with the neutral TS1/TS0 inhibitors, ER is more prone to bind the residues located in the central (β1) and C-terminal (β2) regions of Aβ42 peptide, two key nucleation regions for Aβ intramolecular folding, intermolecular aggregation, and assembly. Notably, ER can strongly bind the charged residues, such as K16, K28, D23, to greatly disturb the potential stabilizer (e.g., salt-bridge, etc.) in metastable Aβ42 oligomers and protofibrils. These results illustrate the strategy of overcoming Alzheimer's disease from inhibiting its early stage Aβ aggregation with two kinds of small molecules to alter their behavior for therapeutic purposes and strongly recommend paying more attention to the engineering and development of negatively charged inhibitors, the long-term underappreciated ones, targeting the early stage Aβ aggregates.

Ameliorative Properties of Boronic Compounds in In Vitro and In Vivo Models of Alzheimer's Disease

Alzheimer's disease (AD) is characterized by amyloid (Aβ) aggregation, hyperphosphorylated tau, neuroinflammation, and severe memory deficits. Reports that certain boronic compounds can reduce amyloid accumulation and neuroinflammation prompted us to compare trans-2-phenyl-vinyl-boronic-acid-MIDA-ester (TPVA) and trans-beta-styryl-boronic-acid (TBSA) as treatments of deficits in in vitro and in vivo models of AD. We hypothesized that these compounds would reduce neuropathological deficits in cell-culture and animal models of AD. Using a dot-blot assay and cultured N₂a cells, we observed that TBSA inhibited Aβ42 aggregation and increased cell survival more effectively than did TPVA. These TBSA-induced benefits were extended to C. elegans expressing Aβ42 and to the 5xFAD mouse model of AD. Oral administration of 0.5 mg/kg dose of TBSA or an equivalent amount of methylcellulose vehicle to groups of six- and 12-month-old 5xFAD or wild-type mice over a two-month period prevented recognition- and spatial-memory deficits in the novel-object recognition and Morris-water-maze memory tasks, respectively, and reduced the number of pyknotic and degenerated cells, Aβ plaques, and GFAP and Iba-1 immunoreactivity in the hippocampus and cortex of these mice. These findings indicate that TBSA exerts neuroprotective properties by decreasing amyloid plaque burden and neuroinflammation, thereby preventing neuronal death and preserving memory function in the 5xFAD mice.

Novel Disassembly Mechanisms of Sigmoid Aβ42 Protofibrils by Introduced Neutral and Charged Drug Molecules

Alzheimer's disease (AD) is characterized by fibrillar deposits of amyloid-β (Aβ) peptides and neurofibrillary tangles of Tau proteins. Aβ peptides are composed of 37-49 residues, among which the Aβ₄₂ isoform is particularly toxic and aggregation-prone and is enriched in the plaques of AD brains and thus considered central to the development of AD. Therefore, disaggregation and disruption provide potential therapeutic approaches to reduce, inhibit, and even reverse Aβ aggregation. Here we capture the atomic-level details of the interactions between sigmoid Aβ₄₂ fibril 2MXU or 5KK3 and either natural tanshinone compounds TS1 or TS0 or negatively charged ER, proposing two unprecedented disassembly mechanisms. Natural TS1 or TS0 prefers to insert into the cavity together with part at the surface of the 2MXU to open up the mouth and twist the conformation, destroying the ordered growth of subsequent monomers along the fibril axis. For the more compact two-fold 5KK3 , attachment of TS1 or TS0 at the surface including some inserted in cavity results in the separation of the two folds. In the two sigmoid fibril systems, it is no longer applicable for the routine criteria to assess Aβ₄₂ fibril disassembly by introduction of these drugs, such as either reduced H-bond number, decreased β-sheet contents, or both. ER, like-charged to Aβ₄₂ fibril, is especially exceptional, and departs utterly from the neutral ones to disassemble Aβ₄₂ fibril. Besides the inapplicable routine criteria, positive binding energy between ER and Aβ₄₂ fibril also deviates from the hypotheses of "ligands exhibiting greater affinity for the β-amyloid peptide are effective at altering its aggregation and inhibiting cell toxicity" ( Cairo et al. , Biochemistry 2002 , 41 , 8620 - 8629 ) but results in stronger disassembly effect on the two kinds of sigmoid Aβ₄₂ fibrils than neutral TS0 or TS1. The disassembly power of charged ER molecules derives from its stronger deformation ability to the conformation of Aβ₄₂ fibril than the neutral ones, twisting the one-fold 2MXU into tapered-shape and separating two-fold 5KK3 in two parts further, which is in great agreement with experimental observations ( Irwin et al. Biomacromolecules 2013 , 14 ( 1 ), 264 - 274 ). The unusual disassembly mechanisms fill the gaps and offer an alternative direction in engineering new inhibitors to treat AD.

The triphenylmethane dye brilliant blue G is only moderately effective at inhibiting amyloid formation by human amylin or at disaggregating amylin amyloid fibrils, but interferes with amyloid assays; Implications for inhibitor design

The development of inhibitors of islet amyloid formation is important as pancreatic amyloid deposition contributes to type-2 diabetes and islet transplant failure. The Alzheimer's Aβ peptide and human amylin (h-amylin), the polypeptide responsible for amyloid formation in type-2 diabetes, share common physio-chemical features and some inhibitors of Aβ also inhibit amyloid formation by h-amylin and vice versa. Thus, a popular and potentially useful strategy to find lead compounds for anti-amylin amyloid agents is to examine compounds that have effects on Aβ amyloid formation. The triphenylmethane dye, brilliant blue G (BBG, Sodium;3-[[4-[(E)-[4-(4-ethoxyanilino)phenyl]-[4-[ethyl-[(3-sulfonatophenyl)methyl]azaniumylidene]-2-methylcyclohexa-2,5-dien-1-ylidene]methyl]-N-ethyl-3-methylanilino]methyl]benzenesulfonate) has been shown to modulate Aβ amyloid formation and inhibit Aβ induced toxicity. However, the effects of BBG on h-amylin have not been examined, although other triphenylmethane derivatives inhibit h-amylin amyloid formation. The compound has only a modest impact on h-amylin amyloid formation unless it is added in significant excess. BBG also remodels preformed h-amylin amyloid fibrils if added in excess, however BBG has no significant effect on h-amylin induced toxicity towards cultured β-cells or cultured CHO-T cells except at high concentrations. BBG is shown to interfere with standard thioflavin-T assays of h-amylin amyloid formation and disaggregation, highlighting the difficulty of interpreting such experiments in the absence of other measurements. BBG also interferes with ANS based assays of h-amylin amyloid formation. The work highlights the differences between inhibition of Aβ and h-amylin amyloid formation, illustrates the limitation of using Aβ inhibitors as leads for h-amylin amyloid inhibitors, and reinforces the difficulties in interpreting dye binding assays of amyloid formation.

Four-repeat tauopathies

Tau is a microtubule-associated protein with versatile functions in the dynamic assembly of the neuronal cytoskeleton. Four-repeat (4R-) tauopathies are a group of neurodegenerative diseases defined by cytoplasmic inclusions predominantly composed of tau protein isoforms with four microtubule-binding domains. Progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease or glial globular tauopathy belong to the group of 4R-tauopathies. The present review provides an introduction in the current concept of 4R-tauopathies, including an overview of the neuropathological and clinical spectrum of these diseases. It describes the genetic and environmental etiological factors, as well as the contemporary knowledge about the pathophysiological mechanisms, including post-translational modifications, aggregation and fragmentation of tau, as well as the role of protein degradation mechanisms. Furthermore, current theories about disease propagation are discussed, involving different extracellular tau species and their cellular release and uptake mechanisms. Finally, molecular diagnostic tools for 4R-tauopathies, including tau-PET and fluid biomarkers, and investigational therapeutic strategies are presented. In summary, we report on 4R-tauopathies as overarching disease concept based on a shared pathophysiological concept, and highlight the challenges and opportunities on the way towards a causal therapy.

From Mitochondrial Function to Neuroprotection-an Emerging Role for Methylene Blue

Methylene blue (MB) is a well-established drug with a long history of use, owing to its diverse range of use and its minimal side effect profile. MB has been used classically for the treatment of malaria, methemoglobinemia, and carbon monoxide poisoning, as well as a histological dye. Its role in the mitochondria, however, has elicited much of its renewed interest in recent years. MB can reroute electrons in the mitochondrial electron transfer chain directly from NADH to cytochrome c, increasing the activity of complex IV and effectively promoting mitochondrial activity while mitigating oxidative stress. In addition to its beneficial effect on mitochondrial protection, MB is also known to have robust effects in mitigating neuroinflammation. Mitochondrial dysfunction has been identified as a seemingly unifying pathological phenomenon across a wide range of neurodegenerative disorders, which thus positions methylene blue as a promising therapeutic. In both in vitro and in vivo studies, MB has shown impressive efficacy in mitigating neurodegeneration and the accompanying behavioral phenotypes in animal models for such conditions as stroke, global cerebral ischemia, Alzheimer's disease, Parkinson's disease, and traumatic brain injury. This review summarizes recent work establishing MB as a promising candidate for neuroprotection, with particular emphasis on the contribution of mitochondrial function to neural health. Furthermore, this review will briefly examine the link between MB, neurogenesis, and improved cognition in respect to age-related cognitive decline.

Comparative studies of the serum half-life extension of a protein via site-specific conjugation to a species-matched or -mismatched albumin

Human serum albumin (HSA) has been investigated as a serum half-life extender of therapeutic proteins thanks to its unusually long serum half-life.

Comparison of disaggregative effect of A-type EGCG dimer and EGCG monomer on the preformed bovine insulin amyloid fibrils

In the present study, the disruptive effects of epigallocatechin-3-gallate (EGCG) and A-type dimeric epigallocatechin-3-gallate (A-type EGCG dimer) on the preformed bovine insulin amyloid fibrils were studied by several biophysical methods including thioflavin-T (ThT) fluorescence assay, 1-anilinonaphthalene-8-sulfonic (ANS) fluorescence assay, Congo red (CR) binding assay, dynamic light scattering (DLS), transmission electron microscopy (TEM), Gel electrophoresis (SDS-PAGE) and Bradford assay. Our results demonstrated that A-type EGCG dimer showed significantly more potential disaggregative effects on the bovine insulin amyloid fibrils than EGCG. A-type EGCG dimer could not only dramatically promote the disaggregation of the preformed bovine insulin amyloid fibrils, but also restructure the amyloid fibrils into amorphous aggregates. While, EGCG could only shorten and thin the fibrils, but induce no small amorphous aggregates. Our present results provided additional evidence for the more potent disaggregation effects of dimeric polyphenols than monomeric polyphenols and suggested that A-type EGCG dimer seems to have potential application as an excellent anti-amyloidogenic agent.

Site-specific albumination of a therapeutic protein with multi-subunit to prolong activity in vivo

Albumin fusion/conjugation (albumination) has been an effective method to prolong in vivo half-life of therapeutic proteins. However, its broader application to proteins with complex folding pathway or multi-subunit is restricted by incorrect folding, poor expression, heterogeneity, and loss of native activity of the proteins linked to albumin. We hypothesized that the site-specific conjugation of albumin to a permissive site of a target protein will expand the utilities of albumin as a therapeutic activity extender to proteins with a complex structure. We show here the genetic incorporation of a non-natural amino acid (NNAA) followed by chemoselective albumin conjugation to prolong therapeutic activity in vivo. Urate oxidase (Uox), a therapeutic enzyme for treatment of hyperuricemia, is a homotetramer with multiple surface lysines, limiting conventional approaches for albumination. Incorporation of p-azido-l-phenylalanine into two predetermined positions of Uox allowed site-specific linkage of dibenzocyclooctyne-derivatized human serum albumin (HSA) through strain-promoted azide-alkyne cycloaddition (SPAAC). The bio-orthogonality of SPAAC resulted in the production of a chemically well-defined conjugate, Uox-HSA, with a retained enzymatic activity. Uox-HSA had a half-life of 8.8 h in mice, while wild-type Uox had a half-life of 1.3 h. The AUC increased 5.5-fold (1657 vs. 303 mU/mL x h). These results clearly demonstrated that site-specific albumination led to the prolonged enzymatic activity of Uox in vivo. Site-specific albumination enabled by NNAA incorporation and orthogonal chemistry demonstrates its promise for the development of long-acting protein therapeutics with high potency and safety.

General interpretation and theory of apparent height in dynamic atomic force microscopy

We provide a general theory and interpretation behind the ubiquitous loss of apparent height of nanostructures in dynamic atomic force microscopy that occurs in the attractive regime irrespective of stiffness.

Cysteine inhibits the fibrillisation and cytotoxicity of amyloid-β 40 and 42: implications for the contribution of the thiophilic interaction

Inhibitors of amyloid fibril formation have been at the centre of intense research efforts for the prevention of amyloidosis. Here, we hypothesise that a specific non-covalent interaction, the thiophilic interaction between the side chain of an aromatic residue in a polypeptide and a sulphur atom of the compound, effectively inhibits amyloid fibril formation. Fluorescence spectroscopy and transmission electron microscopy revealed that sulphur compounds, particularly Cys, inhibit the fibrillisation of amyloid-β 1-40 (Aβ40) and 1-42 (Aβ42). Interestingly, aggregates of Aβ40 and Aβ42 induced by Cys were less cytotoxic than those induced by catechin, which is the most typical inhibitor of amyloid fibril formation. Because the essential amino acid, Cys, is an abundant molecule in the blood and cytosol, our data provide a new basis for the prevention of amyloid-related diseases and the elucidation of the mechanism of these diseases.

Rationally designed peptoids modulate aggregation of amyloid-beta 40

Alzheimer's disease (AD) is the most common form of dementia and the sixth leading cause of death in the United States. Plaques composed of aggregated amyloid-beta protein (Aβ) accumulate between the neural cells in the brain and are associated with dementia and cellular death. Many strategies have been investigated to prevent Aβ self-assembly into disease-associated β-sheet amyloid aggregates; however, a promising therapeutic has not yet been identified. In this study, a peptoid-based mimic of the peptide KLVFF (residues 16-20 of Aβ) was tested for its ability to modulate Aβ aggregation. Peptoid JPT1 includes chiral, aromatic side chains to induce formation of a stable helical secondary structure that allows for greater interaction between the aromatic side chains and the cross β-sheet of Aβ. JPT1 was found to modulate Aβ40 aggregation, specifically decreasing lag time to β-sheet aggregate formation as well as the total number of fibrillar, β-sheet structured aggregates formed. These results suggest that peptoids may be able to limit the formation of Aβ aggregates that are associated with AD.

Atomic force microscopy to study molecular mechanisms of amyloid fibril formation and toxicity in Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disease characterized by dementia and memory loss for which no cure or effective prevention is currently available. Neurodegeneration in AD is linked to formation of amyloid plaques found in brain tissues of Alzheimer's patients during post-mortem examination. Amyloid plaques are composed of amyloid fibrils and small oligomers - insoluble protein aggregates. Although amyloid plaques are found on the neuronal cell surfaces, the mechanism of amyloid toxicity is still not well understood. Currently, it is believed that the cytotoxicity is a result of the nonspecific interaction of small soluble amyloid oligomers (rather than longer fibrils) with the plasma membrane. In recent years, nanotechnology has contributed significantly to understanding the structure and function of lipid membranes and to the study of the molecular mechanisms of membrane-associated diseases. We review the current state of research, including applications of the latest nanotechnology approaches, on the interaction of lipid membranes with the amyloid-β (Aβ) peptide in relation to amyloid toxicity. We discuss the interactions of Aβ with model lipid membranes with a focus to demonstrate that composition, charge and phase of the lipid membrane, as well as lipid domains and rafts, affect the binding of Aβ to the membrane and contribute to toxicity. Understanding the role of the lipid membrane in AD at the nanoscale and molecular level will contribute to the understanding of the molecular mechanism of amyloid toxicity and may aid into the development of novel preventive strategies to combat AD.

Inhibition of human amylin fibril formation by insulin-mimetic vanadium complexes

Inhibition of human amylin fibril formation by insulin-mimetic vanadium complexes.

Halogenation generates effective modulators of amyloid-Beta aggregation and neurotoxicity

Halogenation of organic compounds plays diverse roles in biochemistry, including selective chemical modification of proteins and improved oral absorption/blood-brain barrier permeability of drug candidates. Moreover, halogenation of aromatic molecules greatly affects aromatic interaction-mediated self-assembly processes, including amyloid fibril formation. Perturbation of the aromatic interaction caused by halogenation of peptide building blocks is known to affect the morphology and other physical properties of the fibrillar structure. Consequently, in this article, we investigated the ability of halogenated ligands to modulate the self-assembly of amyloidogenic peptide/protein. As a model system, we chose amyloid-beta peptide (Aβ), which is implicated in Alzheimer’s disease, and a novel modulator of Aβ aggregation, erythrosine B (ERB). Considering that four halogen atoms are attached to the xanthene benzoate group in ERB, we hypothesized that halogenation of the xanthene benzoate plays a critical role in modulating Aβ aggregation and cytotoxicity. Therefore, we evaluated the modulating capacities of four ERB analogs containing different types and numbers of halogen atoms as well as fluorescein as a negative control. We found that fluorescein is not an effective modulator of Aβ aggregation and cytotoxicity. However, halogenation of either the xanthenes or benzoate ring of fluorescein substantially enhanced the inhibitory capacity on Aβ aggregation. Such Aβ aggregation inhibition by ERB analogs except rose bengal correlated well to the inhibition of Aβ cytotoxicity. To our knowledge, this is the first report demonstrating that halogenation of aromatic rings substantially enhance inhibitory capacities of small molecules on Aβ-associated neurotoxicity via Aβ aggregation modulation.