Inhibitory Activity of Insulin on Aβ Aggregation Is Restricted Due to Binding Selectivity and Specificity to Polymorphic Aβ States

Zinc and pH modulate the ability of insulin to inhibit aggregation of islet amyloid polypeptide

Aggregation of the human islet amyloid polypeptide (hIAPP) contributes to the development and progression of Type 2 Diabetes (T2D). hIAPP aggregates within a few hours at few micromolar concentration in vitro but exists at millimolar concentrations in vivo. Natively occurring inhibitors of hIAPP aggregation might therefore provide a model for drug design against amyloid formation associated with T2D. Here, we describe the combined ability of low pH, zinc, and insulin to inhibit hIAPP fibrillation. Insulin dose-dependently slows hIAPP aggregation near neutral pH but had less effect on the aggregation kinetics at acidic pH. We determine that insulin alters hIAPP aggregation in two manners. First, insulin diverts the aggregation pathway to large nonfibrillar aggregates with ThT-positive molecular structure, rather than to amyloid fibrils. Second, soluble insulin suppresses hIAPP dimer formation, which is an important early aggregation event. Further, we observe that zinc significantly modulates the inhibition of hIAPP aggregation by insulin. We hypothesize that this effect arose from controlling the oligomeric state of insulin and show that hIAPP interacts more strongly with monomeric than oligomeric insulin.

Differential Binding and Conformational Dynamics of Tau Microtubule-Binding Repeats with a Preformed Amyloid-β Fibril Seed

Both senile plaques formed by amyloid-β (Aβ) and neurofibrillary tangles (NFTs) comprised of tau are pathological hallmarks of Alzheimer's disease (AD). The accumulation of NFTs better correlates with the loss of cognitive function than senile plaques, but NFTs are rarely observed without the presence of senile plaques. Hence, cross-seeding of tau by preformed Aβ amyloid fibril seeds has been proposed to drive the aggregation of tau and exacerbate AD progression, but the molecular mechanism remains unknown. Here, we first identified cross-interaction hotspots between Aβ and tau using atomistic discrete molecular dynamics simulations (DMD) and confirmed the critical role of the four microtubule-binding repeats of tau (R1-R4) in the cross-interaction with Aβ. We further investigated the binding structure and dynamics of each tau repeat with a preformed Aβ fibril seed. Specifically, R1 and R3 preferred to bind the Aβ fibril lateral surface instead of the elongation end. In contrast, R2 and R4 had higher binding propensities to the fibril elongation end than the lateral surface, enhancing β-sheet content by forming hydrogen bonds with the exposed hydrogen bond donors and acceptors. Together, our results suggest that the four repeats play distinct roles in driving the binding of tau to different surfaces of an Aβ fibril seed. Binding of tau to the lateral surface of Aβ fibril can increase the local concentration, while the binding to the elongation surface promotes β-sheet formation, both of which reduce the free energy barrier for tau aggregation nucleation and subsequent fibrillization.

The Major Components of Cerebrospinal Fluid Dictate the Characteristics of Inhibitors against Amyloid-Beta Aggregation

The main pathological hallmark of Alzheimer's disease (AD) is the aggregation of amyloid-β into amyloid fibrils, leading to a neurodegeneration cascade. The current medications are far from sufficient to prevent the onset of the disease, hence requiring more research to find new alternative drugs for curing AD. In vitro inhibition experiments are one of the primary tools in testing whether a molecule may be potent to impede the aggregation of amyloid-beta peptide (Aβ₄₂). However, kinetic experiments in vitro do not match the mechanism found when aggregating Aβ₄₂ in cerebrospinal fluid. The different aggregation mechanisms and the composition of the reaction mixtures may also impact the characteristics of the inhibitor molecules. For this reason, altering the reaction mixture to resemble components found in cerebrospinal fluid (CSF) is critical to partially compensate for the mismatch between the inhibition experiments in vivo and in vitro. In this study, we used an artificial cerebrospinal fluid that contained the major components found in CSF and performed Aβ₄₂ aggregation inhibition studies using oxidized epigallocatechin-3-gallate (EGCG) and fluorinated benzenesulfonamide VR16-09. This led to a discovery of a complete turnaround of their inhibitory characteristics, rendering EGCG ineffective while significantly improving the efficacy of VR16-09. HSA was the main contributor in the mixture that significantly increased the anti-amyloid characteristics of VR16-09.

EGCG attenuates α-synuclein protofibril-membrane interactions and disrupts the protofibril

The fibrillary aggregates of α-synuclein (α-syn) are closely associated with the etiology of Parkinson's disease (PD). Mounting evidence shows that the interaction of α-syn with biological membranes is a culprit for its aggregation and cytotoxicity. While some small molecules can effectively inhibit α-syn fibrillization in solution, their potential roles in the presence of membrane are rarely studied. Among them, green tea extract epigallocatechin gallate (EGCG) is currently under active investigation. Herein, we investigated the effects of EGCG on α-syn protofibril (an intermediate of α-syn fibril formation) in the presence of a model membrane and on the interactions between α-syn protofibril and the membrane, as well as the underlying mechanisms, by performing microsecond all-atom molecular dynamics simulations. The results show that EGCG has destabilization effects on α-syn protofibril, albeit to a lesser extent than that in solution. Intriguingly, we find that EGCG forms overwhelming H-bonding and cation-π interactions with membrane and thus attenuates protofibril-membrane interactions. Moreover, the decreased protofibril-membrane interactions impede the membrane damage by α-syn protofibril and enable the membrane integrity. These findings provide atomistic understanding towards the attenuation of α-syn protofibril-induced cytotoxicity by EGCG in cellular environment, which is helpful for the development of EGCG-based therapeutic strategies against PD.

Baicalein exhibits differential effects and mechanisms towards disruption of α-synuclein fibrils with different polymorphs

Parkinson's disease (PD) is the second most common neurodegenerative diseases with no cure yet and its major hallmark is α-synuclein fibrillary aggregates. The crucial role of α-synuclein aggregation in PD makes it an attractive target for potential disease-modifying therapies. Disaggregation of α-synuclein fibrils is considered as one of the promising therapeutic strategies to treat PD. The wild type (WT) and mutant α-synuclein fibrils exhibit different polymorphs and provide therapeutic targets for PD. Recent experiments reported that a flavonoid baicalein can disrupt WT α-synuclein fibrils. However, the underlying disruptive mechanism remains largely elusive, and whether BAC is capable of disrupting mutant α-synuclein fibrils is also unknown. Herein, we performed microsecond molecular dynamics simulations on cryo-EM-determined WT and two familial PD-associated mutant (E46K and H50Q) α-synuclein fibrils with and without baicalein. We find that baicalein destructs WT fibril by disrupting E46-K80 salt-bridge and β-sheets, and by remodeling the inter-protofilament interface. And baicalein can also damage E46K and H50Q mutant fibrils, but to different extents and via different mechanisms. The E46K fibril disruption is initiated from E61-K80 salt-bridge and N-terminal β-sheet, while the H50Q fibril disruption starts from the inter-protofilament interface and N-terminal β-sheet. These results reveal that disruptive effects and modes of baicalein on α-synuclein fibrils are polymorphism-dependent. This study suggests that baicalein may be a potential drug candidate to disrupt both WT and E46K/H50Q mutant α-synuclein fibrils and alleviate the pathological process of PD.

A mechanistic survey of Alzheimer's disease

Alzheimer's disease (AD) is the most common, age-dependent neurodegenerative disorder. While AD has been intensively studied from different aspects, there is no effective cure for AD, largely due to a lack of a clear mechanistic understanding of AD. In this mini-review, we mainly focus on the discussion and summary of mechanistic causes of Alzheimer's disease (AD). While different AD mechanisms illustrate different molecular and cellular pathways in AD pathogenesis, they do not necessarily exclude each other. Instead, some of them could work together to initiate, trigger, and promote the onset and development of AD. In a broader viewpoint, some AD mechanisms (e.g., amyloid aggregation mechanism, microbial infection/neuroinflammation mechanism, and amyloid cross-seeding mechanism) could also be applicable to other amyloid diseases including type II diabetes, Parkinson's disease, and prion disease. Such common mechanisms for AD and other amyloid diseases explain not only the pathogenesis of individual amyloid diseases, but also the spreading of pathologies between these diseases, which will inspire new strategies for therapeutic intervention and prevention for AD.

Advancements and future directions in research of the roles of insulin in amyloid diseases

Amyloid diseases, such as type 2 diabetes, Alzheimer's disease and Parkinson's disease are characterized by amyloid aggregates. Insulin is released from the pancreas, and it is known that insulin downstream signaling molecules are located majorly in the regions of cortex and hippocampus. Therefore, insulin plays crucial roles not only in the pancreas, but also in the brain. Recent studies have focused on the role of insulin in amyloid diseases. This review demonstrates the recent studies in which insulin affects amyloid aggregation. Specifically, molecular modeling studies provide insights into the molecular mechanisms of the effects of insulin in amyloid aggregates. Still, experimental studies are required to provide insights into the kinetics effects. This review opens new avenues for future studies on insulin molecules and amyloid aggregation.

Molecular mechanisms of resveratrol and EGCG in the inhibition of Aβ42 aggregation and disruption of Aβ42 protofibril: similarities and differences

The aggregation of amyloid-β protein (Aβ) into fibrillary deposits is implicated in Alzheimer's disease (AD), and inhibiting Aβ aggregation and clearing Aβ fibrils are considered as promising strategies to treat AD. It has been reported that resveratrol (RSV) and epigallocatechin-3-gallate (EGCG), two of the most extensively studied natural polyphenols, are able to inhibit Aβ fibrillization and remodel the preformed fibrillary aggregates into amorphous, non-toxic species. However, the mechanisms by which RSV inhibits Aβ₄₂ aggregation and disrupts Aβ₄₂ protofibril, as well as the inhibitory/disruptive mechanistic similarities and differences between RSV and EGCG, remain mostly elusive. Herein, we performed extensive all-atom molecular dynamics (MD) simulations on Aβ₄₂ dimers (the early aggregation state of Aβ₄₂) and protofibrils (the intermediate of Aβ₄₂ fibril formation and elongation) in the absence/presence of RSV or EGCG molecules. Our simulations show that both RSV and EGCG can bind with Aβ₄₂ monomers and inhibit the dimerization of Aβ₄₂. The binding of RSV with Aβ₄₂ peptide is mostly viaπ-π stacking interactions, while the binding of EGCG with Aβ₄₂ is mainly through hydrophobic, π-π stacking, and hydrogen-bonding interactions. Moreover, both RSV and EGCG disrupt the β-sheet structure and K28-A42 salt bridges, leading to a disruption of Aβ₄₂ protofibril structure. RSV mainly binds with residues whose side-chains point inwards from the surface of the protofibril, while EGCG mostly binds with residues whose side-chains point outwards from the surface of the protofibril. Furthermore, RSV interacts with Aβ₄₂ protofibrils mostly viaπ-π stacking interactions, while EGCG interacts with Aβ₄₂ protofibrils mainly via hydrogen-bonding and hydrophobic interactions. For comparison, we also explore the effects of RSV/EGCG molecules on the aggregation inhibition and protofibril disruption of the Iowa mutant (D23N) Aβ. Our findings may pave the way for the design of more effective drug candidates as well as the utilization of cocktail therapy using RSV and EGCG for the treatment of AD.

Insights into the Mechanistic Perspective Effect of Insulin on the Nonamyloidogenic Component (NAC) and α-Synuclein Aggregation

Insulin plays important functions in the brain, such as neuroprotective effects on neurons, and it is also involved in cognitive functions (e.g., attention, learning and memory). It is proposed that a lack of insulin in the brain may initiate development of neurodegenerative diseases. Herein, we examined the effect of insulin on aggregates of α-synuclein (AS), a protein that is related to Parkinson's disease (PD), and its segment nonamyloidogenic component (NAC), which is known to play a crucial role in AS aggregation. The molecular modeling tools assist us to provide insights into the molecular mechanism of the effect of insulin on fibrillation of NAC and AS. Our research leads to three conclusions. First, the preferred interactions between insulin chain B and the "zipper domain" sequence within both NAC and AS appear at the central domain across the fibril axis or at the edge of the fibril. Second, these interactions do not disrupt the cross-β structure of NAC fibril-like oligomers but disrupt the cross-β structure of AS fibril-like oligomers. Thus, insulin does not inhibit the fibrillation of NAC but may inhibit AS fibrillation. Third, some of the polymorphic NAC and AS fibril-like oligomers bind to chain A in insulin. This is the first study that demonstrates that insulin chain A can also participate in the interactions with amyloid fibril-like oligomers. Our study proposes that insulin plays a crucial role in impeding AS aggregation in the brain and consequently could inhibit the development of PD.

Kinetic and thermodynamic insights into the interaction of Aβ1-42 with astaxanthin and aggregation behavior of Aβ1-42: Surface plasmon resonance, microscopic, and molecular docking studies

Amyloid-β 1-42 (Aβ1-42) aggregation is considered as an important process in the pathology of Alzheimer's disease (AD). Astaxanthin (ATX), a xanthophyll carotenoid, has a broad range of biological activities such as neuroprotective one. The present study aimed to elucidate the interaction of ATX with Aβ1-42, as well as its effect on Aβ1-42 aggregates under different conditions. Based on the surface plasmon resonance (SPR) results, ATX possessed a high affinity towards Aβ1-42 and the binding process was spontaneous, endothermic, and entropy-driven. Additionally, the binding affinity of ATX to Aβ1-42 was glucose and insulin concentration-dependent. Hydrophobic interactions may play an important role in the interaction between ATX and Aβ1-42. The results of SPR, thioflavin T (ThT), and transmission electron microscopy (TEM) analyses represented the dependency of the anti-amyloid activity of ATX on glucose, insulin, and ATX concentrations. Further, molecular docking results indicated the presence of some same binding sites on Aβ1-42 for ATX and glucose, as well as ATX and insulin, which suggests the possible competition between the molecules for Aβ1-42 binding. Furthermore, the MTT results confirmed that ATX effect on the viability of Aβ1-42-treated PC12 cells was dependent on glucose, insulin, and ATX concentrations. In general, the results provided further insights into the interaction between Aβ1-42 and ATX, as well as the effect of ATX on Aβ1-42 aggregates under various conditions.

Serotonin and Melatonin Show Different Modes of Action on Aβ42 Protofibril Destabilization

Alzheimer's disease (AD) is associated with the aberrant self-assembly of amyloid-β (Aβ) protein into fibrillar deposits. The disaggregation of Aβ fibril is believed as one of the major therapeutic strategies for treating AD. Previous experimental studies reported that serotonin (Ser), one of the indoleamine neurotransmitters, and its derivative melatonin (Mel) are able to disassemble preformed Aβ fibrils. However, the fibril-disruption mechanisms are unclear. As the first step to understand the underlying mechanism, we investigated the interactions of Ser and Mel molecules with the LS-shaped Aβ₄₂ protofibril by performing a total of nine individual 500 ns all-atom molecular dynamics (MD) simulations. The simulations demonstrate that both Ser and Mel molecules disrupt the local β-sheet structure, destroy the salt bridges between K28 side chain and A42 COO^-, and consequently destabilize the global structure of Aβ₄₂ protofibril. The Mel molecule exhibits a greater binding capacity than the Ser molecule. Intriguingly, we find that Ser and Mel molecules destabilize Aβ₄₂ protofibril through different modes of action. Ser preferentially binds with the aromatic residues in the N-terminal region through π-π stacking interactions, while Mel binds not only with the N-terminal aromatic residues but also with the C-terminal hydrophobic residues via π-π and hydrophobic interactions. This work reveals the disruptive mechanisms of Aβ₄₂ protofibril by Ser and Mel molecules and provides useful information for designing drug candidates against AD.

Cross-Linked Amyloid β-Protein Oligomers: A Missing Link in Alzheimer's Disease Pathology?

Amyloid β-protein (Aβ) oligomers are broadly viewed as the proximate mediators of toxicity in Alzheimer's disease (AD). Recent studies, however, provide substantial evidence that Aβ is involved in protection and repair of the central nervous system whereby Aβ oligomer and subsequent fibril formation are integral to its normal antimicrobial and antiviral function. These developments raise a question of what exactly makes Aβ oligomers toxic in the context of AD. This Perspective describes a paradigm shift in the search for toxic Aβ oligomer species that involves oxidative-stress-induced stabilization of Aβ oligomers via cross-linking and reviews most recent research elucidating structural aspects of cross-linked Aβ oligomers and potential inhibition of their toxicity.

Epigallocatechin Gallate Destabilizes α-Synuclein Fibril by Disrupting the E46-K80 Salt-Bridge and Inter-protofibril Interface

The accumulation and deposition of fibrillar aggregates of α-synuclein (α-syn) into Lewy bodies are the major hallmarks of Parkinson's disease (PD) for which there is no cure yet. Disrupting preformed α-syn fibrils is considered one of the rational therapeutic strategies to combat PD. Experimental studies reported that epigallocatechin gallate (EGCG), a polyphenol extracted from green tea, can disrupt α-syn fibrils into benign amorphous aggregates. However, the molecular mechanism of action is poorly understood. Herein, we performed molecular dynamics simulations on a newly released Greek-key-like α-syn fibril with or without EGCG to investigate the influence of EGCG on α-syn fibril. Our simulations show that EGCG disrupts the local β-sheet structure, E46-K80 salt-bridge crucial for the stabilization of the Greek-key-like structure, and hydrophobic interactions stabilizing the inter-protofibril interface and destabilizes the global structure of the α-syn fibril. Interaction analyses reveal that hydrophobic and hydrogen-bonding interactions between EGCG and α-syn fibrils play important roles in the destabilization of the fibril. We find that the disruption of the E46-K80 salt-bridge closely correlates with the formation of hydrogen-bonds (H-bonds) between EGCG and E46/K80. Our results provide mechanistic insights into the disruption modes of α-syn fibril by EGCG, which may pave the way for designing drug candidates targeting α-syn fibrillization to treat PD.

Assessments of the Effect of Neurokinin B on Toxic Aβ Aggregates in Alzheimer's Disease with the Molecular Mechanisms' Action

Clinical trials of past and current treatments for Alzheimer's disease (AD) patients on the market suffer from the dual drawbacks of a lack of efficacy and side effects. Neuropeptides have been highlighted by their potential to protect cells against AD and can reverse the toxic effect induced by Aβ in cultured neurons. One of the neuropeptides that has insufficient attention in the literature as a potential treatment for prevention of the progression of AD is neurokinin B (NKB). There are critical and unresolved questions concerning the activation, and the molecular mechanisms underlying NKB effect on prevention of Aβ aggregation remain unknown. The current work identifies for the first time the specific interactions that contribute to the inhibition and prevention of initial seeding of polymorphic early-stage dimers. Three main conclusions are observed in this work. First, NKB inhibits formation of polymorphic early-stage fibrillar Aβ dimers. The efficiency of the inhibition depends on the concentration of NKB (i.e., NKB:Aβ ratio). Second, NKB has an excellent effect of preventing the formation of initial seeding of early-stage nonfibrillar Aβ dimers. Third, NKB peptides may self-assemble to form cross-α fibril-like structure during the inhibition activity of the polymorphic early-stage fibrillar Aβ dimers but not during the prevention activity of early-stage nonfibrillar Aβ dimers. The work provides crucial information for future experimental studies to approve the functional effect of NKB on inhibition and prevention of Aβ polymorphic early-stage oligomers.