Conformationally restricted short peptides inhibit human islet amyloid polypeptide (hIAPP) fibrillization

β-Carboline Alkaloids Resist the Aggregation and Cytotoxicity of Human Islet Amyloid Polypeptide

β-Carboline alkaloids have a variety of pharmacological activities, such as antitumor, antibiosis and antidiabetes. Harmine and harmol are two structurally similar β-carbolines that occur in many medicinal plants. In this work, we chose harmine and harmol to impede the amyloid fibril formation of human islet amyloid polypeptide (hIAPP) associated with type 2 diabetes mellitus (T2DM), by a series of physicochemical and biochemical methods. The results indicate that harmine and harmol effectively prevent peptide fibril formation and alleviate toxic oligomer species. In addition, both small molecules exhibit strong binding affinities with hIAPP mainly through hydrophobic and hydrogen bonding interactions, thus reducing the cytotoxicity induced by hIAPP. Their distinct binding pattern with hIAPP is closely linked to the molecular configuration of the two small molecules, affecting their ability to impede peptide aggregation. The study is of great significance for the application and development of β-carboline alkaloids against T2DM.

Disparate Effect of Hybrid Peptidomimetics Containing Isomers of Aminobenzoic Acid on hIAPP Aggregation

The abnormal misfolding of human islet amyloid polypeptide (hIAPP) in pancreatic β-cells is implicated in the progression of type II diabetes (T2D). With the prevalence of T2D increasing worldwide, preventing the aggregation of hIAPP has been recognized as a promising therapeutic strategy to control this disease. Recently, a class of novel conformationally restricted β-sheet breaker hybrid peptidomimetics (BSBHps) was found to demonstrate efficient inhibitory ability toward amyloid formation of hIAPP. One (Ile26) or more (Gly24 and Ile26) residues in these six-membered peptide sequences, which have been extracted from the amyloidogenic core of hIAPP, N₂₂FGAIL₂₇, are substituted by three different isomers of the conformationally restricted aromatic amino acid, i.e., aminobenzoic acid (β, γ, and δ), to generate these BSBHps. The presence of the nonproteinogenic aminobenzoic acid moiety renders the BSBHps to be more stable toward proteolytic degradation. The different isomeric BSBHps exhibit contrasting influence on the self-assembly of hIAPP. The BSBHps containing β- and γ-aminobenzoic acid can sufficiently prevent hIAPP aggregation, but those with the δ-aminobenzoic group stabilize the β-sheet-rich aggregate of hIAPP. The difference in the angle between the amino and carboxyl groups in the isomers of the aminobenzoic moiety causes the BSBHps to attain discrete conformation and hence leads to variation in their binding preference with hIAPP and ultimately their inhibitory potency. This guides the pathway for the dissimilar effect of BSBHps on peptide aggregation and, therefore, provides insights into the design considerations for novel drugs against T2D.

Exploration of Isoquinoline Alkaloids as Potential Inhibitors against Human Islet Amyloid Polypeptide

Type-2 diabetes mellitus (T2DM) is one of the most concerning public health problems because of its high incidence, multiple complications, and difficult treatment. Human islet amyloid polypeptide (hIAPP) is closely linked to T2DM because its abnormal self-assembly causes membrane damage and cell dysfunction. The development of potential inhibitors to prevent hIAPP fibrillation is a promising strategy for the intervention and treatment of diabetes. Natural isoquinoline alkaloids are used as effective medication that targets different biomolecules. Although studies explored the efficacy of berberine, jatrorrhizine, and chelerythrine in diabetes, the underlying mechanism remains unclear. Herein, three isoquinoline alkaloids are selected to reveal their roles in hIAPP aggregation, disaggregation, and cell protection. All three compounds displayed good inhibitory effects on peptide fibrillation, scattered the preformed fibrils into small oligomers and most monomers, and upregulated cell viability by reducing hIAPP oligomerization. Moreover, combined biophysical analyses indicated that the compounds affected the β-sheet structure and hydrophobicity of polypeptides significantly, and the benzo[c]phenanthridine structure of chelerythrine was beneficial to the inhibition of hIAPP aggregation and their hydrophobic interaction, compared with that of berberine and jatrorrhizine. Our work elaborated the effects of these alkaloids on hIAPP fibrillation and reveals a possible mechanism for these compounds against T2DM.

Human islet amyloid polypeptide: A therapeutic target for the management of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) and other metabolic disorders are often silent and go unnoticed in patients because of the lack of suitable prognostic and diagnostic markers. The current therapeutic regimens available for managing T2DM do not reverse diabetes; instead, they delay the progression of diabetes. Their efficacy (in principle) may be significantly improved if implemented at earlier stages. The misfolding and aggregation of human islet amyloid polypeptide (hIAPP) or amylin has been associated with a gradual decrease in pancreatic β-cell function and mass in patients with T2DM. Hence, hIAPP has been recognized as a therapeutic target for managing T2DM. This review summarizes hIAPP's role in mediating dysfunction and apoptosis in pancreatic β-cells via induction of endoplasmic reticulum stress, oxidative stress, mitochondrial dysfunction, inflammatory cytokine secretion, autophagy blockade, etc. Furthermore, it explores the possibility of using intermediates of the hIAPP aggregation pathway as potential drug targets for T2DM management. Finally, the effects of common antidiabetic molecules and repurposed drugs; other hIAPP mimetics and peptides; small organic molecules and natural compounds; nanoparticles, nanobodies, and quantum dots; metals and metal complexes; and chaperones that have demonstrated potential to inhibit and/or reverse hIAPP aggregation and can, therefore, be further developed for managing T2DM have been discussed.

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Misfolded species of hIAPP form toxic oligomers in pancreatic β-cells.

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hIAPP amyloids has been detected in the pancreas of about 90% subjects with T2DM.

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Inhibitors of hIAPP aggregation can help manage T2DM.

A new strategy to reconcile amyloid cross-seeding and amyloid prevention in a binary system of α-synuclein fragmental peptide and hIAPP

Amyloid cross-seeding and amyloid inhibition are two different research subjects being studied separately for different pathological purposes, in which amyloid cross-seeding targets to study the co-aggregation of different amyloid proteins and potential molecular links between different neurodegenerative diseases, while amyloid inhibition aims to design different molecules for preventing amyloid aggregation. While both amyloid cross-seeding and amyloid inhibition are critical for better understanding the pathological causes of different neurodegenerative diseases including Parkinson disease (PD) and Type 2 diabetes (T2D), less efforts have been made to reconcile the two phenomena. Herein, we proposed a new preventive strategy to demonstrate (a) the cross-seeding of octapeptide TKEQVTNV from α-synuclein (associated with PD) with hIAPP (associated with T2D) and (b) the cross-seeding-promoted hIAPP fibrillization and cross-seeding-reduced hIAPP toxicity. Collective results confirmed that TKEQVTNV can indeed cross-seed with hIAPP monomers and oligomers, not protofibrils, to form β-structure-rich fibrils and to accelerate hIAPP fibrillization. Moreover, such cross-seeding-induced promotion effect by TKEQVTNV also rescued the pancreatic cells from hIAPP-induced cytotoxicity by increasing cell viability and reducing cell apoptosis simultaneously. This work provides a new angle to discover amyloid fragments and use them as amyloid modulators (inhibitors or promotors) to interfere with amyloid aggregation of other amyloid proteins, as well as sequence/structure basis to explore the amyloid cross-seeding between different amyloid proteins that may help explain a potential molecular talk between different neurodegenerative diseases.

Self-Assembled Nanochaperones Inhibit the Aggregation of Human Islet Amyloid Polypeptide Associated with Type 2 Diabetes

Human islet amyloid polypeptide (hIAPP) aggregation is closely associated with dysfunction and apoptosis of pancreatic β-cells in type 2 diabetes (T2D). Accordingly, hIAPP amyloid inhibitors have shown promise against T2D. Here, by mimicking the function of natural molecular chaperones, nanochaperones (nChaps) based on self-assembled polymeric micelles with tunable surface microdomains for T2D treatment are reported. By capturing the aggregation-prone species of hIAPP onto the hydrophobic microdomains and segregating them by hydrophilic PEG chains, this kind of nChaps could effectively prevent hIAPP aggregation, block cell adhesion of hIAPP, facilitate hIAPP aggregates degradation and reduce hIAPP-related cytotoxicity. Therefore, our work will provide useful insights to develop a biomimetic strategy for the treatment of T2D.

Repurposing a Cardiovascular Disease Drug of Cloridarol as hIAPP Inhibitor

Accumulating evidence have shown a strong pathological correlation between cardiovascular disease (CVD) and Type II diabetes (T2D), both of which share many common risk factors (e.g., hyperglycemia, hypertension, hypercoagulability, and dyslipidemia) and mutually contribute to each other. Driven by such strong CVD-T2D correlation and marginal benefits from drug development for T2D, here we proposed to repurpose a CVD drug of cloridarol as human islet amyloid peptide (hIAPP) inhibitor against its abnormal misfolding and aggregation, which is considered as a common and critical pathological event in T2D. To this end, we investigated the inhibition activity of cloridarol on the aggregation and toxicity of hIAPP_1-37 using combined experimental and computational approaches. Collective experimental data from ThT, AFM, and CD demonstrated the inhibition ability of cloridarol to prevent hIAPP aggregation from its monomeric and oligomeric states, leading to the overall reduction of hIAPP fibrils up to 57% at optimal conditions. MTT and LDH cell assays also showed that cloridarol can also effectively increase cell viability by 15% and decrease cell apoptosis by 28%, confirming its protection of islet β-cells from hIAPP-induced cell toxicity. Furthermore, comparative molecular dynamics simulations revealed that cloridarol was preferentially bound to the C-terminal β-sheet region of hIAPP oligomers through a combination of hydrophobic interactions, π-π stacking, and hydrogen bonding. Such multiple site bindings allowed cloridarol to disturb hIAPP structures, reduce β-sheet content, and block the lateral association pathway of hIAPP aggregates, thus explaining experimental findings. Different from other single-target hIAPP inhibitors, cloridarol is unique in that it works as both a CVD drug and hIAPP inhibitor, which can be used as a viable structural template (especially for benzofuran) for the further development of cloridarol-based or benzofuran-based inhibitors of amyloid proteins.

Potential of ATP toward Prevention of hIAPP Oligomerization and Destabilization of hIAPP Protofibrils: An In Silico Perspective

The aggregation of an intrinsically disordered protein, human islet amyloid polypeptide (hIAPP), leads to one of the most prevalent endocrine disorders, type II diabetes mellitus (T2DM). Hence inhibition of hIAPP aggregation provides a possible therapeutic approach for the treatment of T2DM. In this regard, a new aspect of adenosine triphosphate (ATP), which is widely known as the energy source for biological reactions, has recently been discovered, where it can inhibit the formation of protein aggregates and simultaneously dissolve preformed aggregates at a millimolar concentration scale. In this work, we investigate the effect of ATP on the aggregation of an amyloidogenic segment of hIAPP, hIAPP_22-28, and also of the full length sequence. Using all-atom classical molecular dynamics simulations, we observe that the tendency of hIAPP to oligomerize into β-sheet conformers is inhibited by ATP, due to which the peptides remain distant, loosely packed random monomers. Moreover, it can also disassemble preformed hIAPP protofibrils. ATP preferentially interacts with the hydrophobic residues of hIAPP_22-28 fragment and the terminal and turn residues of the full length peptide. The hydrogen bonding, hydrophobic, π-π, and N-H-π stacking interactions are the driving forces for the ATP induced inhibition of hIAPP aggregation. Interestingly, the hydrophobic adenosine of ATP is found to be more in contact with the peptide residues than the hydrophilic triphosphate moiety. The insight into the inhibitory mechanism of ATP on hIAPP aggregation can prove to be beneficial for the design of novel amyloid inhibitors in the future.

Site-specific single point mutation by anthranilic acid in hIAPP8-37 enhances anti-amyloidogenic activity

Amylin or hIAPP, together with insulin, plays a significant role in glucose metabolism. However, it undergoes β-sheet rich amyloid formation associated with pancreatic β-cell dysfunction leading to type-2 diabetes (T2D). Recent studies suggest that restricting β-sheet formation in it may halt amyloid formation, which may limit the risk for the disease. Several peptide-based inhibitors have been reported to prevent aggregation. However, most of them have limitations, including low binding efficiency, active only at higher doses, poor solubility, and proteolytic degradation. Insertion of non-coded amino acids renders proteolytically stable peptides. We incorporated a structurally rigid β-amino acid, Anthranilic acid (Ant), at different sites within the central hydrophobic region of hIAPP and developed two singly mutated hIAPP_8–37 peptidomimetics. These peptidomimetics inhibited the amyloid formation of hIAPP substantially even at low concentration, as evident from in vitro ThT, CD, FT-IR, TEM, and Congo red staining birefringence results. These peptidomimetics also disrupted the preformed aggregates formed by hIAPP into non-toxic species. These β-amino acid-based peptidomimetics can be an attractive scaffold for therapeutic design towards T2D or other protein misfolding diseases.

β-Amino acid based peptidomimetics are attractive scaffolds for therapeutics design towards T2D. They prevent amyloid formation of hIAPP by forming non-fibrillar non-toxic aggregates.

Targeting Human Islet Amyloid Polypeptide Aggregation and Toxicity in Type 2 Diabetes: An Overview of Peptide-Based Inhibitors

Type 2 diabetes (T2D) is a chronic metabolic disease characterized by insulin resistance and a progressive loss of pancreatic islet β-cell mass, which leads to insufficient secretion of insulin and hyperglycemia. Emerging evidence suggests that toxic oligomers and fibrils of human islet amyloid polypeptide (hIAPP) contribute to the death of β-cells and lead to T2D pathogenesis. These observations have opened new avenues for the development of islet amyloid therapies for the treatment of T2D. The peptide-based inhibitors are of great value as therapeutic agents against hIAPP aggregation in T2D owing to their biocompatibility, feasibility of synthesis and modification, high specificity, low toxicity, proteolytic stability (modified peptides), and weak immunogenicity as well as the large size of involved interfaces during self-aggregation of hIAPP. An understanding of what has been done and achieved will provide key insights into T2D pathology and assist in the discovery of more potent drug candidates for the treatment of T2D. In this article, we review various peptide-based inhibitors of hIAPP aggregation, including those derived from the hIAPP sequence and those not based on the sequence, consisting of both natural as well as unnatural amino acids and their derivatives. The present review will be beneficial in advancing the field of peptide medicine for the treatment of T2D.

Sequence and structure-based peptides as potent amyloid inhibitors: A review

Misfolded and natively disordered globular proteins tend to aggregate together in an interwoven fashion to form fibrous, proteinaceous deposits referred to as amyloid fibrils. Formation and deposition of such insoluble fibrils are the characteristic features of a broad group of diseases, known as amyloidosis. Some of these proteins are known to cause several degenerative disorders in humans, such as Amyloid-Beta (Aβ) in Alzheimer's disease (AD), human Islet Amyloid Polypeptide (hIAPP, amylin) in type 2 diabetes, α-synuclein (α-syn) in Parkinson's disease (PD) and so on. The fact that these proteins do not share any significant sequence or structural homology in their native states make therapy quite challenging. However, it is observed that aggregation-prone proteins and peptides tend to adopt a similar type of secondary structure during the formation of fibrils. Rationally designed peptides can be a potent inhibitor that has been shown to disrupt the fibril structure by binding specifically to the amyloidogenic region(s) within a protein. The following review will analyze the inhibitory potency of both sequence-based and structure-based small peptides that have been shown to inhibit amyloidogenesis of proteins such as Aβ, human amylin, and α-synuclein.

Charge effects at nano-bio interfaces: a model of charged gold nanoclusters on amylin fibrillation

The misfolding and abnormal amyloid fibrillation of proteins/peptides are associated with more than 20 human diseases. Although dozens of nanoparticles have been investigated for the inhibition effect on the misfolding and fibrillation of pathogenesis-related proteins/peptides, there are few reports on charge effects of nano inhibitors on amyloid fibrillation. Herein, same-sized gold nanoclusters modified with 2-aminoethanethiol hydrochloride (CSH-AuNCs, positively charged in pH 7.4) or 3-mercaptopropionic acid (MPA-AuNCs, negatively charged in pH 7.4) were synthesized and adopted as models to explore the charge effect of nano inhibitors on amylin fibrillation at the nano-bio interface. ThT fluorescence kinetics analysis, AFM images and circular dichroism (CD) spectra showed that electropositive CSH-AuNCs inhibited the misfolding and fibrillation of amylin in a dosage-dependent manner, but electronegative MPA-AuNCs accelerated the misfolding and fibrillation of amylin in a dosage-dependent manner. Moreover, the theoretical and experimental results revealed the interaction mechanism between amylin and ligands of AuNCs at the nano-bio interfaces. Electropositive CSH-AuNCs could be bound to the main nucleating region of amylin via hydrogen bonding and endowed the nanocomplex with more positive net charges (amylin monomer with a positive +26.23 ± 0.80 mV zeta potential), which would inhibit the misfolding and aggregation of amylin via electrostatic repulsion and steric hindrance. In contrast, electronegative MPA-AuNCs could absorb electropositive amylin via strong electrostatic attractions, which accelerated the fibrillation process of amylin via enhancing local concentrations. Moreover, cell experiments showed that both the charged AuNCs had good biocompatibility and electronegetive MPA-AuNCs showed a better protective effect in the amylin-induced cell model than electropositive CSH-AuNCs. These results provide an insight into structure-based nanodrug design for protein conformational diseases.

Studies on the effect of a Fupenzi glycoprotein on the fibrillation of bovine serum albumin and its antioxidant activity

In this study, the effect of a glycoprotein obtained from Fupenzi (FPZ) (Rubus chingii Hu.) on the fibrillation of bovine serum album (BSA) was investigated by multi-spectroscopic methods and transmission electron microscopy. Moreover, the cytotoxicity of the glycoprotein and the effect of it on H₂O₂-induced cell viability were investigated by cell counting kit and β-galactosidase kit, respectively. The experimental results indicated that the glycoprotein showed very low toxicity to NRK-52E cells and could obviously delay cell senescence and improve cell viability. Moreover, the glycoprotein could effectively inhibit the formation of BSA fibrils and destroy the stability of preformed BSA fibrils in a concentration-dependent manner. Generally, antioxidant capacities are thought to be related to the anti-amyloidogenic activity of inhibitors; therefore, to reveal the inhibitory mechanism, the anti-oxidative property of the glycoprotein was examined by DPPH and ABTS assays. The results demonstrated that FPZ glycoprotein had a remarkable antioxidant activity and the IC₅₀ values of DPPH and ABTS were 0.249 mg mL^-1 and 0.092 mg mL^-1, respectively. This work suggested that the FPZ glycoprotein had the potential to be designed a new therapeutic agent for attenuating aging and preventing the age-related diseases.

γ-AApeptides-based Small Molecule Ligands That Disaggregate Human Islet Amyloid Polypeptide

The abnormal folding and aggregation of functional proteins into amyloid is a typical feature of many age-related diseases, including Type II diabetes. Growing evidence has revealed that the prevention of aggregate formation in culprit proteins could retard the progression of amyloid diseases. Human Amylin, also known as human islet amyloid polypeptide (hIAPP), is the major factor for categorizing Type II diabetes as an amyloid disease. Specifically, hIAPP has a great aggregation potential, which always results in a lethal situation for the pancreas. Many peptide inhibitors have been constructed from the various segments of the full-length hIAPP peptide; however, only a few have their origin from the screening of combinatorial peptidomimetic library. In this study, based on HW-155, which was previously discovered from a one-bead-one compound (OBOC) library to inhibit Aβ40 aggregation, we investigated eight (8) analogues and evaluated their amyloid-prevention capabilities for inhibiting fibrillization of hIAPP. Characterization studies revealed that all analogues of HW-155, as well as HW-155, were effective inhibitors of the fibril formation by hIAPP.

Impact of Dehydroamino Acids on the Structure and Stability of Incipient 310-Helical Peptides

A comparative study of the impact of small, medium-sized, and bulky α,β-dehydroamino acids (ΔAAs) on the structure and stability of Balaram's incipient 3₁₀-helical peptide (1) is reported. Replacement of the N-terminal Aib residue of 1 with a ΔAA afforded peptides 2a-c that maintained the 3₁₀-helical shape of 1. In contrast, installation of a ΔAA in place of Aib-3 yielded peptides 3a-c that preferred a β-sheet-like conformation. The impact of the ΔAA on peptide structure was independent of size, with small (ΔAla), medium-sized (Z-ΔAbu), and bulky (ΔVal) ΔAAs exerting similar effects. The proteolytic stabilities of 1 and its analogs were determined by incubation with Pronase. Z-ΔAbu and ΔVal increased the resistance of peptides to proteolysis when incorporated at the 3-position and had negligible impact on stability when placed at the 1-position, whereas ΔAla-containing peptides degraded rapidly regardless of position. Exposure of peptides 2a-c and 3a-c to the reactive thiol cysteamine revealed that ΔAla-containing peptides underwent conjugate addition at room temperature, while Z-ΔAbu- and ΔVal-containing peptides were inert even at elevated temperatures. These results suggest that both bulky and more accessible medium-sized ΔAAs should be valuable tools for bestowing rigidity and proteolytic stability on bioactive peptides.

Molecular Dynamics Simulations Reveal the Inhibitory Mechanism of Dopamine against Human Islet Amyloid Polypeptide (hIAPP) Aggregation and Its Destabilization Effect on hIAPP Protofibrils

The aberrant self-assembly of human islet amyloid polypeptide (hIAPP) into toxic oligomers, protofibrils, and mature fibrils is associated with the pathogenesis of type 2 diabetes (T2D). Inhibition of hIAPP aggregation and destabilization of preformed hIAPP fibrils are considered as two major therapeutic strategies for treating T2D. Previous experimental studies reported that dopamine prevented the formation of hIAPP oligomers and fibrils. However, the underlying inhibitory mechanism at the atomic level remains elusive. Herein we investigated the conformational ensembles of hIAPP dimer with and without dopamine using replica-exchange molecular dynamics simulations. The simulations demonstrated that dopamine preferentially bound to R11, L12, F15, H18, F23, I26, L27, and Y37 residues, inhibited the formation of β-sheets in the amyloidogenic regions spanning residues ¹¹RLANFLVH¹⁸, ²²NFGAIL²⁷, and ³⁰TNVGSNT³⁶, and resulted in more disordered hIAPP dimers, thus hindering the amyloid formation of hIAPP. Protonated and deprotonated dopamine molecules displayed distinct binding capabilities but bound to similar residue sites on hIAPP. Additional microsecond molecular dynamics simulations showed that dopamine mainly bound to the β1 and turn regions of hIAPP protofibril and destabilized the protofibril structure. This study not only revealed the molecular mechanism of dopamine toward the inhibition of hIAPP aggregation but also demonstrated the protofibril-destabilizing effects of dopamine, which may be helpful for the design of drug candidates to treat T2D.

Azadirachtin inhibits amyloid formation, disaggregates pre-formed fibrils and protects pancreatic β-cells from human islet amyloid polypeptide/amylin-induced cytotoxicity

The human islet amyloid polypeptide (hIAPP) or amylin is the major constituent of amyloidogenic aggregates found in pancreatic islets of type 2 diabetic patients that have been associated with β-cell dysfunction and/or death associated with type 2 diabetes mellitus (T2DM). Therefore, developing and/or identifying inhibitors of hIAPP aggregation pathway and/or compound that can mediate disaggregation of preformed aggregates holds promise as a medical intervention for T2DM management. In the current study, the anti-amyloidogenic potential of Azadirachtin (AZD)—a secondary metabolite isolated from traditional medicinal plant Neem (Azadirachta indica)—was investigated by using a combination of biophysical and cellular assays. Our results indicate that AZD supplementation not only inhibits hIAPP aggregation but also disaggregates pre-existing hIAPP fibrils by forming amorphous aggregates that are non-toxic to pancreatic β-cells. Furthermore, AZD supplementation in pancreatic β-cells (INS-1E) resulted in inhibition of oxidative stress; along with restoration of the DNA damage, lipid peroxidation and the associated membrane damage, endoplasmic reticulum stress and mitochondrial membrane potential. AZD treatment also restored glucose-stimulated insulin secretion from pancreatic islets exposed to hIAPP. All-atom molecular dynamics simulation studies on full-length hIAPP pentamer with AZD suggested that AZD interacted with four possible binding sites in the amyloidogenic region of hIAPP. In summary, our results suggest AZD to be a promising candidate for combating T2DM and related amyloidogenic disorders.

A dual-inhibitor system for the effective antifibrillation of Aβ40 peptides by biodegradable EGCG–Fe(iii)/PVP nanoparticles

By the synergistic effect of dual inhibition, EFPP NPs exhibited a significant effect on the inhibition of Aβ40 fibrillation and on the disaggregation of existing Aβ40 fibrils.

Role and Cytotoxicity of Amylin and Protection of Pancreatic Islet β-Cells from Amylin Cytotoxicity

Amylin, (or islet amyloid polypeptide; IAPP), a 37-amino acid peptide hormone, is released in response to nutrients, including glucose, lipids or amino acids. Amylin is co-stored and co-secreted with insulin by pancreatic islet β-cells. Amylin inhibits food intake, delays gastric emptying, and decreases blood glucose levels, leading to the reduction of body weight. Therefore, amylin as well as insulin play important roles in controlling the level of blood glucose. However, human amylin aggregates and human amylin oligomers cause membrane disruption, endoplasmic reticulum (ER) stress and mitochondrial damage. Since cytotoxicity of human amylin oligomers to pancreatic islet β-cells can lead to diabetes, the protection of pancreatic islet β cells from cytotoxic amylin is crucial. Human amylin oligomers also inhibit autophagy, although autophagy can function to remove amylin aggregates and damaged organelles. Small molecules, including β-sheet breaker peptides, chemical chaperones, and foldamers, inhibit and disaggregate amyloid formed by human amylin, suggesting the possible use of these small molecules in the treatment of diabetes. In this review, we summarize recent findings regarding the role and cytotoxicity of amylin and the protection of pancreatic islet β-cells from cytotoxicity of amylin.

Diabetes Drug Discovery: hIAPP1-37 Polymorphic Amyloid Structures as Novel Therapeutic Targets

Human islet amyloid peptide (hIAPP_1–37) aggregation is an early step in Diabetes Mellitus. We aimed to evaluate a family of pharmaco-chaperones to act as modulators that provide dynamic interventions and the multi-target capacity (native state, cytotoxic oligomers, protofilaments and fibrils of hIAPP_1–37) required to meet the treatment challenges of diabetes. We used a cross-functional approach that combines in silico and in vitro biochemical and biophysical methods to study the hIAPP_1–37 aggregation-oligomerization process as to reveal novel potential anti-diabetic drugs. The family of pharmaco-chaperones are modulators of the oligomerization and fibre formation of hIAPP_1–37. When they interact with the amino acid in the amyloid-like steric zipper zone, they inhibit and/or delay the aggregation-oligomerization pathway by binding and stabilizing several amyloid structures of hIAPP_1–37. Moreover, they can protect cerebellar granule cells (CGC) from the cytotoxicity produced by the hIAPP_1–37 oligomers. The modulation of proteostasis by the family of pharmaco-chaperones A–F is a promising potential approach to limit the onset and progression of diabetes and its comorbidities.

Recombinant human islet amyloid polypeptide forms shorter fibrils and mediates β-cell apoptosis via generation of oxidative stress

Protein misfolding and aggregation play an important role in many human diseases including Alzheimer's, Parkinson's and type 2 diabetes mellitus (T2DM). The human islet amyloid polypeptide (hIAPP) forms amyloid plaques in the pancreas of T2DM subjects (>95%) that are involved in deteriorating islet function and in mediating β-cell apoptosis. However, the detailed mechanism of action, structure and nature of toxic hIAPP species responsible for this effect remains elusive to date mainly due to the high cost associated with the chemical synthesis of pure peptide required for these studies. In the present work, we attempted to obtain structural and mechanistic insights into the hIAPP aggregation process using recombinant hIAPP (rhIAPP) isolated from Escherichia coli. Results from biophysical and structural studies indicate that the rhIAPP self-assembled into highly pure, β-sheet-rich amyloid fibrils with uniform morphology. rhIAPP-mediated apoptosis in INS-1E cells was associated with increased oxidative stress and changes in mitochondrial membrane potential. The transcript levels of apoptotic genes - Caspase-3 and Bax were found to be up-regulated, while the levels of the anti-apoptotic gene - Bcl2 were down-regulated in rhIAPP-treated cells. Additionally, the expression levels of genes involved in combating oxidative stress namely Catalase, SOD1 and GPx were down-regulated. rhIAPP exposure also affected glucose-stimulated insulin secretion from isolated pancreatic islets. The aggregation of rhIAPP also occurred significantly faster when compared with that of the chemically synthesized peptide. We also show that the rhIAPP fibrils were shorter and more cytotoxic. In summary, our study is one among the few to provide comprehensive evaluation of structural, biophysical and cytotoxic properties of rhIAPP.

Inhibition of amyloid oligomerization into different supramolecular architectures by small molecules: mechanistic insights and design rules

Protein misfolding and aggregation have been associated with several human disorders, including Alzheimer’s, Parkinson’s and Huntington’s diseases, as well as senile systemic amyloidosis and Type II diabetes. However, there is no current disease-modifying therapy available for the treatment of these disorders. In spite of extensive academic, pharmaceutical, medicinal and clinical research, a complete mechanistic model for this family of diseases is still lacking. In this review, we primarily discuss the different types of small molecular entities which have been used for the inhibition of the aggregation process of different amyloidogenic proteins under diseased conditions. These include small peptides, polyphenols, inositols, quinones and their derivatives, and metal chelator molecules. In recent years, these groups of molecules have been extensively studied using in vitro, in vivo and computational models to understand their mechanism of action and common structural features underlying the process of inhibition. A salient feature found to be instrumental in the process of inhibition is the balance between the aromatic unit that functions as the amyloid recognition unit and the hydrophilic amyloid breaker unit. The establishment of structure–function relationship for amyloid-modifying therapies by the various functional entities should serve as an important step toward the development of efficient therapeutics.

Inhibitory Mechanism of Epigallocatechin Gallate on Fibrillation and Aggregation of Amidated Human Islet Amyloid Polypeptide

The abnormal fibrillation of human islet amyloid polypeptide (hIAPP) is associated with development of type II diabetes mellitus (T2DM). (-)-Epigallocatechin gallate (EGCG) can bind amyloid proteins to inhibit the fibrillation of these proteins. However, the mechanic detail of EGCG inhibiting amyloid formation is still unclear at the molecular level. In the present work, we sought to investigate the effect of EGCG on amidated hIAPP (hIAPP-NH₂ ) fibrillation and aggregation by using spectroscopic and microscopic techniques, and also sought to gain insights into the interaction of EGCG and hIAPP_22-27 by using spectroscopic experiments and quantum chemical calculations. ThT fluorescence, real-time NMR, and TEM studies demonstrated that EGCG inhibits the formation of hIAPP-NH₂ fibrils, while promoting the formation of hIAPP-NH₂ amorphous aggregates. Phenylalanine intrinsic fluorescence and NMR studies of the EGCG/hIAPP_22-27 complex revealed three important binding sites including the A ring of EGCG, residue Phe23, and residue Ile26. DFT calculations identified the dominant binding structures of EGCG/Phe23 and EGCG/Ile26 complexes, named structure I and structure II, respectively. Our study demonstrates the inhibitory mechanism of EGCG on fibrillation and aggregation of hIAPP-NH₂ in which EGCG interacts with hIAPP-NH₂ through hydrogen bonding and π-π interactions between the A ring and residue Phe23 as well as hydrophobic interactions between the A ring and residue Ile26, which can thus inhibit the interpeptide interaction between hIAPP-NH₂ monomers and finally inhibit fibrillation of hIAPP-NH₂ . This study agrees with and reinforces previous studies and offers an intuitive explanation at both the atomic and molecular levels. Our findings may provide an invaluable reference for the future development of new drugs in the management of diabetes.

Disaggregation of Amylin Aggregate by Novel Conformationally Restricted Aminobenzoic Acid containing α/β and α/γ Hybrid Peptidomimetics

Diabetes has emerged as a threat to the current world. More than ninety five per cent of all the diabetic population has type 2 diabetes mellitus (T2DM). Aggregates of Amylin hormone, which is co-secreted with insulin from the pancreatic β-cells, inhibit the activities of insulin and glucagon and cause T2DM. Importance of the conformationally restricted peptides for drug design against T2DM has been invigorated by recent FDA approval of Symlin, which is a large conformationally restricted peptide. However, Symlin still has some issues including solubility, oral bioavailability and cost of preparation. Herein, we introduced a novel strategy for conformationally restricted peptide design adopting a minimalistic approach for cost reduction. We have demonstrated efficient inhibition of amyloid formation of Amylin and its disruption by a novel class of conformationally restricted β-sheet breaker hybrid peptidomimetics (BSBHps). We have inserted β, γ and δ -aminobenzoic acid separately into an amyloidogenic peptide sequence, synthesized α/β, α/γ and α/δ hybrid peptidomimetics, respectively. Interestingly, we observed the aggregation inhibitory efficacy of α/β and α/γ BSBHps, but not of α/δ analogues. They also disrupt existing amyloids into non-toxic forms. Results may be useful for newer drug design against T2DM as well as other amyloidoses and understanding amyloidogenesis.

Mechanism of Inhibition of Human Islet Amyloid Polypeptide-Induced Membrane Damage by a Small Organic Fluorogen

Human islet amyloid polypeptide (hIAPP) is believed to be responsible for the death of insulin-producing β-cells. However, the mechanism of membrane damage at the molecular level has not been fully elucidated. In this article, we employ coarse- grained dissipative particle dynamics simulations to study the interactions between a lipid bilayer membrane composed of 70% zwitterionic lipids and 30% anionic lipids and hIAPPs with α-helical structures. We demonstrated that the key factor controlling pore formation is the combination of peptide charge-induced electroporation and peptide hydrophobicity-induced lipid disordering and membrane thinning. According to these mechanisms, we suggest that a water-miscible tetraphenylethene BSPOTPE is a potent inhibitor to rescue hIAPP-induced cytotoxicity. Our simulations predict that BSPOTPE molecules can bind directly to the helical regions of hIAPP and form oligomers with separated hydrophobic cores and hydrophilic shells. The micelle-like hIAPP-BSPOTPE clusters tend to be retained in the water/membrane interface and aggregate therein rather than penetrate into the membrane. Electrostatic attraction between BSPOTPE and hIAPP also reduces the extent of hIAPP binding to the anionic lipid bilayer. These two modes work together and efficiently prevent membrane poration.

Influence of Aluminium and EGCG on Fibrillation and Aggregation of Human Islet Amyloid Polypeptide

The abnormal fibrillation of human islet amyloid polypeptide (hIAPP) has been implicated in the development of type II diabetes. Aluminum is known to trigger the structural transformation of many amyloid proteins and induce the formation of toxic aggregate species. The (-)-epigallocatechin gallate (EGCG) is considered capable of binding both metal ions and amyloid proteins with inhibitory effect on the fibrillation of amyloid proteins. However, the effect of Al(III)/EGCG complex on hIAPP fibrillation is unclear. In the present work, we sought to view insight into the structures and properties of Al(III) and EGCG complex by using spectroscopic experiments and quantum chemical calculations and also investigated the influence of Al(III) and EGCG on hIAPP fibrillation and aggregation as well as their combined interference on this process. Our studies demonstrated that Al(III) could promote fibrillation and aggregation of hIAPP, while EGCG could inhibit the fibrillation of hIAPP and lead to the formation of hIAPP amorphous aggregates instead of the ordered fibrils. Furthermore, we proved that the Al(III)/EGCG complex in molar ratio of 1 : 1 as Al(EGCG)(H₂O)₂ could inhibit the hIAPP fibrillation more effectively than EGCG alone. The results provide the invaluable reference for the new drug development to treat type II diabetes.

Self-assembly of a nine-residue amyloid-forming peptide fragment of SARS corona virus E-protein: mechanism of self aggregation and amyloid-inhibition of hIAPP

Molecular self-assembly, a phenomenon widely observed in nature, has been exploited through organic molecules, proteins, DNA, and peptides to study complex biological systems. These self-assembly systems may also be used in understanding the molecular and structural biology which can inspire the design and synthesis of increasingly complex biomaterials. Specifically, use of these building blocks to investigate protein folding and misfolding has been of particular value since it can provide tremendous insights into peptide aggregation related to a variety of protein misfolding diseases, or amyloid diseases (e.g., Alzheimer's disease, Parkinson's disease, type-II diabetes). Herein, the self-assembly of TK9, a nine-residue peptide of the extra membrane C-terminal tail of the SARS corona virus envelope, and its variants were characterized through biophysical, spectroscopic, and simulated studies, and it was confirmed that the structure of these peptides influences their aggregation propensity, hence, mimicking amyloid proteins. TK9, which forms a beta-sheet rich fibril, contains a key sequence motif that may be critical for beta-sheet formation, thus making it an interesting system to study amyloid fibrillation. TK9 aggregates were further examined through simulations to evaluate the possible intra- and interpeptide interactions at the molecular level. These self-assembly peptides can also serve as amyloid inhibitors through hydrophobic and electrophilic recognition interactions. Our results show that TK9 inhibits the fibrillation of hIAPP, a 37 amino acid peptide implicated in the pathology of type-II diabetes. Thus, biophysical and NMR experimental results have revealed a molecular level understanding of peptide folding events, as well as the inhibition of amyloid-protein aggregation are reported.

Effect of sodium salicylate and sodium deoxycholate on fibrillation of bovine serum albumin: comparison of fluorescence, SANS and DLS techniques

Adsorbed sodium salicylate (NaSal) and sodium deoxycholate (NaDC) retard the thermal denaturation of bovine serum albumin.

Inhibition of Alzheimer's amyloid-β peptide aggregation and its disruption by a conformationally restricted α/β hybrid peptide

A novel class of anthranilic acid containing a conformationally restricted β-sheet breaker α/β-hybrid peptide efficiently disrupts preformed fibrillar aggregates of Aβ_1–40in vitro.

Rational design of an orthosteric regulator of hIAPP aggregation

Compounds that can block hIAPP toxic oligomer but not fibril formation have been rationally designed based on the helix aggregation mechanism.