Low pH Acts as Inhibitor of Membrane Damage Induced by Human Islet Amyloid Polypeptide

The Effect of Calcium Ions on hIAPP Channel Activity: Possible Implications in T2DM

The calcium ion (Ca2+) has been linked to type 2 diabetes mellitus (T2DM), although the role of Ca2+ in this disorder is the subject of intense investigation. Serum Ca2+ dyshomeostasis is associated with the development of insulin resistance, reduced insulin sensitivity, and impaired glucose tolerance. However, the molecular mechanisms involving Ca2+ ions in pancreatic β-cell loss and subsequently in T2DM remain poorly understood. Implicated in the decline in β-cell functions are aggregates of human islet amyloid polypeptide (hIAPP), a small peptide secreted by β-cells that shows a strong tendency to self-aggregate into β-sheet-rich aggregates that evolve toward the formation of amyloid deposits and mature fibrils. The soluble oligomers of hIAPP can permeabilize the cell membrane by interacting with bilayer lipids. Our study aimed to evaluate the effect of Ca2+ on the ability of the peptide to incorporate and form ion channels in zwitterionic planar lipid membranes (PLMs) composed of palmitoyl-oleoyl-phosphatidylcholine (POPC) and on the aggregation process of hIAPP molecules in solution. Our results may help to clarify the link between Ca2+ ions, hIAPP peptide, and consequently the pathophysiology of T2DM.

Efficient optical plasmonic tweezer-controlled single-molecule SERS characterization of pH-dependent amylin species in aqueous milieus

It is challenging to characterize single or a few biomolecules in physiological milieus without excluding the influences of surrounding environment. Here we utilize optical plasmonic trapping to construct a dynamic nanocavity, which reduces the diffraction-limited detection volume and provides reproducible electromagnetic field enhancements to achieve high-throughput single-molecule surface-enhanced Raman spectroscopy (SERS) characterizations in aqueous environments. Specifically, we study human Islet Amyloid Polypeptide (amylin, hIAPP) under different physiological pH conditions by combining spectroscopic experiments and molecular dynamics (MD) simulations. Based on a statistically significant amount of time-dependent SERS spectra, two types of low-populated transient species of hIAPP containing either turn or β-sheet structure among its predominant helix-coil monomers are characterized during the early-stage incubation at neutral condition, which play a crucial role in driving irreversible amyloid fibril developments even after a subsequent adjustment of pH to continue the prolonged incubation at acidic condition. Our results might provide profound mechanistic insight into the pH-regulated amyloidogenesis and introduce an alternative approach for investigating complex biological processes at the single-molecule level.

On-Pathway Oligomer of Human Islet Amyloid Polypeptide Induced and Stabilized by Mechanical Rotation during Magic Angle Spinning Nuclear Magnetic Resonance

Intermediates along the fibrillation pathway are generally considered to be the toxic species responsible for the pathologies of amyloid diseases. However, structural studies of these species have been hampered by heterogeneity and poor stability under standard aqueous conditions. Here, we report a novel methodology for producing stable, on-pathway oligomers of the human type-2 diabetes-associated islet amyloid polypeptide (hIAPP or amylin) using the mechanical forces associated with magic angle spinning (MAS). The species were a heterogeneous mixture of globular and short rod-like species with significant β-sheet content and the capability of seeding hIAPP fibrillation. We used MAS nuclear magnetic resonance to demonstrate that the nature of the species was sensitive to sample conditions, including peptide concentration, ionic strength, and buffer. The methodology should be suitable for studies of other aggregating systems.

Modulatory role of copper on hIAPP aggregation and toxicity in presence of insulin

Aggregation of the human islets amyloid polypeptide, or hIAPP, is linked to β-cell death in type II diabetes mellitus (T2DM). Different pancreatic β-cell environmental variables such as pH, insulin and metal ions play a key role in controlling the hIAPP aggregation. Since insulin and hIAPP are co-secreted, it is known from numerous studies that insulin suppresses hIAPP fibrillation by preventing the initial dimerization process. On the other hand, zinc and copper each have an inhibitory impact on hIAPP fibrillation, but copper promotes the production of toxic oligomers. Interestingly, the insulin oligomeric equilibrium is controlled by the concentration of zinc ions when the effect of insulin and zinc has been tested together. Lower zinc concentrations cause the equilibrium to shift towards the monomer and dimer states of insulin, which bind to monomeric hIAPP and stop it from developing into a fibril. On the other hand, the combined effects of copper and insulin have not yet been done. In this study, we have demonstrated how the presence of copper affects hIAPP aggregation and the toxicity of the resultant conformers with or without insulin. For this purpose, we have used a set of biophysical techniques, including NMR, fluorescence, CD etc., in combination with AFM and cell cytotoxicity assay. In the presence and/or absence of insulin, copper induces hIAPP to form structurally distinct stable toxic oligomers, deterring the fibrillation process. More specifically, the oligomers generated in the presence of insulin have slightly higher toxicity than those formed in the absence of insulin. This research will increase our understanding of the combined modulatory effect of two β-cell environmental factors on hIAPP aggregation.

Fibril elongation by human islet amyloid polypeptide is the main event linking aggregation to membrane damage

The aggregation of human islet amyloid polypeptide (hIAPP) is linked to the death of pancreatic β-cells in type II diabetes. The process of fibril formation by hIAPP is thought to cause membrane damage, but the precise mechanisms are still unclear. Previously, we showed that the aggregation of hIAPP in the presence of membranes containing anionic lipids is dominated by secondary nucleation events, which occur at the interface between existing fibrils and the membrane surface. Here, we used vesicles with different lipid composition to explore the connection between hIAPP aggregation and vesicle leakage. We found that different anionic lipids promote hIAPP aggregation to the same extent, whereas remarkably stochastic behaviour is observed on purely zwitterionic membranes. Vesicle leakage induced by hIAPP consists of two distinct phases for any of the used membrane compositions: (i) an initial phase in which hIAPP binding causes a certain level of leakage that is strongly dependent on osmotic conditions, membrane composition and the used dye, and (ii) a main leakage event that we attribute to elongation of hIAPP fibrils, based on seeded experiments. Altogether, our results shed more light on the relationship between hIAPP fibril formation and membrane damage, and strongly suggest that oligomeric intermediates do not considerably contribute to vesicle leakage.

Zn(II) binding to pramlintide results in a structural kink, fibril formation and antifungal activity

The antimicrobial properties of amylin, a 37-amino acid peptide hormone, co-secreted with insulin from the pancreas, are far less known than its antidiabetic function. We provide insight into the bioinorganic chemistry of amylin analogues, showing that the coordination of zinc(II) enhances the antifungal properties of pramlintide, a non-fibrillating therapeutic analogue of amylin. Zinc binds to the N-terminal amino group and His18 imidazole, inducing a kink in the peptide structure, which, in turn, triggers a fibrillization process of the complex, resulting in an amyloid structure most likely responsible for the disruption of the fungal cell.

Analysis of Sheep and Goat IAPP Provides Insight into IAPP Amyloidogenicity and Cytotoxicity

Human islet amyloid polypeptide (hIAPP) plays a role in glucose regulation but forms pancreatic amyloid deposits in type 2 diabetes, and that process contributes to β-cell dysfunction. Not all species develop diabetes, and not all secrete an IAPP that is amyloidogenic in vitro under normal conditions, a perfect correlation currently exists between both. Studies of IAPPs from such organisms can provide clues about the high amyloidogenicity of hIAPP and can inform the design of soluble analogues of hIAPP. Sheep and goat IAPP are among the most divergent from hIAPP, with 13 and 11 substitutions, respectively, including an unusual Tyr to His substitution at the C-terminus. The properties of sheep and goat IAPP were examined in solution and in the presence of anionic vesicles, resulting in no observed amyloid formation, even at increased concentrations. Furthermore, both peptides are considerably less toxic to cultured β-cells than hIAPP. The effect of the Y37H replacements was studied in the context of hIAPP, as was a Y37R substitution. Buffer- and salt-dependent effects were observed. There was little impact on the time to form amyloid in phosphate-buffered saline; however, a significant deceleration was observed in Tris buffer, and amyloid formation was slower in the absence of added salt. The Y37H substitution had little impact on toxicity, while the Y37R replacement led to a 30% decrease in toxicity compared with that of hIAPP. The implications for the amyloidogenicity of hIAPP and the design of soluble analogues of the human peptide are discussed.

Contribution of the 12–17 hydrophobic region of islet amyloid polypeptide in self-assembly and cytotoxicity

The islet amyloid polypeptide (IAPP) is a 37-residue aggregation-prone peptide hormone whose deposition as insoluble fibrils in the islets of Langerhans is associated with type II diabetes. Therapeutic interventions targeting IAPP amyloidogenesis, which contributes to pancreatic β-cell degeneration, remain elusive owing to the lack of understanding of the self-assembly mechanisms and of the quaternary proteospecies mediating toxicity. While countless studies have investigated the contributions of the 20–29 amyloidogenic core in self-assembly, IAPP central region, i.e. positions 11 to 19, has been less studied, notwithstanding its potential key role in oligomerization. In this context, the present study aimed at investigating the physicochemical and conformational properties driving IAPP self-assembly and associated cytotoxicity. Computational tools and all-atom molecular dynamics simulation suggested that the hydrophobic 12–17 segment promotes IAPP self-recognition and aggregation. Alanine scanning revealed that the hydrophobic side chains of Leu12, Phe15 and Val17 are critical for amyloid fibril formation. Destabilization of the α-helical folding by Pro substitution enhanced self-assembly when the pyrrolidine ring was successively introduced at positions Ala13, Asn14 and Phe15, in comparison to respective Ala-substituted counterparts. Modulating the peptide backbone flexibility at position Leu16 through successive incorporation of Pro, Gly and α-methylalanine, inhibited amyloid formation and reduced cytotoxicity, while the isobutyl side chain of Leu16 was not critical for self-assembly and IAPP-mediated toxicity. These results highlight the importance of the 12–17 hydrophobic region of IAPP for self-recognition, ultimately supporting the development of therapeutic approaches to prevent oligomerization and/or fibrillization.

Targeting hIAPP fibrillation: A new paradigm to prevent β-cell death?

Loss of pancreatic β-cell mass is deleterious for type 2 diabetes patients since it reduces insulin production, critical for glucose homeostasis. The main research axis developed over the last few years was to generate new pancreatic β-cells or to transplant pancreatic islets as occurring for some specific type 1 diabetes patients. We evaluate here a new paradigm consisting in preservation of β-cells by prevention of human islet amyloid polypeptide (hIAPP) oligomers and fibrils formation leading to pancreatic β-cell death. We review the hIAPP physiology and the pathology that contributes to β-cell destruction, deciphering the various cellular steps that could be involved. Recent progress in understanding other amyloidosis such as Aβ, Tau, α-synuclein or prion, involved in neurodegenerative processes linked with inflammation, has opened new research lines of investigations to preserve neuronal cells. We evaluate and estimate their transposition to the pancreatic β-cells preservation. Among them is the control of reactive oxygen species (ROS) production occurring with inflammation and the possible implication of the mitochondrial translocator protein as a diagnostic and therapeutic target. The present review also focuses on other amyloid forming proteins from molecular to physiological and physiopathological points of view that could help to better decipher hIAPP-induced β-cell death mechanisms and to prevent hIAPP fibril formation.

Thermodynamic surprises of Cu(II)-amylin analogue complexes in membrane mimicking solutions

Membrane environment often has an important effect on the structure, and therefore also on the coordination mode of biologically relevant metal ions. This is also true in the case of Cu(II) coordination to amylin analogues—rat amylin, amylin_1–19, pramlintide and Ac-pramlintide, which offer N-terminal amine groups and/or histidine imidazoles as copper(II) anchoring sites. Complex stabilities are comparable, with the exception of the very stable Cu(II)–amylin_1–19, which proves that the presence of the amylin C-terminus lowers its affinity for copper(II); although not directly involved, its appropriate arrangement sterically prevents early metal binding. Most interestingly, in membrane-mimicking solution, the Cu(II) affinities of amylin analogues are lower than the ones in water, probably due to the crowding effect of the membrane solution and the fact that amide coordination occurs at higher pH, which happens most likely because the α-helical structure, imposed by the membrane-mimicking solvent, prevents the amides from binding at lower pH, requiring a local unwinding of the α-helix.

Inside the Insulin Secretory Granule

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

Site-Specific Alkylation of the Islet Amyloid Polypeptide Accelerates Self-Assembly and Potentiates Perturbation of Lipid Membranes

The accumulation of insoluble amyloids in the pancreatic islets is a pathological hallmark of type II diabetes and correlates closely with the loss of β-cell mass. The predominant component of these amyloid deposits is the islet amyloid polypeptide (IAPP). The factors contributing to the conversion of IAPP from a monomeric bioactive peptide hormone into insoluble amyloid fibrils remain partially elusive. In this study, we investigated the effect of the oxidative non-enzymatic post-translational modification induced by the reactive metabolite 4-hydroxynonenal (HNE) on IAPP aggregation and cytotoxicity. Incubation of IAPP with exogenous HNE accelerated its self-assembly into β-sheet fibrils and led to the formation of a Michael adduct on the His-18 side chain. To model this covalent modification, the imidazole N(π) position of histidine was alkylated using a close analogue of HNE, the octyl chain. IAPP lipidated at His-18 showed a hastened random coil-to-β-sheet conformational conversion into fibrillar assemblies with a distinct morphology, a low level of binding to thioflavin T, and a high surface hydrophobicity. Introducing an octyl chain on His-18 enhanced the ability of the peptide to perturb synthetic lipid vesicles, to permeabilize the plasma membrane, and to induce the death of pancreatic β-cells. Alkylated IAPP triggered the self-assembly of unmodified IAPP by prompting primary nucleation and increased its capacity to perturb the plasma membrane, indicating that only a small proportion of the modified peptide is necessary to shift the balance toward the formation of proteotoxic species. This study underlines the importance of studying IAPP post-translational modifications induced by oxidative metabolites in the context of pancreatic amyloids.

The Effect of Cholesterol on Membrane-Bound Islet Amyloid Polypeptide

Islet amyloid polypeptide (IAPP) is a proposed cause of the decreased beta-cell mass in patients with type-II diabetes. The molecular composition of the cell-membrane is important for regulating IAPP cytotoxicity and aggregation. Cholesterol is present at high concentrations in the pancreatic beta-cells, and in-vitro experiments have indicated that it affects the amyloid formation of IAPP either by direct interactions or by changing the properties of the membrane. In this study we apply atomistic, unbiased molecular dynamics simulations at a microsecond timescale to investigate the effect of cholesterol on membrane bound IAPP. Simulations were performed with various combinations of cholesterol, phosphatidylcholine (PC) and phosphatidylserine (PS) lipids. In all simulations, the helical structure of monomer IAPP was stabilized by the membrane. We found that cholesterol decreased the insertion depth of IAPP compared to pure phospholipid membranes, while PS lipids counteract the effect of cholesterol. The aggregation propensity has previously been proposed to correlate with the insertion depth of IAPP, which we found to decrease with the increased ordering of the lipids induced by cholesterol. Cholesterol is depleted in the vicinity of IAPP, and thus our results suggest that the effect of cholesterol is indirect.

Investigation of the effects of two major secretory granules components, insulin and zinc, on human-IAPP amyloid aggregation and membrane damage

Human islet amyloid polypeptide (hIAPP) is a highly amyloidogenic peptide found in pancreatic islets of type-2 diabetes (T2D) patients. Under certain conditions, hIAPP is able to form amyloid fibrils that play a role in the progression of T2D. hIAPP is synthesized in the β-cell of the pancreas and stored in the secretory granules before being released into the extracellular compartment. It has been suggested that natural stabilizing agents, such as insulin or zinc present in the secretory granules with hIAPP could prevent hIAPP fibril formation. The difference in the amino acid sequences of IAPP among species strongly correlates with amyloidogenicity and toxicity. The residue histidine at position 18 is known to be important in modulating the fibril formation, membrane leakage and toxicity. In this study, we have synthesized four analogues of hIAPP (H18R-IAPP, H18K-IAPP, H18A-IAPP and H18E-IAPP) and characterized their aggregation with either insulin or zinc in order to determine the effect of the residue-18 on the insulin-IAPP and zinc-IAPP interactions using a variety of biophysical experiments including thioflavin-T fluorescence, transmission electron microscopy imaging, circular dichroism, and NMR spectroscopy. We show that insulin reduced hIAPP fibril formation both in solution and in the presence of membrane and hIAPP-membrane damage and that the interactions are somewhat mediated by the residue-18. In addition, our results reveal that zinc affects the process of hIAPP fibril formation in solution but not in the presence of membrane. Our results indicate that the nature of the residue-18 is important for zinc binding. Based on this observation, we hypothesize that zinc binds to the residues in the N-terminal region of hIAPP, which is not accessible in the presence of membrane due to its strong interaction with lipids.

Mechanisms of Transthyretin Inhibition of IAPP Amyloid Formation

Amyloid-formation by the islet amyloid polypeptide (IAPP), produced by the β-cells in the human pancreas, has been associated with the development of type II diabetes mellitus (T2DM). The human plasma-protein transthyretin (TTR), a well-known amyloid-inhibiting protein, is interestingly also expressed within the IAPP producing β-cells. In the present study, we have characterized the ability of TTR to interfere with IAPP amyloid-formation, both in terms of its intrinsic stability as well as with regard to the effect of TTR-stabilizing drugs. The results show that TTR can prolong the lag-phase as well as impair elongation in the course of IAPP-amyloid formation. We also show that the interfering ability correlates inversely with the thermodynamic stability of TTR, while no such correlation was observed as a function of kinetic stability. Furthermore, we demonstrate that the ability of TTR to interfere is maintained also at the low pH environment within the IAPP-containing granules of the pancreatic β-cells. However, at both neutral and low pH, the addition of TTR-stabilizing drugs partly impaired its efficacy. Taken together, these results expose mechanisms of TTR-mediated inhibition of IAPP amyloid-formation and highlights a potential therapeutic target to prevent the onset of T2DM.

Role of lysine residue of islet amyloid polypeptide in fibril formation, membrane binding, and inhibitor binding

The aggregation of islet amyloid polypeptide (IAPP) is implicated in the pathogenesis of type 2 diabetes (T2D). In T2D, this peptide aggregates to form amyloid fibrils; the mechanism responsible for islet amyloid formation is unclear. However, it is known that the aggregation propensity of IAPP is highly related to its primary sequence. Several residues have been suggested to be critical in modulating IAPP amyloid formation, but role of the sole lysine residue at position 1 (Lys-1) in IAPP has not been discussed. In our previous study, we found that glycated IAPP can form amyloid faster than normal IAPP and induce normal IAPP to expedite the aggregation process. To gain more insight into the contribution of Lys-1 in the kinetics of fibril formation, we synthesized another two IAPP variants, K1E-IAPP and K1Nle-IAPP, in which the Lys residue was mutated to glutamate and norleucine, respectively. Interestingly, we observed that the negative or neutral charged side chain at this position was preferred for amyloid formation. The findings suggested this residue may take part in the inter- or intra-molecular interaction during IAPP aggregation, even though it was proposed not to be in part of fibril core structure. Our data also revealed that the inhibitory mechanism of some inhibitors for IAPP aggregation require reaction with Lys-1. Modifications of Lys-1, such as protein glycation, may affect the effectiveness of the inhibitory action of some potential drugs in the treatment of amyloidosis.

Lipid Modulation in the Formation of β-Sheet Structures. Implications for De Novo Design of Human Islet Amyloid Polypeptide and the Impact on β-Cell Homeostasis

Human islet amyloid polypeptide (hIAPP) corresponds to a 37-residue hormone present in insulin granules that maintains a high propensity to form β-sheet structures during co-secretion with insulin. Previously, employing a biomimetic approach, we proposed a panel of optimized IAPP sequences with only one residue substitution that shows the capability to reduce amyloidogenesis. Taking into account that specific membrane lipids have been considered as a key factor in the induction of cytotoxicity, in this study, following the same design strategy, we characterize the effect of a series of lipids upon several polypeptide domains that show the highest aggregation propensity. The characterization of the C-native segment of hIAPP (residues F₂₃-Y₃₇), together with novel variants F₂₃R and I₂₆A allowed us to demonstrate an effect upon the formation of β-sheet structures. Our results suggest that zwitterionic phospholipids promote adsorption of the C-native segments at the lipid-interface and β-sheet formation with the exception of the F₂₃R variant. Moreover, the presence of cholesterol did not modify this behavior, and the β-sheet structural transitions were not registered when the N-terminal domain of hIAPP (K₁-S₂₀) was characterized. Considering that insulin granules are enriched in phosphatidylserine (PS), the property of lipid vesicles containing negatively charged lipids was also evaluated. We found that these types of lipids promote β-sheet conformational transitions in both the C-native segment and the new variants. Furthermore, these PS/peptides arrangements are internalized in Langerhans islet β-cells, localized in the endoplasmic reticulum, and trigger critical pathways such as unfolded protein response (UPR), affecting insulin secretion. Since this phenomenon was associated with the presence of cytotoxicity on Langerhans islet β-cells, it can be concluded that the anionic lipid environment and degree of solvation are critical conditions for the stability of segments with the propensity to form β-sheet structures, a situation that will eventually affect the structural characteristics and stability of IAPP within insulin granules, thus modifying the insulin secretion.

Acidic Environment Significantly Alters Aggregation Pathway of Human Islet Amyloid Polypeptide at Negative Lipid Membrane

The misfolding and aggregation of human islet amyloid polypeptide (hIAPP) at cell membrane has a close relationship with the development of type 2 diabetes (T2DM). This aggregation process is susceptible to various physiologically related factors, and systematic studies on condition-mediated hIAPP aggregation are therefore essential for a thorough understanding of the pathology of T2DM. In this study, we combined surface-sensitive amide I and amide II spectral signals from the protein backbone, generated simultaneously in a highly sensitive femtosecond broad-band sum frequency generation vibrational spectroscopy system, to examine the effect of environmental pH on the dynamical structural changes of hIAPP at membrane surface in situ and in real time. Such a combination can directly discriminate the formation of β-hairpin-like monomer and oligomer/fibril at the membrane surface. It is evident that, in an acidic milieu, hIAPP slows down its conformational evolution and alters its aggregation pathway, leading to the formation of off-pathway oligomers. When matured hIAPP aggregates are exposed to basic subphase, partial conversion from β-sheet oligomers into ordered β-sheet fibrillar structures is observed. When exposed to acidic environment, however, hIAPP fibrils partially converse into more loosely patterned β-sheet oligomeric structures.

pH-Dependent Aggregation in Intrinsically Disordered Proteins Is Determined by Charge and Lipophilicity

Protein aggregation is associated with an increasing number of human disorders and premature aging. Moreover, it is a central concern in the manufacturing of recombinant proteins for biotechnological and therapeutic applications. Nevertheless, the unique architecture of protein aggregates is also exploited by nature for functional purposes, from bacteria to humans. The relevance of this process in health and disease has boosted the interest in understanding and controlling aggregation, with the concomitant development of a myriad of algorithms aimed to predict aggregation propensities. However, most of these programs are blind to the protein environment and, in particular, to the influence of the pH. Here, we developed an empirical equation to model the pH-dependent aggregation of intrinsically disordered proteins (IDPs) based on the assumption that both the global protein charge and lipophilicity depend on the solution pH. Upon its parametrization with a model IDP, this simple phenomenological approach showed unprecedented accuracy in predicting the dependence of the aggregation of both pathogenic and functional amyloidogenic IDPs on the pH. The algorithm might be useful for diverse applications, from large-scale analysis of IDPs aggregation properties to the design of novel reversible nanofibrillar materials.

The flanking peptides issue from the maturation of the human islet amyloid polypeptide (hIAPP) slightly modulate hIAPP-fibril formation but not hIAPP-induced cell death

Type 2 diabetes mellitus is a disease characterized by the formation of amyloid fibrillar deposits consisting mainly in human islet amyloid polypeptide (hIAPP), a peptide co-produced and co-secreted with insulin. hIAPP and insulin are synthesized by pancreatic β cells initially as prehormones resulting after sequential cleavages in the mature peptides as well as the two flanking peptides (N- and C-terminal) and the C-peptide, respectively. It has been suggested that in the secretory granules, the kinetics of hIAPP fibril formation could be modulated by some internal factors. Indeed, insulin is known to be a potent inhibitor of hIAPP fibril formation and hIAPP-induced cell toxicity. Here we investigate whether the flanking peptides could regulate hIAPP fibril formation and toxicity by combining biophysical and biological approaches. Our data reveal that both flanking peptides are not amyloidogenic. In solution and in the presence of phospholipid membranes, they are not able to totally inhibit hIAPP-fibril formation neither hIAPP-membrane damage. In the presence of INS-1 cells, a rat pancreatic β-cell line, the flanking peptides do not modulate hIAPP fibrillation neither hIAPP-induced cell death while in the presence of human islets, they have a slightly tendency to reduce hIAPP fibril formation but not its toxicity. These data demonstrate that the flanking peptides do not strongly contribute to reduce mature hIAPP amyloidogenesis in solution and in living cells, suggesting that other biochemical factors present in the cells must act on mature hIAPP fibril formation and hIAPP-induced cell death.

hIAPP forms toxic oligomers in plasma

In diabetes, hyperamylinemia contributes to cardiac dysfunction. The interplay between hIAPP, blood glucose and other plasma components is, however, not understood. We show that glucose and LDL interact with hIAPP, resulting in β-sheet rich oligomers with increased β-cell toxicity and hemolytic activity, providing mechanistic insights for a direct link between diabetes and cardiovascular diseases.

Inhibition of Human Amylin Aggregation and Cellular Toxicity by Lipoic Acid and Ascorbic Acid

More than 30 human degenerative diseases result from protein aggregation such as Alzheimer's disease (AD) and type 2 diabetes mellitus (T2DM). Islet amyloid deposits, a hallmark in T2DM, are found in pancreatic islets of more than 90% of T2DM patients. An association between amylin aggregation and reduction in β-cell mass was also established by post-mortem studies. A strategy in preventing protein aggregation-related disorders is to inhibit the protein aggregation and associated toxicity. In this study, we demonstrated that two inhibitors, lipoic acid and ascorbic acid, significantly inhibited amylin aggregation. Compared to amylin (15 μM) as 100%, lipoic acid and ascorbic acid reduced amylin fibril formation to 42.1 ± 17.2% and 42.9 ± 12.8%, respectively, which is confirmed by fluorescence and TEM images. In cell viability tests, both inhibitors protected RIN-m5f β-cells from the toxicity of amylin aggregates. At 10:1 molar ratio of lipoic acid to amylin, lipoic acid with amylin increased the cell viability to 70.3%, whereas only 42.8% RIN-m5f β-cells survived in amylin aggregates. For ascorbic acid, an equimolar ratio achieved the highest cell viability of 63.3% as compared to 42.8% with amylin aggregates only. Docking results showed that lipoic acid and ascorbic acid physically interact with amylin amyloidogenic region (residues Ser20-Ser29) via hydrophobic interactions; hence reducing aggregation levels. Therefore, lipoic acid and ascorbic acid prevented amylin aggregation via hydrophobic interactions, which resulted in the prevention of cell toxicity in vitro.

Zinc-coordination and C-peptide complexation: a potential mechanism for the endogenous inhibition of IAPP aggregation

Aggregation of the highly amyloidogenic IAPP is endogenously inhibited inside beta-cell granules at millimolar concentrations. Combining in vitro experiments and computer simulations, we demonstrated that the stabilization of IAPP upon the formation of zinc-coordinated ion molecular complex with C-peptide might be important for the endogenous inhibition of IAPP aggregation.

Atomistic-level study of the interactions between hIAPP protofibrils and membranes: Influence of pH and lipid composition

The pathology of type 2 diabetes mellitus is associated with the aggregation of human islet amyloid polypeptide (hIAPP) and aggregation-mediated membrane disruption. The interactions of hIAPP aggregates with lipid membrane, as well as the effects of pH and lipid composition at the atomic level, remain elusive. Herein, using molecular dynamics simulations, we investigate the interactions of hIAPP protofibrillar oligomers with lipids, and the membrane perturbation that they induce, when they are partially inserted in an anionic dipalmitoyl-phosphatidylglycerol (DPPG) membrane or a mixed dipalmitoyl-phosphatidylcholine (DPPC)/DPPG (7:3) lipid bilayer under acidic/neutral pH conditions. We observed that the tilt angles and insertion depths of the hIAPP protofibril are strongly correlated with the pH and lipid composition. At neutral pH, the tilt angle and insertion depth of hIAPP protofibrils at a DPPG bilayer reach ~52° and ~1.62 nm with respect to the membrane surface, while they become ~77° and ~1.75 nm at a mixed DPPC/DPPG membrane. The calculated tilt angle of hIAPP at DPPG membrane is consistent with a recent chiral sum frequency generation spectroscopic study. The acidic pH induces a smaller tilt angle of ~40° and a shallower insertion depth (~1.24 nm) of hIAPP at the DPPG membrane surface, mainly due to protonation of His18 near the turn region. These differences mainly result from a combination of distinct electrostatic, van der Waals, hydrogen bonding and salt-bridge interactions between hIAPP and lipid bilayers. The hIAPP-membrane interaction energy analysis reveals that besides charged residues K1, R11 and H18, aromatic residues Phe15 and Phe23 also exhibit strong interactions with lipid bilayers, revealing the crucial role of aromatic residues in stabilizing the membrane-bound hIAPP protofibrils. hIAPP-membrane interactions disturb the lipid ordering and the local bilayer thickness around the peptides. Our results provide atomic-level information of membrane interaction of hIAPP protofibrils, revealing pH-dependent and membrane-modulated hIAPP aggregation at the early stage.

Spontaneous Lipid Nanodisc Fomation by Amphiphilic Polymethacrylate Copolymers

There is a growing interest in the use of lipid bilayer nanodiscs for various biochemical and biomedical applications. Among the different types of nanodiscs, the unique features of synthetic polymer-based nanodiscs have attracted additional interest. A styrene-maleic acid (SMA) copolymer demonstrated to form lipid nanodiscs has been used for structural biology related studies on membrane proteins. However, the application of SMA polymer based lipid nanodiscs is limited because of the strong absorption of the aromatic group interfering with various experimental measurements. Thus, there is considerable interest in the development of other molecular frameworks for the formation of polymer-based lipid nanodiscs. In this study, we report the first synthesis and characterization of a library of polymethacrylate random copolymers as alternatives to SMA polymer. In addition, we experimentally demonstrate the ability of these polymers to form lipid bilayer nanodiscs through the fragmentation of lipid vesicles by means of light scattering, electron microscopy, differential scanning calorimetry, and solution and solid-state NMR experiments. We further demonstrate a unique application of the newly developed polymer for kinetics and structural characterization of the aggregation of human islet amyloid polypeptide (also known as amylin) within the lipid bilayer of the polymer nanodiscs using thioflavin-T-based fluorescence and circular dichroism experiments. Our results demonstrate that the reported new styrene-free polymers can be used in high-throughput biophysical experiments. Therefore, we expect that the new polymer nanodiscs will be valuable in the structural studies of amyloid proteins and membrane proteins by various biophysical techniques.

Implications of peptide assemblies in amyloid diseases

Neurodegenerative disorders and type 2 diabetes are global epidemics compromising the quality of life of millions worldwide, with profound social and economic implications. Despite the significant differences in pathology - much of which are poorly understood - these diseases are commonly characterized by the presence of cross-β amyloid fibrils as well as the loss of neuronal or pancreatic β-cells. In this review, we document research progress on the molecular and mesoscopic self-assembly of amyloid-beta, alpha synuclein, human islet amyloid polypeptide and prions, the peptides and proteins associated with Alzheimer's, Parkinson's, type 2 diabetes and prion diseases. In addition, we discuss the toxicities of these amyloid proteins based on their self-assembly as well as their interactions with membranes, metal ions, small molecules and engineered nanoparticles. Through this presentation we show the remarkable similarities and differences in the structural transitions of the amyloid proteins through primary and secondary nucleation, the common evolution from disordered monomers to alpha-helices and then to β-sheets when the proteins encounter the cell membrane, and, the consensus (with a few exceptions) that off-pathway oligomers, rather than amyloid fibrils, are the toxic species regardless of the pathogenic protein sequence or physicochemical properties. In addition, we highlight the crucial role of molecular self-assembly in eliciting the biological and pathological consequences of the amyloid proteins within the context of their cellular environments and their spreading between cells and organs. Exploiting such structure-function-toxicity relationship may prove pivotal for the detection and mitigation of amyloid diseases.

Identification of Key Interactions in the Initial Self-Assembly of Amylin in a Membrane Environment

Islet amyloid polypeptide, also known as amylin, forms aggregates that reduce the amount of insulin-producing cells in patients with type II diabetes mellitus. Much remains unknown about the process of aggregation and cytotoxicity, but it is known that certain cell membrane components can alter the rate of aggregation. Using atomistic molecular dynamics simulations combined with the highly mobile membrane mimetic model incorporating enhanced sampling of lipid diffusion, we investigate interaction of amylin peptides with the membrane components as well as the self-assembly of amylin. Consistent with experimental evidence, we find that an initial membrane-bound α-helical state folds into stable β-sheet structures upon self-assembly. Our results suggest the following mechanism for the initial phase of amylin self-assembly. The peptides move around on the membrane with the positively charged N-terminus interacting with the negatively charged lipid headgroups. When the peptides start to interact, they partly unfold and break some of the contacts with the membrane. The initial interactions between the peptides are dominated by aromatic and hydrophobic interactions. Oligomers are formed showing both intra- and interpeptide β-sheets, initially with interactions mainly in the C-terminal domain of the peptides. Decreasing the pH to 5.5 is known to inhibit amyloid formation. At low pH, His18 is protonated, adding a fourth positive charge at the peptide. With His18 protonated, no oligomerization is observed in the simulations. The additional charge gives a strong midpoint anchoring of the peptides to negatively charged membrane components, and the peptides experience additional interpeptide repulsion, thereby preventing interactions.

Conformational Dynamics of the Human Islet Amyloid Polypeptide in a Membrane Environment: Toward the Aggregation Prone Form

Human islet amyloid polypeptide (hIAPP) is a 37-residue peptide hormone, which upon misfolding changes from the physiologically active monomer into pathological amyloid fibril aggregates in the pancreas of type 2 diabetes mellitus patients. During this process, the insulin-producing pancreatic β-cells are damaged; however, the underlying mechanism of this mode of cytotoxicity remains elusive. It is known that anionic lipids accelerate amyloid fibril formation, implicating the importance of the cellular membrane in the process, and that a pH close to the level in the β-cell secretory granules (pH 5.5) inhibits amyloid fibril formation. Using all-atom molecular dynamics simulations, we have investigated the membrane-associated monomer state of α-helical hIAPP, analyzed specific interactions of hIAPP with a mixed anionic-zwitterionic lipid membrane and examined the influence of pH on the structure and dynamics of hIAPP and its interaction with the membrane. We find that hIAPP primarily interacts with the membrane by forming favorable interactions between anionic lipids and the positively charged residues in the N-terminal part of the peptide. Rationalizing experimental findings, the simulations show that the N-terminal part of the peptide interacts with the membrane in the lipid headgroup region. At neutral pH, the C-terminal part of the peptide, which contains the residues that initiate fibril formation, displays a highly dynamic, unfolded state, which interacts with the membrane significantly less than the N-terminal part. Such an unfolded form can be proposed to contribute to the acceleration of fibril formation. At low pH, protonation of His18 mediates a stronger interaction of the C-terminal part with the membrane, resulting in the immobilization of the C-terminal part on the membrane surface that might constitute a mechanism by which low pH inhibits fibril formation.

Formation of lamellar micelle-like oligomers and membrane disruption revealed by the study of short peptide hIAPP18–27

An oligomer species with a lamellar crystalline structure is identified to disrupt the membrane containing the anionic component strongly.

Molecular Structure, Membrane Interactions, and Toxicity of the Islet Amyloid Polypeptide in Type 2 Diabetes Mellitus

Human islet amyloid polypeptide (hIAPP) is the major component of the amyloid deposits found in the pancreatic islets of patients with type 2 diabetes mellitus (T2DM). Mature hIAPP, a 37-aa peptide, is natively unfolded in its monomeric state but forms islet amyloid in T2DM. In common with other misfolded and aggregated proteins, amyloid formation involves aggregation of monomers of hIAPP into oligomers, fibrils, and ultimately mature amyloid deposits. hIAPP is coproduced and stored with insulin by the pancreatic islet β-cells and is released in response to the stimuli that lead to insulin secretion. Accumulating evidence suggests that hIAPP amyloid deposits that accompany T2DM are not just an insignificant phenomenon derived from the disease progression but that hIAPP aggregation induces processes that impair the functionality and the viability of β-cells. In this review, we particularly focus on hIAPP structure, hIAPP aggregation, and hIAPP-membrane interactions. We will also discuss recent findings on the mechanism of hIAPP-membrane damage and on hIAPP-induced cell death. Finally, the development of successful antiamyloidogenic agents that prevent hIAPP fibril formation will be examined.

Nanoscale Heterogeneity of the Molecular Structure of Individual hIAPP Amyloid Fibrils Revealed with Tip-Enhanced Raman Spectroscopy

Type 2 diabetes mellitus is characterized by the pathological deposition of fibrillized protein, known as amyloids. It is thought that oligomers and/or amyloid fibrils formed from human islet amyloid polypeptide (hIAPP or amylin) cause cell death by membrane damage. The molecular structure of hIAPP amyloid fibrils is dominated by β-sheet structure, as probed with conventional infrared and Raman vibrational spectroscopy. However, with these techniques it is not possible to distinguish between the core and the surface structure of the fibrils. Since the fibril surface crucially affects amyloid toxicity, it is essential to know its structure. Here the surface molecular structure and amino acid residue composition of hIAPP fibrils are specifically probed with nanoscale resolution using tip-enhanced Raman spectroscopy (TERS). The fibril surface mainly contains unordered or α-helical structures, in contrast to the β-sheet-rich core. This experimentally validates recent models of hIAPP amyloids based on NMR measurements. Spatial mapping of the surface structure reveals a highly heterogeneous surface structure. Finally, TERS can probe fibrils formed on a lipid interface, which is more representative of amyloids in vivo.

Molecular confinement of human amylin in lipidic nanoparticles

Amylin is a pancreatic hormone involved in the regulation of glucose metabolism and homeostasis. Restoration of the post-prandial and basal levels of human amylin in diabetic individuals is a key in controlling glycemia, controlling glucagon, reducing the insulin dose and increasing satiety, among other physiologic functions. Human amylin has a high propensity to aggregate. We have addressed this issue by designing a liposomal human amylin formulation. Nanoparticles of multilamellar liposomes comprising human amylin were obtained with 53% encapsulation efficiency. The in vitro kinetic release assay shows a biphasic profile. The stabilization of the lipidic nanoparticle against freeze-drying was achieved by using mannitol as a cryoprotectant, as evidenced by morphological characterization. The effectiveness of the human amylin entrapped in lipidic nanoparticles was tested by the measurement of its pharmacological effect in vivo after subcutaneous administration in mice. Collectively these results demonstrate the compatibility of human amylin with the lipidic interface as an effective pharmaceutical delivery system.

Detergent-type membrane fragmentation by MSI-78, MSI-367, MSI-594, and MSI-843 antimicrobial peptides and inhibition by cholesterol: a solid-state nuclear magnetic resonance study

Multidrug resistance against the existing antibiotics is becoming a global threat, and any potential drug that can be designed using cationic antimicrobial peptides (AMP) could be an alternate solution to alleviate this existing problem. The mechanism of action of killing bacteria by an AMP differs drastically in comparison to that of small molecule antibiotics. The main target of AMPs is to interact with the lipid bilayer of the cell membrane and disrupt it to kill bacteria. Consequently, the modes of membrane interaction that lead to the selectivity of an AMP are very important to understand. Here, we have used different membrane compositions, such as negatively charged, zwitterionic, or mixed large unilamellar vesicles (LUVs), to study the interaction of four different synthetically designed cationic, linear antimicrobial peptides: MSI-78 (commercially known as pexiganan), MSI-367, MSI-594, and MSI-843. Our solid-state nuclear magnetic resonance (NMR) experiments confirmed that the MSI peptides fragmented LUVs through a detergent-like carpet mechanism depending on the amino acid sequence of the MSI peptide and/or the membrane composition of LUVs. Interestingly, the fragmented lipid aggregates such as SUVs or micelles are sufficiently small to produce an isotropic peak in the (31)P NMR spectrum. These fragmented lipid aggregates contain only MSI peptides bestowed with lipid molecules as confirmed by NMR in conjunction with circular dichroism spectroscopy. Our results also demonstrate that cholesterol, which is present only in the eukaryotic cell membrane, inhibits the MSI-induced fragmentation of LUVs, suggesting that the MSI peptides can discriminate the bacteria and the eukaryotic cell membranes, and this selectivity could be used for further development of novel antibiotics.

Amyloids of alpha-synuclein affect the structure and dynamics of supported lipid bilayers

Interactions of monomeric alpha-synuclein (αS) with lipid membranes have been suggested to play an important role in initiating aggregation of αS. We have systematically analyzed the distribution and self-assembly of monomeric αS on supported lipid bilayers. We observe that at protein/lipid ratios higher than 1:10, αS forms micrometer-sized clusters, leading to observable membrane defects and decrease in lateral diffusion of both lipids and proteins. An αS deletion mutant lacking amino-acid residues 71-82 binds to membranes, but does not observably affect membrane integrity. Although this deletion mutant cannot form amyloid, significant amyloid formation is observed in the wild-type αS clusters. These results suggest that the process of amyloid formation, rather than binding of αS on membranes, is crucial in compromising membrane integrity.

The Effects of Lipid Membranes, Crowding and Osmolytes on the Aggregation, and Fibrillation Propensity of Human IAPP

Type 2 diabetes mellitus (T2DM) is an age-related and metabolic disease. Its development is hallmarked, among others, by the dysfunction and degeneration of β-cells of the pancreatic islets of Langerhans. The major pathological characteristic thereby is the formation of extracellular amyloid deposits consisting of the islet amyloid polypeptide (IAPP). The process of human IAPP (hIAPP) self-association, and the intermediate structures formed as well as the interaction of hIAPP with membrane systems seem to be, at least to a major extent, responsible for the cytotoxicity. Here we present a summary and comparison of the amyloidogenic propensities of hIAPP in bulk solution and in the presence of various neutral and charged lipid bilayer systems as well as biological membranes. We also discuss the cellular effects of macromolecular crowding and osmolytes on the aggregation pathway of hIAPP. Understanding the influence of different cellular factors on hIAPP aggregation will provide more insight into the onset of T2DM and help to develop novel therapeutic strategies.

Charge-Based Inhibitors of Amylin Fibrillization and Toxicity

To test the hypothesis that electrostatic repulsion is an important force opposing amyloid fibril assembly, we designed peptides that substitute strings of positively or negatively charged residues into the sequence of the amyloidogenic hormone amylin, which contributes to type 2 diabetes pathology. Arg-1 and Arg-2 substitute four positively charged arginines for segments that in structural models of amylin fibrils form the end of strand β1 and the beginning of strand β2, respectively. Mem-T substitutes negatively charged aspartates for the peptide segment with the largest avidity for membranes. All three charge-loaded peptides fibrillize poorly on their own and inhibit fibril elongation of WT-amylin at physiological ionic strength. The inhibition of WT-amylin fibril elongation rates is salt-dependent indicating that the analogs act through electrostatic interactions. Arg-1 protects against WT-amylin cytotoxicity towards a MIN6 mouse model of pancreatic β-cells, and Arg-2 protects at higher concentrations, whereas Mem-T has no effect. The most effective variant, Arg-1, inhibits WT-amylin fibril elongation rates with an IC50 of ~1 µM and cytotoxicity with an IC50 of ~50 µM, comparable to other types of fibrillization inhibitors reported in the literature. Taken together, these results suggest that electrostatic interactions can be exploited to develop new types of inhibitors of amyloid fibrillization and toxicity.