Structure and assembly mechanisms of toxic human islet amyloid polypeptide oligomers associated with copper

Modular Protein Fibers with Outstanding High-Strength and Acid-Resistance Performance Mediated by Copper Ion Binding and Imine Networking

Engineered protein fibers are promising biomaterials with diverse applications due to their tunable protein structure and outstanding mechanical properties. However, it remains challenging at the molecular level to achieve satisfied mechanical properties and environmental tolerance simultaneously, especially under extreme acid conditions. Herein, the construction of artificial fibers comprising chimeric proteins made of rigid amyloid peptide and flexible cationic elastin-like protein (ELP) module is reported. The amyloid peptide readily assembles into highly organized β-sheet structures that can be further strengthened by the coordination of Cu2+, while the flexible ELP module allows the formation of imine-based crosslinking networks. These double networks synergistically enhance the mechanical properties of the fibers, leading to a high tensile strength and toughness, overwhelming many reported recombinant spidroin fibers. Notably, the coordination of Cu2+ with serine residues could stabilize β-sheet structures in the fibers under acidic conditions, which makes the fibers robust against acid, thus enabling their successful utilization in gastric perforation suturing. This work highlights the customization of double networks at the molecular level to create tailored high-performance protein fibers for various application scenarios.

Copper binding and protein aggregation: a journey from the brain to the human lens

Metal ions have been implicated in several proteinopathies associated to degenerative and neurodegenerative diseases. While the molecular mechanisms for protein aggregation are still under investigation, recent findings from Cryo-EM point out to polymorphisms in aggregates obtained from patients, as compared to those formed in vitro, suggesting that several factors may impact aggregation in vivo. One of these factors could be the direct binding of metal ions to the proteins engaged in aggregate formation. In this opinion article, three case studies are discussed to address the question of how metal ion binding to a peptide or protein may impact its conformation, folding, and aggregation, and how this may be relevant in understanding the polymorphic nature of the aggregates related to disease. Specifically, the impact of Cu2+ ions in the amyloid aggregation of amyloid-β and amylin (or IAPP- islet amyloid polypeptide) are discussed and then contrasted to the case of Cu2+-induced non-amyloid aggregation of human lens γ-crystallin proteins. For the intrinsically disordered peptides amyloid-β and IAPP, the impact of Cu2+ ion binding is highly dependent on the relative location of the metal binding site and the hydrophobic regions involved in β-sheet folding and amyloid formation. Further structural studies of how Cu2+ binding impacts amyloid aggregation pathways and the molecular structure of the final amyloid fibril, both, in vitro and in vivo, will certainly shed light into the molecular origins of the polymorphisms observed in diseased tissue. Finally, contrasting these cases to that of Cu2+-induced non-amyloid aggregation of γ-crystallins, it is evident that, although the impact in aggregation - and the nature of the aggregate - may differ in each system, at the molecular level there is a competition between metal ion coordination and the stability of β-sheet structures. Considering the importance of the β-sheet fold in biology, it is fundamental to understand the energetics and molecular details behind such competition. This opinion article aims to highlight future research directions in the field that can help tackle the important question of how metal ion binding may impact protein folding and aggregation and how this relates to disease.

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.

The Role of Heme and Copper in Alzheimer's Disease and Type 2 Diabetes Mellitus

Beyond the well-explored proposition of protein aggregation or amyloidosis as the central event in amyloidogenic diseases like Alzheimer's Disease (AD), and Type 2 Diabetes Mellitus (T2Dm); there are alternative hypotheses, now becoming increasingly evident, which suggest that the small biomolecules like redox noninnocent metals (Fe, Cu, Zn, etc.) and cofactors (Heme) have a definite influence in the onset and extent of such degenerative maladies. Dyshomeostasis of these components remains as one of the common features in both AD and T2Dm etiology. Recent advances in this course reveal that the metal/cofactor-peptide interactions and covalent binding can alarmingly enhance and modify the toxic reactivities, oxidize vital biomolecules, significantly contribute to the oxidative stress leading to cell apoptosis, and may precede the amyloid fibrils formation by altering their native folds. This perspective highlights this aspect of amyloidogenic pathology which revolves around the impact of the metals and cofactors in the pathogenic courses of AD and T2Dm including the active site environments, altered reactivities, and the probable mechanisms involving some highly reactive intermediates as well. It also discusses some in vitro metal chelation or heme sequestration strategies which might serve as a possible remedy. These findings might open up a new paradigm in our conventional understanding of amyloidogenic diseases. Moreover, the interaction of the active sites with small molecules elucidates potential biochemical reactivities that can inspire designing of drug candidates for such pathologies.

The influence of aluminium and copper upon the early aggregatory behaviour and size of Islet amyloid polypeptide under simulated physiological conditions

BACKGROUND AND AIM

Islet amyloid polypeptide/amylin deposition in the form of amyloid plaques is a common pathological feature observed in the pancreatic tissue of those with Type II Diabetes Mellitus. Its propensity to form amyloid fibrils and the resultant toxicity of this peptide in vivo is influenced by both the concentration and species of metal present in situ. Herein, we examine the influence of Al (III) and Cu (II), applied at equimolar and supra-stoichiometric concentrations on the initial aggregatory behaviour of amylin under near physiological conditions.

METHODS

Dynamic light scattering measurements, which monitored the aggregation status and size of the peptide in real time, were performed during the early lag-phase of fibrillogenesis (T ≤ 30 min) in the absence or presence of metal ions.

RESULTS

Islet amyloid polypeptide (10 µM) rapidly aggregated when introduced into a physiological medium favouring the formation of large, agglomerated structures (> 1000 nm) after 30 min incubation. Neither the addition of equimolar or excess metals significantly influenced the size of the peptide when intensity distributions were consulted; however, number distributions indicated that both Al (III) and Cu (II) may have had, an albeit temporary, stabilising influence upon the conformations present within solution.

CONCLUSION

These results infer that small oligomeric species are likely transient entities that are rapidly incorporated into large agglomerates during the very initial stages of fibrillogenesis. While both Al (III) and Cu (II) both inhibited agglomeration to some degree, their stabilising affect upon peptide aggregation was limited over the juncture of the experiments performed herein; hence, it is difficult to say whether these metal ions play a role in enhancing the toxicity of these peptides through influencing their aggregation in the short-term.

Antidiabetic-activity sulfated polysaccharide from Chaetomorpha linum: Characteristics of its structure and effects on oxidative stress and mitochondrial function

A water-soluble polysaccharide from the green alga Chaetomorpha linum, designated CHS2, was obtained by water extraction, preparative anion-exchange and size-exclusion chromatography. Results of chemical and spectroscopic analyses showed that CHS2 was a sulfated rhamnogalactoarabinan, and its backbone was mainly constituted by 4-linked and 3,4-linked β-l-arabinopyranose with sulfate groups at C-2/C-3 of 4-linked β-l-arabinopyranose. The branching contained 4-linked, 6-linked β-d-galactopyranose and terminal rhamnose residues. Based on the inhibition of human islet amyloid polypeptide (hIAPP) aggregation and morphology change of hIAPP aggregates in in vitro tests, it was proved that CHS2 effectively inhibited the hIAPP aggregation and possessed strong antidiabetic activity. CHS2 was nearly no toxicity in NIT-1 cells and could attenuate hIAPP-induced cytotoxicity. CHS2 may significantly reduce the generation of intracellular reactive oxygen species and hIAPP aggregation-induced oxidative stress in NIT-1 cells. CHS2 was co-localized with mitochondria, and largely protected mitochondria function from hIAPP aggregation-induced damage through stabilizing mitochondrial membrane potential and enhancing the mitochondrial complex I, II or III activity and ATP level. The data demonstrated that CHS2 could have potential prospect to become an antidiabetic drug for type 2 diabetes mellitus treatment.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Ion Mobility Mass Spectrometry Analysis of Oxygen Affinity-Associated Structural Changes in Hemoglobin

Hemoglobin (Hb) is a major oxygen-transporting protein with allosteric properties reflected in the structural changes that accompany binding of O₂. Glycated hemoglobin (GHb), which is a minor component of human red cell hemolysate, is generated by a nonenzymatic reaction between glucose and hemoglobin. Due to the long lifetime of human erythrocytes (∼120 days), GHb is widely used as a reliable biomarker for monitoring long-term glucose control in diabetic patients. Although the structure of GHb differs from that of Hb, structural changes relating to the oxygen affinity of these proteins remain incompletely understood. In this study, the oxygen-binding kinetics of Hb and GHb are evaluated, and their structural dynamics are investigated using solution small-angle X-ray scattering (SAXS), electrospray ionization mass spectrometry equipped with ion mobility spectrometry (ESI-IM-MS), and molecular dynamic (MD) simulations to understand the impact of structural alteration on their oxygen-binding properties. Our results show that the oxygen-binding kinetics of GHb are diminished relative to those of Hb. ESI-IM-MS reveals structural differences between Hb and GHb, which indicate the preference of GHb for a more compact structure in the gas phase relative to Hb. MD simulations also reveal an enhancement of intramolecular interactions upon glycation of Hb. Therefore, the more rigid structure of GHb makes the conformational changes that facilitate oxygen capture more difficult creating a delay in the oxygen-binding process. Our multiple biophysical approaches provide a better understanding of the allosteric properties of hemoglobin that are reflected in the structural alterations accompanying oxygen binding.

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.

Optimized experimental pre-treatment strategy for temporary inhibition of islet amyloid polypeptide aggregation

Islet amyloid polypeptide (IAPP) is a neuroendocrine hormone from pancreatic β-cells. Misfolded, aggregated IAPP is believed to be toxic to islet cells and amyloid deposits in the pancreas are pathological hallmarks of type 2 diabetes. Rapid fibrillization of this peptide makes it difficult to study in its soluble form, impeding a better understanding of its role. In this study, a variety of popular pretreatment methods were tested for their ability to delay aggregation of IAPP, including solutions of hexafluoroisopropanol, sodium hydroxide, hydrochloric acid, phosphate buffered saline, ammonium hydroxide, as well as tris buffer at different pH and containing either calcium (II), zinc (II), or iron (II). Aggregation was assessed using the thioflavin T fluorescence assay as well as by transmission electron microscopy. Tris buffer at pH 8.1 containing Zn(II) was found to have the best balance of temporary inhibition of aggregation and biological relevance.

Regulation of divalent metal ions to the aggregation and membrane damage of human islet amyloid polypeptide oligomers

The accumulation of human islet amyloid polypeptide (hIAPP) on the surface of pancreatic β cells is closely related to the death of the cells. Divalent metal ions play a significant role in the cytotoxicity of hIAPP. In this study, we examined the roles played by the divalent metal ions of zinc, copper and calcium in the aggregation of both hIAPP18-27 fragment and full-length hIAPP and the ability of their oligomers to damage the membrane of POPC/POPG 4 : 1 LUVs using the ThT fluorescence, TEM, AFM, CD, ANS binding fluorescence and dye leakage experiments. We prepared metal-free and metal-associated oligomers that are similar in size and aggregate slowly using the short peptide and confirmed that the ability of the peptide oligomers to damage the lipid membrane is reduced by the binding to the metal ions, which is closely linked to the reducing hydrophobic exposure of the metal-associated oligomers. The study on the full-length hIAPP showed that the observed membrane damage induced by hIAPP oligomers is either mitigated at a peptide-to-metal ratio of 1 : 0.33 or aggravated at a peptide-to-metal ratio of 1 : 1 in the presence of Zn(ii) and Cu(ii), while the surface hydrophobicity of hIAPP oligomers was reduced at both peptide-to-metal ratios. The observed results of the membrane damage were attributed to the counteraction between a decrease in the disruptive ability of metal-associated oligomer species and an increase in the quantity of oligomers promoted by the binding of the metal ions to hIAPP oligomers. The former could play a predominant role in reducing the membrane damage at a peptide-to-metal ratio of 1 : 0.33, while the latter could play a predominant role in enhancing the membrane damage at a peptide-to-metal ratio of 1 : 1. This study shows that an enhanced membrane damage could be caused by the oligomer species with a decreased instead of an increased disruptive ability, given that the abundance of the oligomer species is high enough.

Their is a counteraction between a decrease in the disruptive ability of metal-associated oligomer species and an increase in the quantity of oligomers promoted by the metal binding in the activity of hIAPP induced membrane damage.

Interaction of ApoMyoglobin with Heme-hIAPP complex

Human Islet Amyloid Polypeptide (hIAPP) or amylin, can bind heme and the resultant complexes are prone to generate partially reduced oxygen species (PROS). The formation of PROS and the related oxidative stress highlight the importance of Heme-hIAPP in the onset and development of Type 2 Diabetes mellitus (T2Dm) in humans. In this study, the interaction of Heme-hIAPP with apomyoglobin (ApoMb) has been investigated using a combination of spectroscopic and electrophoresis techniques. Absorption, resonance Raman data and gel electrophoresis results confirm that ApoMb can uptake heme from Heme-hIAPP and constitute a six-coordinate high-spin ferric heme active site identical to that of myoglobin (Mb). The heme transfer reaction has two distinct kinetic steps. A possible mechanism of this reaction involves heme transfer to the apoprotein in the first step followed by a reorganisation of the protein chain to form the active site of native Mb. Increase in the pH of the reaction medium enhances the rate of the second step of heme transfer. This possibly corresponds to the deprotonation of a propionate side chain of the heme moiety at high pH which facilitates secondary interactions with the conserved distal Lys45 residue of horse heart Mb. Additionally, ApoMb sequesters ligand bound heme from Heme-hIAPP. After the heme transfer reaction, the amount of PROS formed by Heme-hIAPP complex diminishes significantly. This not only potentially diminishes heme-induced toxicity in the pancreatic β-cells but also produces Mb which has well-documented functions throughout the respiratory system and can thereby likely reduce the risks associated with T2Dm.

Sclerotiorin Stabilizes the Assembly of Nonfibrillar Abeta42 Oligomers with Low Toxicity, Seeding Activity, and Beta-sheet Content

The self-assembly of the 42-residue amyloid-β peptide, Aβ42, into fibrillar aggregates is associated with neuronal dysfunction and toxicity in Alzheimer's disease (AD) patient brains, suggesting that small molecules acting on this process might interfere with pathogenesis. Here, we present experimental evidence that the small molecule sclerotiorin (SCL), a natural product belonging to the group of azaphilones, potently delays both seeded and nonseeded Aβ42 polymerization in cell-free assays. Mechanistic biochemical studies revealed that the inhibitory effect of SCL on fibrillogenesis is caused by its ability to kinetically stabilize small Aβ42 oligomers. These structures exhibit low β-sheet content and do not possess seeding activity, indicating that SCL acts very early in the amyloid formation cascade before the assembly of seeding-competent, β-sheet-rich fibrillar aggregates. Investigations with NMR WaterLOGSY experiments confirmed the association of Aβ42 assemblies with SCL in solution. Furthermore, using ion mobility-mass spectrometry, we observed that SCL directly interacts with a small fraction of Aβ42 monomers in the gas phase. In comparison to typical amyloid fibrils, small SCL-stabilized Aβ42 assemblies are inefficiently taken up into mammalian cells and have low toxicity in cell-based assays. Overall, these mechanistic studies support a pathological role of stable, β-sheet-rich Aβ42 fibrils in AD, while structures with low β-sheet content may be less relevant.

His18 promotes reactive oxidative stress production in copper-ion mediated human islet amyloid polypeptide aggregation

Copper ions play a critical role in human islet amyloid polypeptide (hIAPP) aggregation, which has been found in more than 90% of patients with type-2 diabetes (T2D). The role of Cu(ii) in the cell cytotoxicity with hIAPP has been explored in two aspects: inhibiting the formation of fibrillar structures and stimulating the generation of reactive oxygen species (ROS). In this work, we carried out spectroscopic studies of Cu(ii) interacting with several hIAPP fragments and their variants as well. Electron paramagnetic resonance (EPR) measurements and Amplex Red analysis showed that the amount of H₂O₂ generated in hIAPP(11-28) solution co-incubated with Cu(ii) was remarkably more than hIAPP(1-11) and hIAPP(28-37). Furthermore, the H₂O₂ level was seriously reduced when His18 of hIAPP(11-28) was replaced by Arg(R) or Ser(S), indicating that His18 is the key residue of Cu(ii) binding to hIAPP(11-28) to promote H₂O₂ generation. This is likely because the donation of electrons from the peptide to Cu(ii) ions would result in the formation of the redox-active complexes, which could stimulate the formation of H₂O₂. Overall, this study provides further insight into the molecular mechanism of Cu(ii) induced ROS generation.

His18 promotes H₂O₂ production in copper-ion mediated hIAPP aggregation.

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.

Gas-phase conformations of intrinsically disordered proteins and their complexes with ligands: Kinetically trapped states during transfer from solution to the gas phase

Flexible structures of intrinsically disordered proteins (IDPs) are crucial for versatile functions in living organisms, which involve interaction with diverse partners. Electrospray ionization ion mobility mass spectrometry (ESI-IM-MS) has been widely applied for structural characterization of apo-state and ligand-associated IDPs via two-dimensional separation in the gas phase. Gas-phase IDP structures have been regarded as kinetically trapped states originated from conformational features in solution. However, an implication of the states remains elusive in the structural characterization of IDPs, because it is unclear what structural property of IDPs is preserved. Recent studies have indicated that the conformational features of IDPs in solution are not fully reproduced in the gas phase. Nevertheless, the molecular interactions captured in the gas phase amplify the structural differences between IDP conformers. Therefore, an IDP conformational change that is not observed in solution is observable in the gas-phase structures obtained by ESI-IM-MS. Herein, we have presented up-to-date researches on the key implications of kinetically trapped states in the gas phase with a brief summary of the structural dynamics of IDPs in ESI-IM-MS.

X-Ray Absorption Spectroscopy Measurements of Cu-ProIAPP Complexes at Physiological Concentrations

The amyloidogenic islet amyloid polypeptide (IAPP) and the associated pro-peptide ProIAPP1–48 are involved in cell death in type 2 diabetes mellitus. It has been observed that interactions of this peptide with metal ions have an impact on the cytotoxicity of the peptides as well as on their deposition in the form of amyloid fibrils. In particular, Cu(II) seems to inhibit amyloid fibril formation, thus suggesting that Cu homeostasis imbalance may be involved in the pathogenesis of type 2 diabetes mellitus. We performed X-ray Absorption Spectroscopy (XAS) measurements of Cu(II)-ProIAPP complexes under near-physiological (10 μM), equimolar concentrations of Cu(II) and peptide. Such low concentrations were made accessible to XAS measurements owing to the use of the High Energy Resolved Fluorescence Detection XAS facility recently installed at the ESRF beamline BM16 (FAME-UHD). Our preliminary data show that XAS measurements at micromolar concentrations are feasible and confirm that ProIAPP1–48-Cu(II) binding at near-physiological conditions can be detected.

Aggregation of the diabetes-related peptide ProIAPP1-48 measured by dynamic light scattering

Islet amyloid polypeptide (IAPP_1-37) or amylin is implicated in the aetiology of diabetes. It is found as amyloid along with its precursor ProIAPP_1-48 in the islets of Langerhans in the pancreas. Metals have been implicated in amyloidogenesis of both IAPP and ProIAPP. Herein we have used dynamic light scattering (DLS) to investigate how Al(III) and Cu(II) influence aggregation of ProIAPP_1-48 under near-physiological conditions and in a biologically-relevant timeframe. ProIAPP_1-48 formed primarily sub-micron particles within 5 min (e.g. 470 nm at 15μM peptide) that grew to micron-sized particles (1310 nm) within a 30 min timeframe. Equimolar Al(III) had little influence upon particle size at either 5 (656 nm) or 30 min (1250 nm) while Cu(II) tended to increase particle size over the same time period (731-1300 nm). It is suggested that any effects of Al(III) and Cu(II) reflected their well known tendencies to support β-sheet or amorphous aggregates of ProIAPP_1-48 respectively.

Cu and Zn coordination to amyloid peptides: From fascinating chemistry to debated pathological relevance

Several diseases share misfolding of different peptides and proteins as a key feature for their development. This is the case of important neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and type II diabetes mellitus. Even more, metal ions such as copper and zinc might play an important role upon interaction with amyloidogenic peptides and proteins, which could impact their aggregation and toxicity abilities. In this review, the different coordination modes proposed for copper and zinc with amyloid-β, α-synuclein and IAPP will be reviewed as well as their impact on the aggregation, and ROS production in the case of copper. In addition, a special focus will be given to the mutations that affect metal binding and lead to familial cases of the diseases. Different modifications of the peptides that have been observed in vivo and could be relevant for the coordination of metal ions are also described.

NFGAIL Amyloid Oligomers: The Onset of Beta-Sheet Formation and the Mechanism for Fibril Formation

The hexapeptide NFGAIL is a highly amyloidogenic peptide, derived from the human islet amyloid polypeptide (hIAPP). Recent investigations indicate that presumably soluble hIAPP oligomers are one of the cytotoxic species in type II diabetes. Here we use thioflavin T staining, transmission electron microscopy, as well as ion mobility-mass spectrometry coupled to infrared (IR) spectroscopy to study the amyloid formation mechanism and the quaternary and secondary structure of soluble NFGAIL oligomers. Our data reveal that at neutral pH NFGAIL follows a nucleation dependent mechanism to form amyloid fibrils. During the lag phase, highly polydisperse, polymorph, and compact oligomers (oligomer number n = 2-13) as well as extended intermediates (n = 4-11) are present. IR secondary structural analysis reveals that compact conformations adopt turn-like structures, whereas extended oligomers exhibit a significant amount of β-sheet content. This agrees well with previous molecular dynamic simulations and provides direct experimental evidence that unordered off-pathway NFGAIL aggregates up to the size of at least the 13-mer as well as partially folded β-sheet containing oligomers are coexisting.

Molecular interactions of amyloid nanofibrils with biological aggregation modifiers: implications for cytotoxicity mechanisms and biomaterial design

Amyloid nanofibrils are ubiquitous biological protein fibrous aggregates, with a wide range of either toxic or beneficial activities that are relevant to human disease and normal biology. Protein amyloid fibrillization occurs via nucleated polymerization, through non-covalent interactions. As such, protein nanofibril formation is based on a complex interplay between kinetic and thermodynamic factors. The process entails metastable oligomeric species and a highly thermodynamically favoured end state. The kinetics, and the reaction pathway itself, can be influenced by third party moieties, either molecules or surfaces. Specifically, in the biological context, different classes of biomolecules are known to act as catalysts, inhibitors or modifiers of the generic protein fibrillization process. The biological aggregation modifiers reviewed here include lipid membranes of varying composition, glycosaminoglycans and metal ions, with a final word on xenobiotic compounds. The corresponding molecular interactions are critically analysed and placed in the context of the mechanisms of cytotoxicity of the amyloids involved in diverse pathologies and the non-toxicity of functional amyloids (at least towards their biological host). Finally, the utilization of this knowledge towards the design of bio-inspired and biocompatible nanomaterials is explored.

Copper Coordination Features of Human Islet Amyloid Polypeptide: The Type 2 Diabetes Peptide

Human islet amyloid polypeptide (hIAPP) is the major component of amyloid deposits found in pancreatic β-cells of patients with type 2 diabetes (T2D). Copper ions have an inhibitory effect on the amyloid aggregation of hIAPP, and they may play a role in the etiology of T2D. However, deeper knowledge of the structural details of the copper-hIAPP interaction is required to understand the molecular mechanisms involved. Here, we performed a spectroscopic study of Cu(II) binding to hIAPP and several variants, using electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), electronic absorption, and circular dichroism (CD) in the UV-vis region in combination with Born-Oppenheimer molecular dynamics (BOMD) and density functional theory geometry optimizations. We find that Cu(II) binds to the imidazole N1 of His18, the deprotonated amides of Ser19 and Ser20, and an oxygen-based ligand provided by Ser20, either via its hydroxyl group or its backbone carbonyl, while Asn22 might also play a role as an axial ligand. Ser20 plays a crucial role in stabilizing Cu(II) coordination toward the C-terminal, providing a potential link between the S20G mutation associated with early onset of T2D, its impact in Cu binding properties, and hIAPP amyloid aggregation. Our study defines the nature of the coordination environment in the Cu(II)-hIAPP complex, revealing that the amino acid residues involved in metal ion binding are also key residues for the formation of β-sheet structures and amyloid fibrils. Cu(II) binding to hIAPP may lead to the coexistence of more than one coordination mode, which in turn could favor different sets of Cu-induced conformational ensembles. Cu-induced hIAPP conformers would display a higher energetic barrier to form amyloid fibrils, hence explaining the inhibitory effect of Cu ions in hIAPP aggregation. Overall, this study provides further structural insights into the bioinorganic chemistry of T2D.