Co-assembly of human islet amyloid polypeptide (hIAPP)/insulin

The pathogenesis of type II diabetes can be linked to cosecreted hIAPP/insulin interacting with cell membranes. Here we investigate the nanostructures by co-assembling hIAPP and insulin on surfaces. By tuning the hIAPP/insulin ratio, atomic force microscopy reveals the resulting nanostructure morphology changes from fibrils to oligomers, to annular. Implications for in vivo studies are discussed.     

二价铜离子对人胰岛淀粉样蛋白(hIAPP)(11～28)多肽聚集的影响

人胰岛淀粉样蛋白(hIAPP)与Ⅱ型糖尿病(T2DM)密切相关,被认为是导致胰岛β细胞凋亡的致病因素之一,研究发现环境因素(如金属离子、pH值和温度等)对hIAPP的聚集过程有很大影响。本文采用多种生物物理的实验方法,研究了二价铜离子对hIAPP及其片段聚集的影响。原子力显微镜(AFM)和硫代黄素T(ThT)荧光的测量表明,铜离子能够明显地抑制hIAPP(11~28)聚集成纤维,其抑制程度随铜离子浓度的增加而明显加剧。显微傅里叶变换红外光谱(Micro-FTIR)的结果表明,铜离子能够抑制hIAPP多肽中α螺旋结构向β折叠的转变。另外,氨基酸定点突变实验结果表明,hIAPP(11~28)中的组氨酸(His18)可能对多肽的聚集行为和金属铜离子的相互作用起到了决定性的影响。     

Aβ40和hIAPP在溶液和表面的成核与交叉成核聚集行为

阿尔茨海默氏病(AD)和2型糖尿病(T2DM)是常见的由蛋白质错误折叠引起的疾病，作为与此二者相关的致病蛋白，淀粉样β蛋白(Aβ)和人胰岛淀粉样多肽(hIAPP)的交叉聚集行为暗示了AD和T2DM的相关性。然而，Aβ和hIAPP在体内的交叉聚集过程尚不明确。为了更好地模拟体内环境特征，即同时存在不同形式的淀粉样蛋白聚集体，且少量的聚集体附着在血管壁上会成为聚集过程的种子，本文以硫代黄素T荧光测定，原子力显微镜，圆二色光谱，石英晶体微天平以及MTT法作为研究手段，探究了Aβ和hIAPP在溶液和固体表面的成核与交叉成核聚集行为。结果表明，少量的Aβ40和hIAPP种子(单体浓度的1/50)即可显著改变异源聚集的聚集路径，形成具有不同形态且含有更多β-折叠结构的异源聚集体，导致更高的细胞毒性。溶液和固体表面上的结果均证明异源成核聚集效率低于同源聚集，且异源聚集的特征很大程度上取决于种子类型。此外，不同于溶液中所得结果，hIAPP种子在固体表面的交叉成核聚集效率显著高于Aβ40种子，证明了界面性质对交叉聚集过程的影响。这些结论对于理解淀粉样蛋白交叉聚集过程具有重要意义。     

Structural Effects of L16Q,S20G,and L16Q‐S20G Mutations on hIAPP: A Comparative Molecular Dynamics Study

The conformation change picture of human islet amyloid polypeptide (hIAPP) is outlined using molecular dynamics simulation, and the structural influences of L16Q, S20G, and L16Q‐S20G mutations on the conformation of hIAPP are analyzed. Particularly, the conformational changes of the amyloidogenic‐related regions of residues 15–17 and 20–29 are emphasized. Our studies find that, for WT hIAPP, residues 15–17 always maintain a stable α‐helix structure, residues 20–25 structurally fluctuate between turn and 5‐helix, and residues 26–29 mainly adopt coil and bend structures. The hydrogen bonds between the polar groups of hIAPP, long‐rang van der Waals forces between the residues, and hydrophobic interactions between the residues of hIAPP are important driving forces to maintain the secondary structure of hIAPP. The replacement of leucine16 by glutamine stabilizes the helix structure of residues 15–17 and 20–23 of hIAPP monomer, and the structure of residues 24–29 fluctuates between helix and turn. The relatively stable helix structures of residues 15–17 and 20–29 are supposed to be beneficial for L16Q hIAPP to resist the aggregation as observed in the experiment. The substitution of serine20 by glycine drives residues 15–17 and 20–29 of hIAPP to transform from helix structure to β‐strands or coil structures with higher extension and flexibility, which may promote the aggregation of hIAPP as the experiments reported. These results are significant to understand the aggregation mechanism of hIAPP monomer into the dimer, trimer, oligomers and fibrils associated with the type 2 diabetes at the atomic level.     

Studies on the cross‐interaction between hIAPP and Aβ25–35 and the aggregation process in binary mixture by electrospray ionization‐ion mobility‐mass spectrometry

A wealth of epidemiological evidence indicates a strong link between type 2 diabetes (T2D) and Alzheimer's disease (AD). The fiber deposition with cross‐β‐sheet structure formed by self‐aggregation and misfolding of amyloidogenic peptides is a common hallmark of both diseases. For the patients with T2D, the fibrils are mainly found in the islets of Langerhans that results from the accumulation of human islet amyloid polypeptide (hIAPP). The major component of aggregates located in the brain of AD patients is amyloid‐β (Aβ). Many biophysical and physiological properties are shared by hIAPP and Aβ, and both peptides show similar cytotoxic mechanisms. Therefore, it is meaningful to investigate the possible cross‐interactions of hIAPP and Aβ in both diseases. In this article, the segment 25–35 of Aβ was selected because Aβ_25–35 was a core region in the process of amyloid formation and showed similar aggregation tendency and toxicity with full‐length Aβ. The electrospray ionization‐ion mobility‐mass spectrometry analysis and thioflavin T fluorescence kinetic analysis combined with transmission electron microscopy were used to explore the effects of the coexistence of Aβ_25–35 and hIAPP on the self‐aggregation of both peptides and whether there was co‐assembly in fibrillation. The results indicated that the aggregation of hIAPP and Aβ_25–35 had two nucleation stages in the binary mixtures. hIAPP and Aβ_25–35 had a high binding affinity and a series of hetero‐oligomers formed in the mixtures of hIAPP and Aβ_25–35 in the early stage. The cross‐reaction between hIAPP monomers and Aβ_25–35 monomers as well as a little of oligomers during primary nucleation stage could accelerate the aggregation of Aβ_25–35. However, owing to the obvious difference in aggregation ability between hIAPP and Aβ_25–35, this cross‐interaction had no significant impact on the self‐assembly of hIAPP. Our study may offer a better understanding for exploring the molecular mechanism of the association between AD and T2D observed in clinical and epidemiological studies and developing therapeutic strategies against amyloid diseases.     

The amyloid formation mechanism in human IAPP: dimers have β-strand monomer-monomer interfaces

Early oligomerization of human IAPP (hIAPP) is responsible for β-cell death in the pancreas and is increasingly considered a primary pathological process linked to Type II Diabetes (T2D). Yet, the assembly mechanism remains poorly understood, largely due to the inability of conventional techniques to probe distributions or detailed structures of early oligomeric species. Here, we describe the first experimental data on the isolated and unmodified dimers of human (hIAPP) and nonamyloidogenic rat IAPP (rIAPP). The experiments reveal that the human IAPP dimers are more extended than those formed by rat IAPP and likely descend from extended monomers. Independent all-atom molecular dynamics simulations show that rIAPP forms compact helix and coil rich dimers, whereas hIAPP forms β-strand rich dimers that are generally more extended. Also, the simulations reveal that the monomer-monomer interfaces of the hIAPP dimers are dominated by β-strands and that β-strands can recruit coil or helix structured regions during the dimerization process. Our β-rich interface contrasts with an N-terminal helix-to-helix interface proposed in the literature but is consistent with existing experimental data on the self-interaction pattern of hIAPP, mutation effects, and inhibition effects of the N-methylation in the mutation region.     

Amyloid fiber formation and membrane disruption are separate processes localized in two distinct regions of IAPP, the type-2-diabetes-related peptide

Aggregation of Islet Amyloid Polypeptide (IAPP) has been implicated in the development of type II diabetes. Because IAPP is a highly amyloidogenic peptide, it has been suggested that the formation of IAPP amyloid fibers causes disruption of the cellular membrane and is responsible for the death of beta-cells during type II diabetes. Previous studies have shown that the N-terminal 1-19 region, rather than the amyloidogenic 20-29 region, is primarily responsible for the interaction of the IAPP peptide with membranes. Liposome leakage experiments presented in this study confirm that the pathological membrane disrupting activity of the full-length hIAPP is also shared by hIAPP 1-19. The hIAPP 1-19 fragment at a low concentration of peptide induces membrane disruption to a near identical extent as the full-length peptide. At higher peptide concentrations, the hIAPP 1-19 fragment induces a greater extent of membrane disruption than the full-length peptide. Similar to the full-length peptide, hIAPP 1-19 exhibits a random coil conformation in solution and adopts an alpha-helical conformation upon binding to lipid membranes. However, unlike the full-length peptide, the hIAPP 1-19 fragment did not form amyloid fibers when incubated with POPG vesicles. These results indicate that membrane disruption can occur independently from amyloid formation in IAPP, and the sequences responsible for amyloid formation and membrane disruption are located in different regions of the peptide.     

Modeling the interface between islet amyloid polypeptide and insulin-based aggregation inhibitors: correlation to aggregation kinetics and membrane damage

Human islet amyloid polypeptide (hIAPP) forms cytotoxic fibrils in type-2 diabetes and insulin is known to inhibit formation of these aggregates. In this study, a series of insulin-based inhibitors were synthesized and assessed for their ability to slow aggregation and impact hIAPP-induced membrane damage. Computational studies were employed to examine the underlying mechanism of inhibition. Overall, all compounds were able to slow aggregation at sufficiently high concentrations (10× molar excess); however, only two peptides showed any inhibitory capability at the 1:1 molar ratio (EALYLV and VEALYLV). The results of density functional calculations suggest this is due to the strength of a salt bridge formed with the Arg11 side chain of hIAPP and the inhibitors' ability to span from the Arg11 to past the Phe15 residue of hIAPP, blocking one of the principal amyloidogenic regions of the molecule. Unexpectedly, slowing fibrillogenesis actually increased damage to lipid membranes, suggesting that the aggregation process itself, rather than the fibrilized peptide, may be the cause of cytotoxicity in vivo.     

Tracking fiber formation in human islet amyloid polypeptide with automated 2D-IR spectroscopy

Amyloid forming proteins have been implicated in many human diseases. The kinetics of amyloid fiber formation are of particular interest because evidence points to intermediate folding structures as potential cytotoxic species. The standard methods for monitoring the kinetics are to use fluorescence or circular dichroism spectroscopy, which do not uniquely resolve secondary structures. In this work, we use a new technology for rapidly scanning 2D-IR spectra that allows us to follow the fiber formation kinetics of the human islet amyloid polypeptide (hIAPP) that is involved in type II diabetes. Spectroscopic markers are identified that uniquely monitor random coil versus beta-sheet secondary structures as well as probe beta-sheet elongation and stacking. Our measurements provide more rigorous kinetics for the secondary structure evolution of amyloid formation than is available with other techniques.     

Effect of a Short Peptide with Alternating L- and D-Amino Acid on the Aggregation and Membrane Damage of hIAPP

Alpha-sheet is believed to be a significant structural component, formed in the fibrillation process of the amyloid peptide. However, the knowledge about the role of α-sheet played in the amyloidosis and toxicity is lack. In this work, we modified a short peptide derived from the core region of human islet amyloid polypetide(hIAPP, hIAPP_18-27) with an alternating D-amino acid replacement and investigated the effects of the L/D alternating peptide on the fibrillar aggregation and the membrane damage of hIAPP using NMR, ThT fluorescence assay, circular dichroism(CD), transmission electron microscopy(TEM) and leakage assay, and compared the results with those of hIAPP_18-27without D-amino acid replacement. We show that the short peptide with alternating L- and D-amino acids forms an α-sheet structure and is more potent in promoting the fibrillation of hIAPP and reducing the ability of hIAPP to disrupt the membrane composed of POPG and POPC[1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine] 1:4 lipids than the short peptide with all L-amino acids in a random coil structure. The higher potency of the D/L alternating peptide in these activities is attributed to its ability to induce the α-sheet-like structure in the core region of hIAPP and block the interaction of hIAPP with the membrane more effectively.     

Molecular dynamics simulation study on the inhibitory effects of choline-O-sulfate on hIAPP protofibrilation

Type 2 diabetes mellitus (T2Dm) is a neurodegenerative disease, which occurs due to the self-association of human islet amyloid polypeptide (hIAPP), also known as human amylin. It was reported experimentally that choline-O-sulfate (COS), a small organic molecule having a tertiary amino group and sulfate group, can prevent the aggregation of human amylin without providing the mechanism of the action of COS in the inhibition process. In this work, we investigate the influence of COS on the full-length hIAPP peptide by performing 500 ns classical molecular dynamics simulations. From pure water simulation (without COS), we have identified the residues 11–20 and 23–36 that mainly participate in the fibril formation, but in the presence of 1.07 M COS these residues become totally free of β-sheet conformation. Our results also show that the sulfate oxygen of COS directly interacts with the peptide backbone, which leads to the local disruption of peptide–peptide interaction. Moreover, the presence of favorable peptide-COS vdW interaction energy and high coordination number of COS molecules in the first solvation shell of the peptide indicates the hydrophobic solvation of the peptide residues by COS molecules, which also play a crucial role in the prevention of β-sheet formation. Finally, from the potential of mean force (PMFs) calculations, we observe that the free energy between two peptides is more negative in the absence of COS and with increasing concentration of COS, it becomes unfavorable significantly indicating that the peptide dimer formation is most stable in pure water, which becomes less favorable in the presence of COS. © 2019 Wiley Periodicals, Inc.     

Environmental factors differently affect human and rat IAPP: conformational preferences and membrane interactions of IAPP17-29 peptide derivatives

Interest in the 37-residue human islet amyloid polypeptide (hIAPP) is related to its ability to form amyloid deposits in patients affected by type II diabetes. Attempts to unravel the molecular features of this disease have indicated several regions of this polypeptide to be responsible for either the ability to form insoluble fibrils or the abnormal interaction with membranes. To extend these studies to peptides that enclose His18, whose ionization state is believed to play a key role in the aggregation of hIAPP, we report on the synthesis of two peptides, hIAPP17-29 and rIAPP17-29, encompassing the 17-29 sequences of human and rat IAPP, respectively, as well as on their conformational features in water and in several membrane-mimicking environments as revealed by circular dichroism (CD) and 2D-NMR studies. hIAPP17-29 adopts a beta-sheet structure in water and its solubility increases at low pH. Anionic sodium dodecyl sulfate (SDS) micelles promoted the formation of an alpha-helical structure in the peptide chain, which was poorly influenced by pH variations. rIAPP17-29 was soluble and unstructured in all the environments investigated, with a negligible effect of pH. The membrane interactions of hIAPP17-29 and rIAPP17-29 were assessed by recording differential scanning calorimetry (DSC) measurements aimed at elucidating the peptide-induced changes in the thermotropic behaviour of zwitterionic (DPPC) and negatively charged (DPPC/DPPS 3:1) model membranes (DPPC=1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPS=1,2-dipalmitoyl-sn-glycero-3-phosphoserine). Results of DSC experiments demonstrated the high potential of hIAPP17-29 to interact with DPPC membranes. hIAPP17-29 exhibited a negligible affinity for negatively charged DPPC/DPPS model membranes at neutral pH. On the other hand, rIAPP17-29 did not interact with neutral or negatively charged membranes. The role played by His18 in the modulation of the biophysical properties of this hIAPP region was assessed by synthesising and studying the R18HrIAPP17-29 peptide; the replacement of a single Arg with a His residue is not sufficient to induce either amyloidogenic propensity or membrane interaction in this region. The results show that the 17-29 domain of hIAPP has many properties of the full-length protein "in vitro" and this opens up new perspectives for both research and eventually therapy.     

Rapid Diagnosis of Type II Diabetes Using Fourier Transform Mid-Infrared Attenuated Total Reflection Spectroscopy Combined with Support Vector Machine

Type II diabetes was diagnosed by Fourier transform mid-infrared (FTMIR) attenuated total reflection (ATR) spectroscopy in combination with support vector machine (SVM). Spectra of serum samples from 65 patients with clinical confirmed type II diabetes mellitus and 55 healthy volunteers were acquired using ATR-FTMIR and were first pretreated by three pretreatments (Savitzky–Golay smoothing, multiple scattering correction, and wavelet transforms algorithms) to reduce the interfering information before establishing the SVM models. The parameters of SVM (penalty factor C and kernel function parameter gamma) were optimized to improve the generalization abilities of the models. A grid search method (GS), genetic algorithm (GA), and particle swarm optimization (PSO) algorithm, were used to find out the optimal parameter values. The results showed that the maximum accuracies were 95.74, 97.87, and 89.36% for the optimized GS, GA, and PSO algorithms. The maximum sensitivities were 96, 100, and 92, and the maximum specificity were 95.45, 95.45, and 86.36%, respectively. The results indicated that the accuracy of type II diabetes was improved using the GS, GA, and PSO algorithms for optimizing the SVM parameters. The GA was found to be slightly better than the GS and PSO. The results of the experiment confirmed that the combination of the ATR-FTMIR spectroscopy and SVM was able to rapidly and accurately diagnose type II diabetes without reagents.     

Chiral sum frequency generation spectroscopy for characterizing protein secondary structures at interfaces

In situ and real-time characterization of protein secondary structures at interfaces is important in biological and bioengineering sciences, yet remains technically challenging. In this study, we used chiral sum frequency generation (SFG) spectroscopy to establish a set of vibrational optical markers for characterizing protein secondary structures at interfaces. We discovered that the N-H stretches along the peptide backbones of α-helices can be detected in chiral SFG spectra. We further observed that the chiral vibrational signatures of the N-H stretch together with the peptide amide I are unique to α-helix, β-sheet, and random coil at interfaces. Using these chiral vibrational signatures, we studied the aggregation of human islet amyloid polypeptide (hIAPP), which is implicated in type II diabetes. We observed in situ and in real time the misfolding of hIAPP from random coils to α-helices and then β-sheets upon interaction with a lipid-water interface. Our findings show that chiral SFG spectroscopy is a powerful tool to follow changes in protein conformations at interfaces and identify interfacial protein secondary structures that elude conventional techniques.     

Molecular Mechanisms of Amylin Turnover,Misfolding and Toxicity in the Pancreas

Amyloidosis is a common pathological event in which proteins self-assemble into misfolded soluble and insoluble molecular forms, oligomers and fibrils that are often toxic to cells. Notably, aggregation-prone human islet amyloid polypeptide (hIAPP), or amylin, is a pancreatic hormone linked to islet β-cells demise in diabetics. The unifying mechanism by which amyloid proteins, including hIAPP, aggregate and kill cells is still matter of debate. The pathology of type-2 diabetes mellitus (T2DM) is characterized by extracellular and intracellular accumulation of toxic hIAPP species, soluble oligomers and insoluble fibrils in pancreatic human islets, eventually leading to loss of β-cell mass. This review focuses on molecular, biochemical and cell-biology studies exploring molecular mechanisms of hIAPP synthesis, trafficking and degradation in the pancreas. In addition to hIAPP turnover, the dynamics and the mechanisms of IAPP–membrane interactions; hIAPP aggregation and toxicity in vitro and in situ; and the regulatory role of diabetic factors, such as lipids and cholesterol, in these processes are also discussed.     

Probing amyloid fibril formation of the NFGAIL peptide by computer simulations

Amyloid fibril formation, as observed in Alzheimer's disease and type II diabetes, is currently described by a nucleation-condensation mechanism, but the details of the process preceding the formation of the nucleus are still lacking. In this study, using an activation-relaxation technique coupled to a generic energy model, we explore the aggregation pathways of 12 chains of the hexapeptide NFGAIL. The simulations show, starting from a preformed parallel dimer and ten disordered chains, that the peptides form essentially amorphous oligomers or more rarely ordered beta-sheet structures where the peptides adopt a parallel orientation within the sheets. Comparison between the simulations indicates that a dimer is not a sufficient seed for avoiding amorphous aggregates and that there is a critical threshold in the number of connections between the chains above which exploration of amorphous aggregates is preferred.     

New Insight in Copper‐Ion Binding to Human Islet Amyloid: The Contribution of Metal‐Complex Speciation To Reveal the Polypeptide Toxicity

Type‐2 diabetes (T2D) is considered to be a potential threat on a global level. Recently, T2D has been listed as a misfolding disease, such as Alzheimer's and Parkinson's diseases. Human islet amyloid polypeptide (hIAPP) is a molecule cosecreted in pancreatic β cells and represents the main constituent of an aggregated amyloid found in individuals affected by T2D. The trace‐element serum level is significantly influenced during the development of diabetes. In particular, the dys‐homeostasis of Cu²⁺ ions may adversely affect the course of the disease. Conflicting results have been reported on the protective role played by complex species formed by Cu²⁺ ions with hIAPP or its peptide fragments in vitro. The histidine (His) residue at position 18 represents the main binding site for the metal ion, but contrasting results have been reported on other residues involved in metal‐ion coordination, in particular those toward the N or C terminus. Sequences that encompass regions 17–29 and 14–22 were used to discriminate between the two models of the hIAPP coordination mode. Due to poor solubility in water, poly(ethylene glycol) (PEG) derivatives were synthesized. A peptide fragment that encompasses the 17–29 region of rat amylin (rIAPP) in which the arginine residue at position 18 was substituted by a histidine residue was also obtained to assess that the PEG moiety does not alter the peptide secondary structure. The complex species formed by Cu²⁺ ions with Ac‐PEG‐hIAPP(17–29)‐NH₂, Ac‐rIAPP(17–29)R18H‐NH₂, and Ac‐PEG‐hIAPP(14–22)‐NH₂ were studied by using potentiometric titrations coupled with spectroscopic methods (UV/Vis, circular dichroism, and EPR). The combined thermodynamic and spectroscopic approach allowed us to demonstrate that hIAPP is able to bind Cu²⁺ ions starting from the His18 imidazole nitrogen atom toward the N‐terminus domain. The stability constants of copper(II) complexes with Ac‐PEG‐hIAPP(14–22)‐NH₂ were used to simulate the different experimental conditions under which aggregate formation and oxidative stress of hIAPP has been reported. Speciation unveils: 1) the protective role played by increased amounts of Cu²⁺ ions on the hIAPP fibrillary aggregation, 2) the effect of adventitious trace amounts of Cu²⁺ ions present in phosphate‐buffered saline (PBS), and 3) a reducing fluorogenic probe on H₂O₂ production attributed to the polypeptide alone.     

Complex of calcium receptor blocker nifedipine with glycyrrhizic acid

Physicochemical methods were used to explore the regularities of complexing between the calcium channel blocker nifedipine (NF) and pharmaceutically acceptable complex-forming glycyrrhizic acid (GA) in view of the discovered influence of GA on the therapeutic activity of NF. 1H NMR (including relaxation measurements) and UV-vis spectra have produced illustrative evidence that NF forms stable complexes with GA within a wide concentration range, from 0.05 to 5 mM. At low GA concentrations, below 0.5 mM, NF forms an inclusion complex where each NF molecule is bound by two molecules of GA. Computer simulations of the NMR experimental data have shown that, in aqueous solution, the stability constant of this complex, K, is about 10(5) M(-1). At higher concentrations, GA forms large micelle-like aggregates which increase the water solubility of NF. Quenching of chemically induced dynamic nuclear polarization effects in the photoinduced interaction of the NF-GA complex with tyrosine suggests that complex formation with GA completely blocks the single electron-transfer step between NF and the amino acid. This, arguably, could explain the increased therapeutic activity of GA complexes, since GA might protect the drug molecule from the reaction with amino acid residues of the receptor binding site.     

Structural insights into the polymorphism of amyloid-like fibrils formed by region 20-29 of amylin revealed by solid-state NMR and X-ray fiber diffraction

Many unrelated proteins and peptides can assemble into amyloid or amyloid-like nanostructures, all of which share the cross-beta motif of repeat arrays of beta-strands hydrogen-bonded along the fibril axis. Yet, paradoxically, structurally polymorphic fibrils may derive from the same initial polypeptide sequence. Here, solid-state nuclear magnetic resonance (SSNMR) analysis of amyloid-like fibrils of the peptide hIAPP 20-29, corresponding to the region S (20)NNFGAILSS (29) of the human islet amyloid polypeptide amylin, reveals that the peptide assembles into two amyloid-like forms, (1) and (2), which have distinct structures at the molecular level. Rotational resonance SSNMR measurements of (13)C dipolar couplings between backbone F23 and I26 of hIAPP 20-29 fibrils are consistent with form (1) having parallel beta-strands and form (2) having antiparallel strands within the beta-sheet layers of the protofilament units. Seeding hIAPP 20-29 with structurally homogeneous fibrils from a 30-residue amylin fragment (hIAPP 8-37) produces morphologically homogeneous fibrils with similar NMR properties to form (1). A model for the architecture of the seeded fibrils is presented, based on the analysis of X-ray fiber diffraction data, combined with an extensive range of SSNMR constraints including chemical shifts, torsional angles, and interatomic distances. The model features a cross-beta spine comprising two beta-sheets with an interface defined by residues F23, A25, and L27, which form a hydrophobic zipper. We suggest that the energies of formation for fibril form containing antiparallel and parallel beta-strands are similar when both configurations can be stabilized by a core of hydrophobic contacts, which has implications for the relationship between amino acid sequence and amyloid polymorphism in general.     

Histidine Tautomerism Driving Human Islet Amyloid Polypeptide Aggregation in the Early Stages of Diabetes Mellitus Progression: Insight at the Atomistic Level

Early oligomerization of human islet amyloid polypeptide (hIAPP), which is accountable for β-cell death, has been implicated in the progression of type 2 diabetes mellitus. Some researches have shown the connection between hIAPP and Alzheimer's disease as well. However, the mechanism of peptide accumulation and associated cytotoxicity remains unclear. Due to the unique properties and significant role of histidine in protein sequences, here for the first time, the tautomeric effect of histidine at the early stages of amylin misfolding was investigated via molecular dynamics simulations. Considering Tau and Pi tautomeric forms of histidine (Tau and Pi tautomers are denoted as ϵ and δ, respectively), simulations were performed on two possible isomers of amylin. Our analysis revealed a higher probability of transient α-helix generation in the δ isomer in monomeric form. In dimeric forms, the δδ and δϵ conformations showed an elevated amount of α-helix and lower coil in comparison to the ϵϵ dimer. Due to the significant role of α-helix in membrane disruption and transition to β-sheet structure, these results may imply a noticeable contribution of the δ isomer and the δδ and δϵ dimers rather than ϵ and ϵϵ conformations in the early stages of diabetes initiation. Our results may aid in elucidating the hIAPP self-association process in the etiology of amyloidosis.