Assembly behavior of amylin fragment hIAPP19-37 regulated by Au(III) complexes

https://doi.org/10.1016/j.jinorgbio.2019.110807Get rights and content

Highlights

  • Au complexes inhibited the aggregation of amylin fragment 19-37.

  • Au complexes upregulated the peptide induced cytotoxicity.

  • The oligomer formation was resisted by Au complexes.

  • Metal coordination was weakened due to the lack of histidine residue.

Abstract

Human islet amyloid polypeptide (hIAPP, amylin) may self-aggregate and rupture the membrane of β cells, which is closely correlated with the pathogenesis of type 2 diabetes mellitus (T2DM). Hence, suppressing amyloidogenic hIAPP may be beneficial for the treatment of diabetes. As an important part of hIAPP, the fragment hIAPP19-37 was studied in this work to explore their disaggregation and cellular behavior regulation by some selected Au complexes, as follows: dichloro diethyl dithiocarbamate Au complex [AuCl2(DDTC)] (1), dichloro pyrrolidine dithiocarbamate Au complex [AuCl2(PDT)] (2), dichloro 4-4′-dimethyl-2,2′-bipyridyl Au(III) chloride [AuCl2(Me)2bpy]Cl (3), and dichloro 4-4-di-tert-butyl-2,2′-bipyridyl Au(III) chloride [AuCl2(t-Bu)2bpy]Cl (4). The peptide aggregation was observed and analyzed by fluorescence assay, atomic force microscopy (AFM), dynamic light scattering (DLS) and other methods. The assembly behaviors of hIAPP19-37 affected by the four Au complexes indicated that these complexes could effectively inhibit the fibrillation of the peptide and depolymerized the aged peptide into nanoscale particles. These Au compounds also remarkably reduced membrane leakage and cytotoxicity caused by peptide oligomers. An interaction study revealed that the complexes were predominantly bound with hIAPP19-37 through hydrophobic and electrostatic interactions, and metal coordination. The differences among various complexes were compared according to their binding affinity, inhibitory effect, and cellular behavior. Our study offers a potential path for the possible utilization of Au compounds as amyloidosis inhibitors.

Graphical abstract

Au(III) complexes resisted the assembly behavior of amylin fragment hIAPP19-37 (a fragment of hIAPP, from residue 19 to 37).

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Introduction

The amyloid deposition of some proteins and peptides are associated with a series of diseases, such as α-synuclein with Parkinson's disease, amyloid-β protein (Aβ) and tau protein with Alzheimer's disease, and human islet amyloid polypeptide with T2DM [1,2]. hIAPP consists of 37 amino acids and is co-secreted with insulin by β-cells [3,4]. The hIAPP fragment hIAPP19-37 (SSNNFGAILSSTNVGSNTY-NH2) is an important component of the full peptide hIAPP1-37. The differences in the species-specific amino acid sequence of IAPP mainly focus on the sequence 20-29. Compared with the fragment of nonamyloidogenic rat amylin rat-IAPP20-29, hIAPP20-29 has a tendency to spontaneously aggregate. Moreover, there are three aromatic residues in the sequence of hIAPP, two of them are located in the fragment hIAPP19-37, phenylalanine 23 (Phe23) and tyrosine 37 (Tyr37). These aromatic residues may interact with each other and with aromatic compounds by π–π stacking. It has been reported that Phe-Phe interaction within the hydrophobic core make the fibers more stable. Therefore, the fragment hIAPP19-37 acts as a key regulatory sequence to produce amyloidosis [[5], [6], [7], [8], [9]].

Inhibitors of amylin deposition have been extensively studied. Many small organic molecules, natural products, and metal compounds affect the aggregation and depolymerization of amylin through hydrophobic and electrostatic interactions and hydrogen bonding [[9], [10], [11], [12]]. Metals and their complexes, such as the anticancer cisplatin, the anti-inflammatory auranofin, and masses of metal diagnostic reagents, are widely utilized for disease treatment and demonstrate diverse biological functions [[13], [14], [15], [16], [17], [18], [19]]. Metal ions can interact with amylin and have a favorable effect on the speed, strength, and morphology of amylin aggregation [15,20]. Zn(II) ions may resist hIAPP aggregation at low concentration, but they increase hIAPP aggregation at high concentration [15]. Given their advantages of distinct metal center and ligand with different configurations, ruthenium complexes have become potential inhibitors of amyloidosis [21,22]. Au has been widely used for the treatment of various diseases since the ancient times. Au(III), Au(I) compounds, and Au nanoparticles exhibit good antibacterial, antitumor, and anti-amyloidosis activities [[23], [24], [25], [26], [27], [28]]. For example, Au complexes may inhibit hIAPP aggregation and the prion protein (PrP) neuropeptide PrP106-126 via metal coordination as a major binding force. The compound [Au(bipy)Cl2][PF6] regulates hIAPP aggregation through a dimer transient state [29]. However, the effects of Au center and ligand configuration on the assembly behavior and binding affinity of different peptides remain obscure and need further exploration.

In this study, we synthesized four Au complexes (Scheme 1), dichloro diethyl dithiocarbamate Au complex [AuCl2(DDTC)] (1), dichloro pyrrolidine dithiocarbamate Au complex [AuCl2(PDT)] (2), dichloro 4-4′-dimethyl-2,2′-bipyridyl Au(III) chloride [AuCl2(Me)2bpy]Cl (3), dichloro 4-4-di-tert-butyl-2,2′-bipyridyl Au(III) chloride [AuCl2(t-Bu)2bpy]Cl (4), and studied their inhibition on the aggregation of the hIAPP core fragment hIAPP19-37. We aimed to further understand the effect of Au complexes on hIAPP. We also explored the interaction mechanism between the peptide and Au complexes. Although many metal ions and metal complexes affect the peptides' amyloidosis, the effects are twofold, the inhibition or the promotion on peptide aggregation. Gold complexes better inhibit the aggregation of amyloid peptides mainly by metal coordination. Researches also find that some small aromatic organic molecules may inhibit the aggregation and depolymerize the mature amyloid fibrils through hydrophobic and electrostatic interactions. Therefore, considering Au(III) ion which provides good metal coordination, and small organic molecules which provide effective ligands to modify the complex's steric configuration, we chose the four tetra-coordinated Au(III) complexes with different ligand features. These complexes possibly possess favorable conformation advantages as amyloid inhibitors and may show good prospects for biological utilization [[30], [31], [32], [33], [34], [35]]. We employed the thioflavin T (ThT) fluorescence assay, AFM, DLS to detect the effects of Au complexes on hIAPP19-37 aggregation and the depolymerization of the aged peptide aggregates. The intrinsic fluorescence method and electrospray ionization mass spectrometry (ESI-MS) were performed to elucidate the binding affinity of the complexes with hIAPP19-37. Furthermore, liposomal experiments, cell viability determination, and antibody immune analysis were conducted to reveal the influence of these Au complexes on the cellular behaviors of hIAPP19-37 and compare them with other reported metal compounds.

Section snippets

Materials

The sample hIAPP19-37 was chemically synthesized by SynPeptide Co., Ltd. (Shanghai, China). It had >95% purity and was identified by high-performance liquid chromatography and ESI-MS. hIAPP19–37 was treated with hexafluoroisopropanol before use.

Complexes 1, 2, 3, and 4 were synthetized in accordance with previous reports [36,37]. The ligands diethyl dithiocarbamate (DDTC), pyrrolidine dithiocarbamate (PDT), and 4-4′-dimethyl-2,2′-bipyridyl ((Me)2bpy), 4-4′-di-tert-butyl-2,2′-bipyridyl ((t-Bu)2

Synthesis of Au complexes

Au complexes 1, 2, 3, and 4 were synthetized and identified as previously described [36,37]. The UV–vis spectra of these complexes are shown in Fig. S1. The NMR and IR results (data not shown) identified that the products were consistent with the literature and could be used for further experiments.

Influence of Au complexes on hIAPP19-37 aggregation

The dye ThT can bind the peptide aggregates and show intense fluorescent emission peak at 485 nm [41]. Changes in the ThT fluorescence intensity were used to reflect the degree of hIAPP19-37

Discussion

The deposition of hIAPP is a vital factor correlated with T2DM, and the peptide aggregation to form fibers is attained through structural transformation. Peptide conformation after aggregation is distinct from the original state, and additional β-sheet components have been found [52]. Hence, inhibiting the aggregation and disassembling the aggregates of hIAPP may be a feasible way to manage T2DM. As a crucial part of hIAPP, the peptide hIAPP19-37 plays an important role in the deposition of

Conclusion

We studied the inhibition and depolymerization abilities of the Au complexes on hIAPP19-37. The four compounds well inhibited the peptide aggregation and scattered the aggregates into nanoscale particles. Complexes 1 and 2 had relatively better binding affinity with hIAPP19-37 and inhibitory effects on peptide aggregation than complexes 3 and 4. They bind to the peptide mainly through metal coordination, hydrophobic interaction, and electrostatic interaction, as indicated by the experiment's

Abbreviations

    amyloid-β protein

    T2DM

    type 2 diabetes mellitus

    hIAPP

    human islet amyloid polypeptide

    Phe

    phenylalanine

    Tyr

    tyrosine

    PrP

    prion protein

    ThT

    thioflavin T

    AFM

    atomic force microscopy images

    DLS

    dynamic light scattering analysis

    ESI-MS

    electrospray ionization mass spectrometry

    DDTC

    dichloro diethyl dithiocarbamate

    PDT

    dichloro pyrrolidine dithiocarbamate

    (Me)2bpy

    4-4′-dimethyl-2,2′-bipyridyl

    (t-Bu)2bpy

    4-4-Di-tert-butyl-2,2′-bipyridyl

    UV‐Vis

    ultraviolet–visible

    DOPG

    1, 2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium

Acknowledgements

The project is supported by the National Natural Science Foundation of China, China (Nos. 21473251 & 21271185).

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