Misfolding of a Human Islet Amyloid Polypeptide at the Lipid Membrane Populates through β-Sheet Conformers without Involving α-Helical Intermediates

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.

Chiral Water Superstructures around Antiparallel β-Sheets Observed by Chiral Vibrational Sum Frequency Generation Spectroscopy

Hydration modulates every aspect of protein structure and function. However, studying water structures in hydration shells remains challenging mostly due to overwhelming background from bulk water. We used vibrational sum frequency generation (SFG) spectroscopy to characterize hydrated films of an antiparallel β-sheet peptide (LK7β) adsorbed on glass slides. The hydrated films give chiral SFG response from water only when the peptide self-assembles into antiparallel β-sheets. Experiments of isotopic labeling, isotopic dilution of water, and H2O-D2O exchange kinetics corroborate the assignments of the chiral SFG response to water stretching modes. Because individual water molecules are achiral, the chiral SFG response indicates formation of chiral superstructures of water around the antiparallel β-sheet, implying that a protein secondary structure can imprint its chirality onto the surrounding water. This result demonstrates chiral SFG spectroscopy as a promising tool for probing water structures in protein hydration and addressing fundamental questions of protein structure-function.

Effect of Imidazolium-Based Ionic Liquids on the Structure and Phase Behavior of Palmitoyl-oleoyl-phosphatidylethanolamine

Among various applications, ionic liquids (ILs) have been used as antimicrobial agents in laboratories, possibly through induction of the leakage of bacteria. A molecular-level understanding of the mechanism that describes how ILs enhance the permeation of membranes is still lacking. In this study, the effects of imidazolium-based ILs with different alky chain lengths on the structure and phase behavior of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE), which is a representative bacteria-membrane-rich lipid, have been investigated. By employing differential scanning calorimetry and synchrotron small- and wide-angle X-ray scattering techniques, we found that ILs with longer alkyl chains influenced the phase behavior more effectively, and lower IL concentrations are needed to induce phase separation for both lamellar liquid crystalline phase and nonlamellar inverted hexagonal phase of POPE. Interestingly, the IL with an alkyl chain of 12 carbon atoms ([C₁₂mim]Cl) shows a difference. It exhibits a stronger disturbing effect on the POPE bilayer structure than [C₁₆mim]Cl, indicating that the ability of ILs to influence the membrane structures is dependent not only on the alkyl chain length of ILs, but also on the degree of matching of the alkyl chain lengths of ILs and lipids. The new lamellar and nonlamellar structures induced by ILs both have smaller repeat distances than that of pure POPE, implying thinner membrane structures. Data of the fluorescence-based vesicle dye leakage assay are consistent with these results, particularly the defects caused by IL-induced phase separation can enhance the membrane permeability markedly. The present work may shed light on our understanding of the antimicrobial mechanism of ILs.

Amyloid Self-Assembly of hIAPP8-20 via the Accumulation of Helical Oligomers, α-Helix to β-Sheet Transition, and Formation of β-Barrel Intermediates

The self-assembly of human islet amyloid polypeptide (hIAPP) into β-sheet-rich nanofibrils is associated with the pathogeny of type 2 diabetes. Soluble hIAPP is intrinsically disordered with N-terminal residues 8-17 as α-helices. To understand the contribution of the N-terminal helix to the aggregation of full-length hIAPP, here the oligomerization dynamics of the hIAPP fragment 8-20 (hIAPP8-20) are investigated with combined computational and experimental approaches. hIAPP8-20 forms cross-β nanofibrils in silico from isolated helical monomers via the helical oligomers and α-helices to β-sheets transition, as confirmed by transmission electron microscopy, atomic force microscopy, circular dichroism spectroscopy, Fourier transform infrared spectroscopy, and reversed-phase high performance liquid chromatography. The computational results also suggest that the critical nucleus of aggregation corresponds to hexamers, consistent with a recent mass-spectroscopy study of hIAPP8-20 aggregation. hIAPP8-20 oligomers smaller than hexamers are helical and unstable, while the α-to-β transition starts from the hexamers. Converted β-sheet-rich oligomers first form β-barrel structures as intermediates before aggregating into cross-β nanofibrils. This study uncovers a complete picture of hIAPP8-20 peptide oligomerization, aggregation nucleation via conformational conversion, formation of β-barrel intermediates, and assembly of cross-β protofibrils, thereby shedding light on the aggregation of full-length hIAPP, a hallmark of pancreatic beta-cell degeneration.

Formation of α-helical and β-sheet structures in membrane-bound human IAPP monomer and the resulting membrane deformation

Upon adsorption on membrane, human IAPP monomer takes conformational changes from coils to α-helices and β-sheets. The helices inserted and β on surface cause different types of membrane deformation, implying two distinct aggregation mechanisms.