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

Abstract

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.

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

“…In addition to c-peptide, Zn displays an ability to complex with small molecule amyloid inhibitor epigallocatechin-gallate (EGCG) and markedly enhance its anti-aggregative and anti-toxic activity against toxic forms of hIAPP [ 117 ]. Zn ions also act as an essential cofactor for the inhibitory action of insulin against hIAPP aggregation [ 118 , 119 ]. Similar to Zn, copper (Cu) (another essential microelement) was found to prevent hIAPP aggregation and toxicity in pancreatic β-cells by preventing its structural transitions from random coil to β-sheet-enriched oligomers and aggregates [ 120 , 121 ].…”

Tuning the rate of aggregation of hIAPP into amyloid using small-molecule modulators of assembly

“…While both YX-I-1 and YX-A-1 significantly alter the aggregation kinetics of wt hIAPP under the conditions employed here, it is important to note that other components in vivo such as metal ions 80 , 81 , insulin 82 , and membranes 83 , can affect the aggregation of hIAPP. It will be interesting to explore how YX-I-1 and YX-A-1 affect hIAPP aggregation in the presence of those components in the future.…”

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

“…In the case of pramlintide, and its membrane disrupting fragment, amylin 1–19 , the zinc(II) ions are coordinated by the imidazole nitrogen at position 18 and the N-terminal amino group of Lys1, leading to the bending of the peptide backbone 16 , 25 . However, as shown in a recent study by Khemtemourian et al, conducted in the presence of a lipid membrane using amylin analogues in which His18 was replaced by other amino acid residues, the presence of zinc(II) ions does not affect the fibrillation process of amylin 26 . It has been confirmed in Brender’s NMR experiment which showed the N-terminus is involved in interactions with the membrane and becomes inaccessible to zinc(II) 27 .…”