Charge manipulation of the human insulin B chain C-terminal to shed light on the complex mechanism of insulin fibrillation

Insulin fibrillation poses a significant challenge in the development and treatment of diabetes. Current efforts to unravel its mechanisms have thus far remained incomplete. To shed light on the intricate processes behind insulin fibrillation, we employed mutagenesis techniques to introduce additional positive charge residues into the C-terminal region of the insulin B chain which plays an important role in insulin dimerization. We employed our investigation with various spectroscopic methods, electron microscopy, and molecular dynamics simulations. These methods allowed us to explore the structure and fibrillation behavior of the engineered B chains following their expression in a bacterial host and successful purification. This manipulation had a pronounced impact on the oligomerization behavior of the insulin B chain. It appears that these mutations delay the formation of the dimeric state in the process of transitioning to larger oligomers, consequently, leading to an alteration in the kinetics of fibrillation. Our findings also indicated that the mutant insulin B chains (Di-R, Di-K, and Di-H) displayed resistance to the initiation of fibrillation. This resistance can be attributed to the repulsive forces generated by the introduced positive charges, which disrupt the attractive interactions favoring nucleation. Notably, the mutant B chains formed shorter and less abundant oligomers and fibrils, which can be ascribed to the alterations induced by repulsion. Our engineered mutant B chains exhibited enhanced stability against stressinduced fibrillation, hinting at their potential utility in the development of new insulin analogs. This study underscores the significance of the C-terminal region in the initial stages of insulin B chain fibrillation, providing valuable insights into the intricate mechanisms involved and their potential pharmaceutical applications.