Unraveling the underlying mechanisms of reduced amyloidogenic properties in human calcitonin via double mutations

The therapeutic efficacy of peptide-based drugs is commonly hampered by the intrinsic propensity to aggregation. A notable example is human calcitonin (hCT), a peptide hormone comprising 32 amino acids, which is synthesized and secreted by thyroid gland parafollicular cells (C cells). This hormone plays a vital role in regulating blood calcium levels and upholding bone integrity. Despite its physiological importance, utilizing hCT as a drug is hampered by its inclination to form amyloid. To address this limitation, an alternative is provided by salmon calcitonin (sCT), which possesses a lower aggregation propensity. Although sharing the same disulfide bond at the N terminus as hCT, sCT differs from hCT at a total of 16 amino acid positions. However, due to the dissimilarity in sequences, using sCT as a clinical replacement occasionally results in adverse side effects in patients. Earlier investigations have highlighted the significant roles of Tyr-12 and Asn-17 in inducing the formation of amyloid fibrils. By introducing double mutations at these sites, the ability to hinder aggregation can be significantly augmented. This study delves into the oligomerization and helical structure formation of the hCT double mutant (Y12LN17H hCT, noted as DM hCT), as well as two single mutants (Y12L and N17H), aiming to elucidate the mechanism behind hCT fibrillization. In addition, computational prediction tools were employed again to identify potential substitutes. Although the results yielded were not entirely satisfactory, a comparison between the newly examined and previously found hCT double mutants provides insights into the reduced aggregation propensity of the latter. This research endeavor holds the promise of informing the design of more effective therapeutic peptide drugs in the future.