Understanding the Mechanism of Somatostatin-14 Amyloid Formation In Vitro

Abstract

Somatostatin-14 (SST-14) is a small cyclic peptide hormone secreted by hypothalamus, one of the well-known functions of which in the body is growth hormone inhibition. SST-14 has previously been reported to spontaneously self-assemble in water containing 150 mM NaCl into amyloid-like nanofibrils. Recently, it was shown that SST-14 forms amyloid in presence of glycosaminoglycan heparin at secretory granule relevant conditions. However, the pathway of its amyloid formation in not understood yet. In the present work, we studied the mechanism of SST-14 amyloid formation in vitro as well as in silico, in the presence and absence of heparin. Our in vitro data suggests that SST-14 in presence of heparin, transforms from its monomeric ‘random coil’ conformation to a secondary structure representing amyloid, via a ‘helix-rich intermediate. It was observed in this study that SST-14 adopts a ‘nucleation-dependent polymerization’ mechanism of amyloid formation. We also studied the disulphide bond reduced ‘non-cyclic’ SST-14 (ncSST) form for amyloid formation, and observed that ncSST formed amyloid fibrils instantaneously in presence of heparin compared to the cyclic form. Furthermore, the ncSST fibrils were observed to possess higher conformational and thermal stability relative to the cyclic SST-14 fibrils.Using all-atom molecular dynamics simulations, we studied molecular interactions among SST-14 peptides in the presence and absence of heparin. Our in silico results suggest that heparin induces aggregation of SST-14 by increasing local concentration of the peptides and promotes interpeptide contacts and secondary structure transformation in the peptides. Overall, this study offers the mechanism of somatostatin-14 oligomerization and amyloid formation, which has substantial value in understanding its storage within secretory granules.