Structural features of proinsulin C‐peptide oligomeric and amyloid states

Abstract

The formation and structure of proinsulin C-peptide oligomers has been investigated by PAGE, NMR spectroscopy and dynamic light scattering. The results obtained show that C-peptide forms oligomers of different sizes, and that their formation and size distribution is altered by salt and divalent metal ions, which indicates that the aggregation process is mediated by electrostatic interactions. It is further demonstrated that the size distribution of the C-peptide oligomers, in agreement with previous studies, is altered by insulin, which supports a physiologically relevant interaction between these two peptides. A small fraction of oligomers has previously been suggested to be in equilibrium with a dominant fraction of soluble monomers, and this pattern also is observed in the present study. The addition of modest amounts of sodium dodecyl sulphate at low pH increases the relative amount of oligomers, and this effect was used to investigate the details of both oligomer formation and structure by a combination of biophysical techniques. The structural properties of the SDS-induced oligomers, as obtained by thioflavin T fluorescence, CD spectroscopy and IR spectroscopy, demonstrate that soluble aggregates are predominantly in b-sheet conformation, and that the oligomerization process shows characteristic features of amyloid formation. The formation of large, insoluble, b-sheet amyloid-like structures will alter the equilibrium between monomeric C-peptide and oligomers. This leads to the conclusion that the oligomerization of C-peptide may be relevant also at low concentrations.

Effect of Mid-infrared Free-Electron Laser Irradiation on Refolding of Amyloid-Like Fibrils of Lysozyme into Native Form

“…For example, amyloid fibrils are formed by amyloid beta Aβ (40–43) [7, 21], tau protein [11], polyglutamine [22], transthyretin [35], α-synuclein [24], prion protein [19], and β 2 -microgulobulin [15]. In addition, it has been recently reported that sequentially non-homologous proteins, insulin [34], proinsulin c-peptide [14], lysozyme [6], calcitonin [8], and myoglobin [3] may form amyloid-like fibrils under certain conditions. These observations imply that amyloid structures are common to the intrinsic structures of various proteins, although the mechanism of their formation is not completely understood.…”

pH-Dependent Interaction between C-Peptide and Phospholipid Bicelles

“…Further, residues Q9-L12, residues G15-A18 and residues Q22-A25 were all shown to have structural preferences in the NMR-derived ensemble average [13]. It has recently been demonstrated that C-peptide also has the ability, under certain conditions, such as low pH, to form β -sheet structure, resembling amyloid structures [14, 15]. The peptide forms predominantly low-order oligomers [14], but very low concentrations of amyloid-like structures may also form [15].…”

“…It has recently been demonstrated that C-peptide also has the ability, under certain conditions, such as low pH, to form β -sheet structure, resembling amyloid structures [14, 15]. The peptide forms predominantly low-order oligomers [14], but very low concentrations of amyloid-like structures may also form [15]. The formation of amyloid structure can be enhanced in the presence of subcritical micelle concentration (CMC) amounts of SDS (at low pH), while SDS in amounts above the CMC, on the other hand, promote a more α -helical structure [15].…”

“…The peptide forms predominantly low-order oligomers [14], but very low concentrations of amyloid-like structures may also form [15]. The formation of amyloid structure can be enhanced in the presence of subcritical micelle concentration (CMC) amounts of SDS (at low pH), while SDS in amounts above the CMC, on the other hand, promote a more α -helical structure [15]. …”

Encephalopathies Accompanying Type 1 and Type 2 Diabetes

“…The net effect of insulin resistance and hyperinsulinemia is therefore increased levels of intracellular and extracellular Aβ levels, respectively. Interestingly, recent data suggest that oligomeric C-peptide may promote amyloid states [ 116 ]. Infl ammation with activation of microglia promotes Aβ accumulation and amyloid precursor protein (APP) expression and cleavage increase with oxidative stress [ 117 , 118 ].…”