Viktoria Babenko

Chiral superstructures of insulin amyloid fibrils

Hydrodynamic forces are capable of inducing structural order in dispersed solid phases, and of causing symmetry-breaking when chiral crystals precipitate from an achiral liquid phase. Until it was observed upon vortex-assisted fibrillation of insulin, such behavior had been thought to be confined to few unbiological systems. In this paper we are discussing chiroptical properties of two chiral variants of insulin amyloid, termed +ICD and -ICD, which form during the process of chiral bifurcation in vortexed solutions of aggregating insulin. As conventional measurements of circular dichroism of solid, anisotropic substances are particularly vulnerable to overlapping influences of linear birefringence and linear dichroism, we have employed complementary tools including dedicated universal chiroptical spectrophotometer to rule out such artifacts. We propose that the strong chiroptical properties of +ICD and -ICD insulin fibrils are an aspect of genuine superstructural chirality of amyloid fibrils and of powerful excitonic couplings taking place within them. A comparison of thioflavin T complexes with fibrils formed by insulin and polyglutamic acid suggests that the extrinsic Cotton effect stemming from the level of single twisted dye molecules is weaker, although diagnostically useful, and cannot account for the overall magnitude of ICD of the dye bound to ±ICD insulin amyloid.

Thioflavin T: Electronic Circular Dichroism and Circularly Polarized Luminescence Induced by Amyloid Fibrils

The circularly polarized luminescence (CPL) spectrum of thioflavin T (ThT) bound to insulin amyloid fibrils has been measured for the first time. It has been found that the samples exhibiting induced circular dichroism (CD) retain the optical activity in the CPL spectra, with the same sign of the rotatory strength. The fluorescence dissymmetry factor is substantial (of the order of magnitude 10(-2) ). Unlike in the corresponding CD and absorption spectra, there is no shift of the CPL band with respect to the fluorescence band. It has been verified that the measured CPL spectra are free from artifacts from circularly polarized scattering of emitted light by conducting additional measurements in a medium with a refractive index similar to insulin (methylsalicylate). The CD and CPL spectra have been interpreted by means of density functional calculations carried out for ThT in its ground and first excited states in different dielectric environments and for ThT interacting with an aromatic ring. It has been found that the presence of an aromatic ring close to the ThT molecule induces Cotton effects of the same order of magnitude as the stabilization of one enantiomeric conformer. Thus, it is expected that both mechanisms contribute to the induced CD and CPL effect to a similar degree.

Dimethyl Sulfoxide Induced Destabilization and Disassembly of Various Structural Variants of Insulin Fibrils Monitored by Vibrational Circular Dichroism

Dimethyl sulfoxide (DMSO) induced destabilization of insulin fibrils has been previously studied by Fourier transform infrared spectroscopy and interpreted in terms of secondary structural changes. The variation of this process for fibrils with different types of higher-order morphological structures remained unclear. Here, we utilize vibrational circular dichroism (VCD), which has been reported to provide a useful biophysical probe of the supramolecular chirality of amyloid fibrils, to characterize changes in the macroscopic chirality following DMSO-induced disassembly for two types of insulin fibrils formed under different conditions, at different reduced pH values with and without added salt and agitation. We confirm that very high concentrations of DMSO can disaggregate both types of insulin fibrils, which initially maintained a β-sheet conformation and eventually changed their secondary structure to a disordered form. The two types responded to varying concentrations of DMSO, and disaggregation followed different mechanisms. Interconversion of specific insulin fibril morphological types also occurred during the destabilization process as monitored by VCD. With transmission electron microscopy, we were able to correlate the changes in VCD sign patterns to alteration of morphology of the insulin fibrils.

Vortex-induced amyloid superstructures of insulin and its component A and B chains.

Insulin is an amyloid-forming polypeptide built of two disulfide-linked chains (A and B), both themselves amyloidogenic. An interesting property of insulin is that agitation strongly influences the course of its aggregation, resulting in characteristic chiral superstructures of amyloid fibrils. Here, we investigate the self-assembly of these superstructures by comparing the quiescent and vortex-assisted aggregation of insulin and its individual A and B chains in the presence or absence of reducing agent tris(2-carboxyethyl)phosphine (TCEP). Our study shows that only the B chain in the presence of TCEP is converted into aggregates with morphology (according to atomic force microscopy) and optical activity (manifested as an extrinsic Cotton effect induced in bound thioflavin T) characteristic of amyloid superstructures that are normally formed by insulin in the absence of TCEP. In contrast to more rigid B-peptide fibrils, elongated aggregates of the A peptide become amorphous upon agitation. Moreover, the aggregation of equimolar mixture of both peptides does not produce highly ordered entities. Our results suggest that the dynamics of the B chain are the driving force for the assembly of superstructures, with the A chain being complicit as long as its own dynamics are controlled by the firm attachment to the B chain provided by the intact covalent structure of insulin.

Conformational memory effect reverses chirality of vortex-induced insulin amyloid superstructures.

Formation of amyloid fibrils is often associated with intriguing far-from-equilibrium phenomena such as conformational memory effects or flow-driven self-assembly. Insulin is a model amyloidogenic polypeptide forming distinct structural variants of fibrils, which self-propagate through seeding. According to infrared absorption, fibrils from bovine insulin ([BI]) and Lys(B31)-Arg(B32) human insulin analogue ([KR]) cross-seed each other and imprint distinct structural features in daughter fibrils. In the absence of preformed [KR] amyloid seeds, bovine insulin agitated at 60 °C converts into chiral amyloid superstructures exhibiting negative extrinsic Cotton effect in bound thioflavin T. However, when agitated bovine insulin is simultaneously cross-seeded with [KR] amyloid, daughter fibrils reveal a positive extrinsic Cotton effect. Our study indicates that dramatic changes in global properties of amyloid superstructures may emerge from subtle conformational-level variations in single fibrils (e.g., alignment and twist of β-strands) that are encoded by memory effects.

Amino acid sequence determinants in self-assembly of insulin chiral amyloid superstructures: Role of C-terminus of B-chain in association of fibrils

HEWL and HEWLbind by atomic force microscopy (View interaction)

On the DMSO-dissolved state of insulin: a vibrational spectroscopic study of structural disorder.

Upon dissolving in dimethyl sulfoxide (DMSO), native insulin and insulin amyloid fibrils convert into an identical disordered structural state based on IR spectral characteristics. Here, we investigate the DMSO-denatured state of insulin using a number of spectroscopic methods: near-UV circular dichroism, infrared absorption spectroscopy, vibrational circular dichroism (VCD), Raman scattering, and Raman optical activity (ROA), as well as by carrying out 140-ns-long molecular dynamics (MD) simulations of DMSO-dissolved native insulin monomers. According to this work, the DMSO-solvated state of insulin is an ensemble of conformations including polyproline II-type helix and possibly a residual α-helical structure. Effects of DMSO-specific solvation and conformation-restricting covalent structure of insulin (including the three intact disulfide bridges) are argued to play important roles in stabilizing the disordered state of the protein. A comparison of ROA spectra of insulin dissolved in fully deuterated and nondeuterated DMSO suggested transfer of chirality from the protein to the otherwise ROA-silent solvent. Our study provides an example of a biological protein that acquires a substantial population of PP II conformation in an entirely nonaqueous environment. The DMSO-unfolded state of insulin and its dynamics are also discussed in the context of the established link between PP II conformation and protein misfolding.

Insulin Amyloid Superstructures as Templates for Surface Enhanced Raman Scattering

Nanostructuring of noble metal surfaces with biomorphic and biological templates facilitates a variety of applications of surface enhanced Raman scattering (SERS). Here we show that the newly reported insulin amyloid superstructures may be employed as stable nanoscaffolds for metallic Au films providing an effective substrate for SERS on covalently bound molecules of 4-mercaptobenzoic acid (4-MBA). The vortex-aligned insulin fibrils are capable of templating nanopatterns in sputtered Au layers without overlapping the SERS spectra of 4-MBA with vibrational bands stemming from the protein. This holds true regardless of whether the incident laser beam is directly backscattered from the 4-MBA layer, or after passage through the insulin amyloid layer.

Master and slave relationship between two types of self-propagating insulin amyloid fibrils.

Cross-seeding of fibrils of bovine insulin (BI) and Lys(B31)-Arg(B32) human insulin analog (KR) induces self-propagating amyloid variants with infrared features inherited from mother seeds. Here we report that when native insulin (BI or KR) is simultaneously seeded with mixture of equal amounts of both templates (i.e., of separately grown fibrils of BI and KR), the phenotype of resulting daughter fibrils is as in the case of the purely homologous seeding: heterologous cotemplates accelerate the fibrillation but do not determine infrared traits of the daughter amyloid. This implies that fibrillation-promoting and structure-imprinting properties of heterologous seeds become uncoupled in the presence of homologous seeds. We argue that explanation of such behavior requires that insulin molecules partly transformed through interactions with heterologous fibrils are subsequently recruited by homologous seeds. The selection bias toward homologous daughter amyloid is exceptional: more than 200-fold excess of heterologous seed is required to imprint its structural phenotype upon mixed seeding. Our study captures a snapshot of elusive docking interactions in statu nascendi of elongation of amyloid fibril and suggests that different types of seeds may collaborate in sequential processing of soluble protein into fibrils.

Highly Amyloidogenic Two-chain Peptide Fragments Are Released upon Partial Digestion of Insulin with Pepsin

Background: Insulin is a model amyloidogenic protein. Results: Limited proteolysis of bovine insulin dimers with pepsin releases highly fibrillation-prone two-chain fragments. Conclusion: Dynamics of the disulfide-bonded N-terminal fragments of A- and B-chains may strongly contribute to insulin amyloidogenesis. Significance: Highly aggregation-prone regions of protein molecules may be revealed by partial proteolysis of the native state. Proteases play a well recognized role in the emergence of highly aggregation-prone protein fragments in vivo, whereas in vitro limited proteolysis is often employed to probe different phases of amyloidogenic pathways. Here, we show that addition of moderate amounts of pepsin to acidified bovine insulin at close to physiological temperature results in an abrupt self-assembly of amyloid-like fibrils from partially digested insulin fragments. Biochemical analysis of the pepsin-induced fibrils implicates peptide fragments (named H) consisting of the 13 or 15 N-terminal residues of the A-chain and 11 or 13 N-terminal residues of the B-chain linked by the disulfide bond between Cys-7A–Cys-7B as the main constituents. There are up to eight pepsin-cleavage sites remaining within the double chain peptide, which become protected upon fast fibrillation unless concentration of the enzyme is increased resulting in complete digestion of insulin. Controlled re-association of H-peptides leads to “explosive” fibrillation only under nonreducing conditions implying the key role of the disulfide bond in their amyloidogenicity. Such re-assembled amyloid is similar in terms of morphology and infrared features to typical bovine insulin fibrils, although it lacks the ability to seed the intact protein.