Probing the mechanism of insulin aggregation with added metalloporphyrins

Biocompatible zeolite-dye composites with anti-amyloidogenic properties

One of the most attractive approaches in biomedicine and pharmacy is the application of multifunctional materials. The mesoporous structure of clinoptilolite (CZ) absorbs various types of substances and can be used as a model for studying the carriers for targeted drug delivery with controlled release. CZ-dye composites are fabricated by incorporation into clinoptilolite pores commonly used dyes, aluminum phthalocyanine, zinc porphine, and hypericin. We examined and compared the effect of pure dyes and CZ-dye composites on insulin amyloidogenesis. The formation of insulin amyloid fibrils and the disassembly of preformed fibrils is significantly affected by any of the three compounds, however, the strongest effect is observed for aluminum phthalocyanine indicating a structurally-dependent anti-amyloidogenic activity of the dyes. The incorporation of dyes into CZ particles resulted in enhanced anti-amyloidogenic activity in comparison to pure CZ particles. The cell metabolic activity, biocompatibility and fluorescence biodistribution of the dyes entrapped in the composites were tested in vitro (U87 MG cells) and in vivo in the quail chorioallantoic membrane model. Considering the photoactive properties of the dyes used, we assume their applicability in photodiagnostics and photodynamic therapy. It can also be expected that their anti-amyloidogenic potential can be enhanced by photodynamic effect.

Amyloid fibril reduction through covalently modified lysine in HEWL and insulin

Proteins possess a variety of nucleophiles, which can carry out different reactions in the functioning cells. Proteins endogenously and synthetically can be modified through their nucleophilic sites. The roles of these chemical modifications have not been completely revealed. These modifications can alter the protein folding process. Protein folding directly affects the function of proteins. If an error in protein folding occurs, it may cause protein malfunction leading to several neurodegenerative disorders such as Alzheimer's and Parkinson's. In this study, Hen Egg White Lysozyme (HEWL) and bovine insulin, as model proteins for studying the amyloid formation, were covalently attached with 5(6)-thiophenolfluorescein. The amyloid formation of the covalently labeled lysozyme and insulin were compared with the native proteins. Interestingly, the results indicated that the covalent attachment of fluorescein slowed down the amyloid formation of HEWL and insulin significantly. The amyloid formation was examined using Thioflavin T (ThT) fluorescence assay, circular dichroism, FTIR, and gel electrophoresis. Tandem mass spectrometry was employed to identify the sites of covalent modifications in HEWL. It turned out that two surface lysine residues (K97 and K 116) in HEWL were modified. Computational studies, including docking and molecular simulations, revealed that 5(6)-thiophenolfluorescein makes several non-covalent interactions with HEWL residues, including Lys 97, leading to the reduction of the β-sheet in the protein. Additionally, AFM analysis confirmed the amyloid fibril reduction of lysine-modified bovine insulin and HEWL. Altogether, our results expand mechanistic insights into preventing amyloid formation by providing an approach for reducing amyloid formation by modifying specific lysine residues in the proteins.

Mechanisms behind the Fibrillation and Toxicity of Insulin Fibrils on Neuron Cells by Engineered Curcumin Analogs

Among foods, the use of plant derivatives as promising drugs and/or excipients has been considered from various perspectives. In the present study, curcumin, which is one of the most important plant derivatives for biological uses, and four curcumin-based pyrido[2,3-d]pyrimidine analogs (C2-C5) were used for investigating the mechanism of insulin fibrillation and evaluating the cytotoxicity of insulin fibrils. The synthesized analogs differed in terms of hydrophobicity and electrostatic charge. The analogs with more hydrophobicity (C1 and C4) in both acidic and neutral environments were able to reduce the rate of insulin fibrillation and the degree of cross-linking in the produced fibrils. Additionally, the toxicity of these fibrils for neural cells (N2a cell line) was very low. However, they did not show any significant effects on the toxicity of non-neural cells (HEK293 cell line), indicating the effect of the biochemical surface diversity on determining the vulnerability to fibrils and even the mechanism of action of additives on cell line survival. Although negatively charged analogs were able to reduce insulin fibrillation in the acidic environment, they indicated an opposite effect in the neutral environment. The resultant fibrils in the acidic medium appeared with a well-distinguished filament, but they were very close at neutral pH levels. Moreover, such fibrils indicated very poor toxicity against the N2a cell line and had no significant effects on HEK293 cells. Considering the docking studies, by creatively using the size exclusion chromatography, it was suggested that analogs C2 and C3 were capable of binding to the C-terminal end of the insulin B chain (low affinity) and His^B10 (high affinity). Hence, it was suggested that different compounds could play different protecting and/or destroying roles in cell toxicity by blocking some ligands at the surface of neuron cells.

Structure and mechanism behind the inhibitory effect of water soluble metalloporphyrins on Aβ1-42 aggregation

Although the exact molecular mechanism of the pathogenesis of Alzheimer’s disease (AD) is still unclear, compounds that can inhibit the aggregation of amyloid-β peptide (Aβ1-42) or scavenge the highly toxic...

Hydroxy-Porphyrin as an Effective, Endogenous Molecular Clamp during Early Stages of Amyloid Fibrillization

Amyloid fibril formation of proteins is of great concern in neurodegenerative disease and can be detrimental to the storage and stability of biologics. Recent evidence suggests that insulin fibril formation reduces the efficacy of type II diabetes management and may lead to several complications. To develop anti-amyloidogenic compounds of endogenous origin, we have utilized the hydrogen bond anchoring, π stacking ability of porphyrin, and investigated its role on the inhibition of insulin amyloid formation. We report that hydroxylation and metal removal from the heme moiety yields an excellent inhibitor of insulin fibril formation. Thioflavin T, tyrosine fluorescence, Circular Dichorism (CD) spectroscopy, Field emission scanning electron microscopy (FESEM) and molecular dynamics (MD) simulation studies suggest that hematoporphyrin (HP) having hydrogen bonding ability on both sides is a superior inhibitor compared to hemin and protoporphyrin (PP). Experiments with hen egg white lysozyme (HEWL) amyloid fibril formation also validated the efficacy of endogenous porphyrin based small molecules. Our results will help to decipher a general therapeutic strategy to counter amyloidogenesis.

Insulin fibrillation: toward strategies for attenuating the process

The environmental factors affecting the rate of insulin fibrillation. The factors are representative.

Study of tetraphenylporphyrins as modifiers of insulin amyloid aggregation

Amyloid fibrils are rigid β-pleated protein aggregates that are connected with series of harmful diseases and at the same time are promising as base for novel nanomaterials. Thus, design of compounds able to inhibit or redirect those aggregates formation is important both for the biomedical aims and for nanotechnology applications. Here, we studied the effect of tetraphenylporphyrins (metal free, their Cu and Pd complexes, and those functionalized by carboxy and amino groups on periphery) on insulin amyloid self-assembling. The strongest impact on insulin aggregation was demonstrated by a metal-free porphyrin bearing four carboxy groups. This compound strongly suppresses insulin aggregation (about 88% reduction in amyloid-sensitive probe emission) inducing formation of fibrils with the length close to this of free insulin (1.7 ± 0.6 μm as compared with 1.4 ± 0.4 μm, respectively) with an essentially reduced tendency to lateral aggregation. Contrarily, the presence of tetraphenylporphyrin containing four amino groups only slightly affects fibrils' morphology and makes weaker impact on insulin aggregation yield (about 44% reduction). This is explained by the ability of aromatic carboxy groups of 5,10,15,20-(tetra-4-carboxyphenyl)porphyrin to interact with complementary protein-binding groups and thus stabilize the supramolecular complex. For 5,10,15,20-(tetra-4-aminophenyl)porphyrin, full protonation takes place in acidic medium of protein aggregation reaction; this results in the high positive charge of TPPN4 (equal or close to +6) and hence higher contribution of coulombic repulsion to interaction of TPPN4 with insulin. One more possible mechanism of the lower inhibition effect of TPPN4 as compared with TPPC4 could be the more restricted possibility of the former as compared with the latter to form H bonds with insulin groups. It was also shown that metal-free, Pd-containing, and Cu-containing tetraphenylporphyrins without peripheral substituents make almost the same impact on the protein self-assembling. We suppose this to be due to coordination saturation of these metal atoms.

Embelin, a Potent Molecule for Alzheimer's Disease: A Proof of Concept From Blood-Brain Barrier Permeability, Acetylcholinesterase Inhibition and Molecular Docking Studies

Embelin is well-known in ethnomedicine and reported to have central nervous system activities. However, there is no report on blood-brain barrier (BBB) permeability of embelin. Here the BBB permeability of embelin was evaluated using in vitro primary porcine brain endothelial cell (PBEC) model of the BBB. Embelin was also evaluated for acetylcholinesterase (AChE) inhibitory activity and docking prediction for interaction with AChE and amyloid beta (Aβ) binding sites. Embelin was found to be non-toxic to the PBECs and did not disturb the PBEC barrier function. The PBECs showed restrictive tight junctions with average transendothelial electrical resistance of 365.37 ± 113.00 Ω.cm², for monolayers used for permeability assays. Permeability assays were conducted from apical-to-basolateral direction (blood-to-brain side). Embelin showed apparent permeability (P_app) value of 35.46 ± 20.33 × 10⁻⁶ cm/s with 85.53% recovery. In vitro AChE inhibitory assay demonstrated that embelin could inhibit the enzyme. Molecular docking study showed that embelin binds well to active site of AChE with CDOCKER interaction energy of −65.75 kcal/mol which correlates with the in vitro results. Docking of embelin with Aβ peptides also revealed the promising binding with low CDOCKER interaction energy. Thus, findings from this study indicate that embelin could be a suitable molecule to be further developed as therapeutic molecule to treat neurological disorders particularly Alzheimer's disease.

Nano-cage-mediated refolding of insulin by PEG-PE micelle

Insulin aggregation has pronounced pharmaceutical implications and biological importance. Deposition of insulin aggregates is associated with type II diabetes and instability of pharmaceutical formulations. We present in this study the renaturation effect of PEG-PE micelle on dithiothreitol (DTT)-denatured insulin revealed by techniques including turbidity assay, circular dichroism (CD), thioflavinT (ThT) binding assay, bis-ANS binding assay, agarose gel electrophoresis and MALDI-TOF MS. The obtained results show that PEG-PE micelle having a hydrophilic nano-cage-like structure in which with a negative charge layer, can capture DTT-induced insulin A and B chains, and block their hydrophobic interaction, thereby preventing aggregation. The reduced insulin A and B chain in the nano-cage are capable of recognizing each other and form the native insulin with yields of ∼30% as measured by hypoglycemic activity analysis in mice. The observed insulin refolding assisted by PEG-PE micelle may be applicable to other proteins.

Inhibitory effects of arginine on the aggregation of bovine insulin

Static and dynamic light scattering were used to investigate the effects of L-arginine, commonly used to inhibit protein aggregation, on the initial aggregation kinetics of solutions of bovine insulin in 20% acetic acid and 0.1 M NaCl as a model system for amyloidosis. Measurements were made as a function of insulin concentration (0.5–2.0 mM), quench temperature (60–85°C), and arginine concentration (10–500 mM). Aggregation kinetics under all conditions had a lag phase, whose duration decreased with increasing temperature and with increasing insulin concentration but which increased by up to a factor of 8 with increasing added arginine. Further, the initial growth rate after the lag phase also slowed by up to a factor of about 20 in the presence of increasing concentrations of arginine. From the temperature dependence of the lag phase duration, we find that the nucleation activation energy doubles from 17 ± 5 to 36 ± 3 kcal/mol in the presence of 500 mM arginine.

Differential interactions of isomeric amino sugars with insulin studied under electrospray ionisation mass spectrometry

Protein-ligand interactions were studied for bovine insulin-amino sugar systems under electrospray ionisation mass spectrometry conditions. The isomeric amino sugars showed differences in the relative abundance of 1:1 protein-ligand complex formation. The electrospray ionisation and tandem mass spectrometry results of the complex clearly demonstrated that the differences in the interaction of isomeric sugars with insulin are mainly due to the differences in their gas-phase basicity. The same phenomenon is replicated in the formation of complexes between insulin and other ligands, such as amino acids, as well as in the binding of the amino sugars with amyloid β 1-40 peptide.

Influence of electrostatic interactions on the fibrillation process of human serum albumin

The fibrillation propensity of the multidomain protein human serum albumin (HSA) has been analyzed under physiological and acidic conditions at room and elevated temperatures with varying ionic strengths by different spectroscopic techniques. The kinetics of fibril formation under the different solution conditions and the structures of resulting fibrillar aggregates were also determined. In this way, we have observed that fibril formation is largely affected by electrostatic shielding: at physiological pH, fibrillation is progressively more efficient and faster in the presence of up to 50 mM NaCl; meanwhile, at larger salt concentrations, excessive shielding and further enhancement of the solution hydrophobicity might involve a change in the energy landscape of the aggregation process, which makes the fibrillation process difficult. In contrast, under acidic conditions, a continuous progressive enhancement of HSA fibrillation is observed as the electrolyte concentration in solution increases. Both the distinct ionization and initial structural states of the protein before incubation may be the origin of this behavior. CD, FT-IR, and tryptophan fluorescence spectra seem to confirm this picture by monitoring the structural changes in both protein tertiary and secondary structures along the fibrillation process. On the other hand, the fibrillation of HSA does not show a lag phase except at pH 3.0 in the absence of added salt. Finally, differences in the structure of the intermediates and resulting fibrils under the different conditions are also elucidated by TEM and FT-IR.

Development of sensitive metalloporphyrin probes for chemiluminescent imaging detection of serum proteins

The development of metalloporphyrin- (ferric protoporphyrin IX chloride (FePP), cobalt (III) protoporphyrin IX chloride, copper (II) protoporphyrin IX) enhanced chemiluminescent (CL) imaging detection of serum proteins after PAGE is described in this article. The detection is based on the catalytic activity of metalloporphyrins, especially FePP, in the CL reaction of the luminol-H2O2 system. Some relatively low abundant proteins such as hemopexin (Hpx) and complement C4 are sensitively detected by FePP-enhanced CL imaging. Other proteins such as haptoglobin, apolipoprotein A-1, complement C3, and alpha-1-antitrypsin are also detected and identified by MS and MS/MS techniques. Detection limit of Hpx is as low as 20 ng, without the need of expensive antibodies or tedious immunoassay procedures. The mechanism of the proposed method is discussed employing standard proteins. The application to the analysis of different protein patterns in healthy people and in Thalassemia patients is being investigated.

Existence of different structural intermediates on the fibrillation pathway of human serum albumin

The fibrillation propensity of the multidomain protein human serum albumin (HSA) was analyzed under different solution conditions. The aggregation kinetics, protein conformational changes upon self-assembly, and structure of the different intermediates on the fibrillation pathway were determined by means of thioflavin T (ThT) fluorescence and Congo Red absorbance; far- and near-ultraviolet circular dichroism; tryptophan fluorescence; Fourier transform infrared spectroscopy; x-ray diffraction; and transmission electron, scanning electron, atomic force, and microscopies. HSA fibrillation extends over several days of incubation without the presence of a lag phase, except for HSA samples incubated at acidic pH and room temperature in the absence of electrolyte. The absence of a lag phase occurs if the initial aggregation is a downhill process that does not require a highly organized and unstable nucleus. The fibrillation process is accompanied by a progressive increase in the beta-sheet (up to 26%) and unordered conformation at the expense of alpha-helical conformation, as revealed by ThT fluorescence and circular dichroism and Fourier transform infrared spectroscopies, but changes in the secondary structure contents depend on solution conditions. These changes also involve the presence of different structural intermediates in the aggregation pathway, such as oligomeric clusters (globules), bead-like structures, and ring-shaped aggregates. We suggest that fibril formation may take place through the role of association-competent oligomeric intermediates, resulting in a kinetic pathway via clustering of these oligomeric species to yield protofibrils and then fibrils. The resultant fibrils are elongated but curly, and differ in length depending on solution conditions. Under acidic conditions, circular fibrils are commonly observed if the fibrils are sufficiently flexible and long enough for the ends to find themselves regularly in close proximity to each other. These fibrils can be formed by an antiparallel arrangement of beta-strands forming the beta-sheet structure of the HSA fibrils as the most probable configuration. Very long incubation times lead to a more complex morphological variability of amyloid mature fibrils (i.e., long straight fibrils, flat-ribbon structures, laterally connected fibers, etc.). We also observed that mature straight fibrils can also grow by protein oligomers tending to align within the immediate vicinity of the fibers. This filament + monomers/oligomers scenario is an alternative pathway to the otherwise dominant filament + filament manner of the protein fibril's lateral growth. Conformational preferences for a certain pathway to become active may exist, and the influence of environmental conditions such as pH, temperature, and salt must be considered.

Annular structures as intermediates in fibril formation of Alzheimer Abeta17-42

We report all-atom molecular dynamics simulations of annular beta-amyloid (17-42) structures, single- and double-layered, in solution. We assess the structural stability and association force of Abeta annular oligomers associated through different interfaces, with a mutated sequence (M35A), and with the oxidation state (M35O). Simulation results show that single-layered annular models display inherent structural instability: one is broken down into linear-like oligomers, and the other collapses. On the other hand, a double-layered annular structure where the two layers interact through their C-termini to form an NC-CN interface (where N and C are the N and C termini, respectively) exhibits high structural stability over the simulation time due to strong hydrophobic interactions and geometrical constraints induced by the closed circular shape. The observed dimensions and molecular weight of the oligomers from atomic force microscopy (AFM) experiments are found to correspond well to our stable double-layered model with the NC-CN interface. Comparison with K3 annular structures derived from the beta 2-microglobulin suggests that the driving force for amyloid formation is sequence specific, strongly dependent on side-chain packing arrangements, structural morphologies, sequence composition, and residue positions. Combined with our previous simulations of linear-like Abeta, K3 peptide, and sup35-derived GNNQQNY peptide, the annular structures provide useful insight into oligomeric structures and driving forces that are critical in amyloid fibril formation.