A Small Molecule Impedes Insulin Fibrillation: Another New Role of Phenothiazine Derivatives

Decoding Anti-Amyloidogenic and Fibril Neutralizing Action of Gut Microbiota-Derived Indole 3-Acetic Acid on Insulin Fibrillation through Multispectroscopic, Machine Learning, and Hybrid Quantum Mechanics/Molecular Mechanics Approaches

Insulin fibrillation inflicts both economic and clinical challenges by causing bioactivity loss, inflammation, and adverse effects during storage, transport, and injection. The present study explores antiamyloidogenic and fibril-disaggregating effects of a gut microbiota-derived indole metabolite, indole-3-acetic acid (IAA) on insulin fibrillation. According to Thioflavin T (ThT) fluorescence assays and transmission electron microscopy (TEM), IAA significantly inhibited both primary and seed-induced fibrillation of insulin. We note that IAA reduced insulin aggregate sizes as evident from the scattering profiles, while circular dichroism studies confirmed that IAA preserves native α-helical structure possibly minimizing the exposed surface hydrophobicity of insulin. Additionally, IAA showed effectiveness in breaking apart preformed fibrils, indicated by a time-dependent decrease in ThT fluorescence and further confirmed by TEM. Our biolayer interferometry interaction studies revealed a moderate 2:1 binding affinity between IAA and insulin. Two key binding sites on insulin were identified via machine-learning-based-docking and hybrid QM/MM studies, where IAA interacts. Site I (Leu13A, Tyr14A, Glu17A, Phe1B) showed more favorable interaction energetics than site II (Tyr19A, Phe25B, Thr27B) based on SAPT0 residue-wise interaction energy analysis. IAA also protected cells from fibril-induced cytotoxicity and hemolysis, thereby offering a promising therapeutic option for amyloid-related disorders, with dual action in preventing fibril formation and promoting fibril disaggregation.

Synthesis of a highly thermostable insulin by phenylalanine conjugation at B29 Lysine

Globally, millions of diabetic patients require daily life-saving insulin injections. Insulin heat-lability and fibrillation pose significant challenges, especially in parts of the world without ready access to uninterrupted refrigeration. Here, we have synthesized four human insulin analogs by conjugating ε-amine of B29 lysine of insulin with acetic acid, phenylacetic acid, alanine, and phenylalanine residues. Of these, phenylalanine-conjugated insulin, termed FHI, was the most stable under high temperature (65 °C), elevated salt stress (25 mM NaCl), and varying pH levels (ranging from highly acidic pH 1.6 to physiological pH 7.4). It resists fibrillation for a significantly longer duration with sustained biological activity in in vitro, ex vivo, and in vivo and displays prolonged stability over its native counterpart. We further unravel the critical interactions, such as additional aromatic π-π interactions and hydrogen bonding in FHI, that are notably absent in native insulin. These interactions confer enhanced structural stability of FHI and offer a promising solution to the challenges associated with insulin heat sensitivity.

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.

Poly(trehalose methacrylate) as an Excipient for Insulin Stabilization: Mechanism and Safety

Insulin, the oldest U.S. Food and Drug Administration (FDA)-approved recombinant protein and a World Health Organization (WHO) essential medicine for treating diabetes globally, faces challenges due to its storage instability. One approach to stabilize insulin is the addition of poly(trehalose methacrylate) (pTrMA) as an excipient. The polymer increases the stability of the peptide to heat and mechanical agitation and has a low viscosity suitable for injection and pumps. However, the safety and stabilizing mechanism of pTrMA is not yet known and is required to understand the potential suitability of pTrMA as an insulin excipient. Herein is reported the immune response, biodistribution, and insulin plasma lifetime in mice, as well as investigation into insulin stabilization. pTrMA alone or formulated with ovalbumin did not elicit an antibody response over 3 weeks in mice, and there was no observable cytokine production in response to pTrMA. Micropositron emission tomography/microcomputer tomography of 64Cu-labeled pTrMA showed excretion of 78-79% ID/cc within 24 h and minimal liver accumulation at 6-8% ID/cc when studied out to 120 h. Further, the plasma lifetime of insulin in mice was not altered by added pTrMA. Formulating insulin with 2 mol equiv of pTrMA improved the stability of insulin to standard storage conditions: 46 weeks at 4 °C yielded 87.0% intact insulin with pTrMA present as compared to 7.8% intact insulin without the polymer. The mechanism by which pTrMA-stabilized insulin was revealed to be a combination of inhibiting deamidation of amino acid residues and preventing fibrillation, followed by aggregation of inactive and immunogenic amyloids all without complexing insulin into its hexameric state, which could delay the onset of insulin activity. Based on the data reported here, we suggest that pTrMA stabilizes insulin as an excipient without adverse effects in vivo and is promising to investigate further for the safe formulation of insulin.

Molecular Aspects of Insulin Aggregation and Various Therapeutic Interventions

Protein aggregation leading to the formation of amyloid fibrils has various adverse effects on human health ranging from fatigue and numbness to organ failure and death in extreme cases. Insulin, a peptide hormone commonly used to treat diabetes, undergoes aggregation at the site of repeated injections in diabetic patients as well as during its industrial production and transport. The reduced bioavailability of insulin due to aggregation hinders the proper control of glucose levels in diabetic patients. Thus, it is necessary to develop rational approaches for inhibiting insulin aggregation, which in turn requires a detailed understanding of the mechanism of fibrillation. Given the relative simplicity of insulin and ease of access, insulin has also served as a model system for studying amyloids. Approaches to inhibit insulin aggregation have included the use of natural molecules, synthetic peptides or small molecules, and bacterial chaperone machinery. This review focuses on insulin aggregation with an emphasis on its mechanism, the structural features of insulin fibrils, and the reported inhibitors that act at different stages in the aggregation pathway. We discuss molecules that can serve as leads for improved inhibitors for use in commercial insulin formulations. We also discuss the aggregation propensity of fast- and slow-acting insulin biosimilars, commonly administered to diabetic patients. The development of better insulin aggregation inhibitors and insights into their mechanism of action will not only aid diabetic therapies, but also enhance our knowledge of protein amyloidosis.

Strategies for interference of insulin fibrillogenesis: challenges and advances

The discovery of insulin came up with very high hopes for diabetic patients. In the year 2021, the world celebrated the 100 th anniversary of the discovery of this vital hormone. However, external use of insulin is highly affected by its aggregating tendency that occurs during its manufacturing, transportation, and improper handling which ultimately leads its pharmaceutically and biologically ineffective form. In this review, we aim to discuss the various approaches used for decelerating insulin aggregation which results in the enhancement of its overall structural stability and usage. The approaches that are discussed are broadly classified as either a measure through excipient additions or by intrinsic modifications in the insulin native structure.

Design, Synthesis, and Validation of a Novel [11C]Promethazine PET Probe for Imaging Abeta Using Autoradiography

Promethazine, an antihistamine drug used in the clinical treatment of nausea, has been demonstrated the ability to bind Abeta in a transgenic mouse model of Alzheimer’s disease. However, so far, all of the studies were performed in vitro using extracted tissues. In this work, we report the design and synthesis of a novel [¹¹C]promethazine PET radioligand for future in vivo studies. The [¹¹C]promethazine was isolated by RP-HPLC with radiochemical purity >95% and molar activity of 48 TBq/mmol. The specificity of the probe was demonstrated using human hippocampal tissues via autoradiography.

Halogenation of the N-Terminus Tyrosine 10 Promotes Supramolecular Stabilization of the Amyloid-β Sequence 7-12

Here, we demonstrate that introduction of halogen atoms at the tyrosine 10 phenol ring of the DSGYEV sequence derived from the flexible amyloid-β N-terminus, promotes its self-assembly in the solid state. In particular, we report the crystal structures of two halogen-modified sequences, which we found to be stabilized in the solid state by halogen-mediated interactions. The structural study is corroborated by Non-Covalent Interaction (NCI) analysis. Our results prove that selective halogenation of an amino acid enhances the supramolecular organization of otherwise unstructured biologically-relevant sequences. This method may develop as a general strategy for stabilizing highly polymorphic peptide regions.

Monocyclic phenolic compounds stabilize human insulin and suppress its amorphous aggregation: In vitro and in vivo study

Insulin is a small protein with 51 residues that mediates glucose uptake, and an interesting model for studying protein misfolding and aggregation. The aggregated forms of insulin undergo loss of activity and can provoke unwanted immune responses. Use of small molecules is considered to be an affordable method to counteract this aggregation process and stabilize insulin. In this study, aggregated forms of human recombinant insulin have been produced following exposure to high temperature. Aggregation process was followed over time by checking absorbance with spectrophotometry in presence and absence of various concentrations of small phenolic compounds including eugenol and epinephrine. Effects of these compounds on the structure and function of incubated insulin were evaluated by spectrofluorimetry, melting temperature (Tm) measurement and insulin tolerance test on Wistar rats. Formation of heat-induced insulin aggregation can be effectively inhibited by 1 mM eugenol and epinephrine and both compounds were found to preserve insulin activity to a considerable extent. In conclusion, simple aromatic compounds could be tailored to act as potent anti-aggregation compounds for insulin.

A Small Molecule Impedes Insulin Fibrillation: Another New Role of Phenothiazine Derivatives

Protein misfolding is interrelated to several diseases, including neurodegenerative diseases and type II diabetes. Misfolded/unfolded proteins produce soluble oligomers that accumulate into "amyloid plaques". Inhibition of amyloid-plaque formation by those misfolded/unfolded proteins will lead to the invention of new therapeutic approaches for amyloid-related diseases. Herein, methylene blue (MB), a well-defined drug against multiple diseases and disorders, is used to impede insulin fibrillation. In this study, we perform an array of in vitro experiments to monitor the effects of MB on the fibrillation of bovine insulin. Our results confirm that MB distresses the kinetics of insulin fibrillation by interacting with insulin in its monomeric form. A thioflavin T assay indicates that insulin fibrillation is interrupted upon the addition of MB. The same results are confirmed by circular dichroism, dynamic light scattering (DLS), and size-exclusion chromatography (SEC). According to the DLS data, the insulin fibrils are 800 nm in diameter, and the addition of MB reduces the size of the fibrils, which remain 23 nm in size, and this indicates that no fibrillation of insulin occurs in the presence of MB. This data is also supported by SEC. Saturation transfer difference NMR spectroscopy and molecular dynamics simulations demonstrate the interactions between insulin and MB at the atomic level.