Tracking the heterogeneous distribution of amyloid spherulites and their population balance with free fibrils

Chemical Modification of the Amino Groups of Human Insulin: Investigating Structural Properties and Amorphous Aggregation of Acetylated Species

The efficacy of human recombinant insulin can be affected by its aggregation. Effects of acetylation were observed on insulin structure, stability, and aggregation at 37 and 50 °C and pH of 5.0 and 7.4 with the use of spectroscopy, circular dichroism (CD), dynamic light scattering (DLS), and atomic force microscopy (AFM). Raman and FTIR results were indicative of structural changes in AC-INS, and CD analyses showed a slight increase in β-sheet content in AC-INS. Melting temperature (T_m) measurements indicated an overall more stable structure and spectroscopic assessment showed a more compact one. Formation of amorphous aggregates was followed over time and kinetics parameters showed a longer nucleation phase (higher t* amount) and lower aggregates amount (lower A_lim) for acetylated insulin (AC-INS) compared to native (N-INS) in all tested conditions. The results of amyloid-specific probes approved the formation of amorphous aggregates. Size particle and microscopic analysis suggested that AC-INS was less prone to form aggregates, which were smaller if formed. In conclusion, this study has demonstrated that controlled acetylation of insulin may lead to its higher stability and lower propensity toward amorphous aggregation and has provided insight into the result of this type of post-translational protein modification.

Reproducible Formation of Insulin Superstructures: Amyloid-Like Fibrils, Spherulites, and Particulates

Inducing protein aggregation in vitro under various formulation and stress conditions may lead to an increased understanding of the different association routes a protein can undergo. However, a range of factors can affect the aggregation process, often leading to heterogenous samples and experimental irreproducibility between labs. Here, we present detailed methods to reproducibly form homogenous samples of superstructures: amyloid-like fibrils, spherulites, and particulates from human insulin. We discuss pitfalls and good practice in the lab, with the aim of creating awareness on the potential sources of artefacts for protein stability and aggregation studies.

Subtle pH variation around pH 4.0 affects aggregation kinetics and aggregate characteristics of recombinant human insulin

Insulin is a biotherapeutic protein, which, depending on environmental conditions such as pH, has been shown to form a large variety of aggregates with different structures and morphologies. This work focuses on the formation and characteristics of insulin particulates, dense spherical aggregates having diameters spanning from nanometre to low-micron size. An in-depth investigation of the system is obtained by applying a broad range of techniques for particle sizing and characterisation. An interesting observation was achieved regarding the formation kinetics and aggregate characteristics of the particulates; a subtle change in the pH from pH 4.1 to pH 4.3 markedly affected the kinetics of the particulate formation and led to different particulate sizes, either nanosized or micronsized particles. Also, a clear difference between the secondary structure of the protein particulates formed at the two pH values was observed, where the nanosized particulates had an increased content of aggregated β-structure compared to the micronsized particles. The remaining characteristics of the particles were identical for the two particulate populations. These observations highlight the importance of carefully studying the formulation design space and of knowing the impact of parameters such as pH on the aggregation to secure a drug product in control. Furthermore, the identification of particles only varying in few parameters, such as size, are considered highly valuable for studying the effect of particle features on the immunogenicity potential.

Morphological integrity of insulin amyloid-like aggregates depends on preparation methods and post-production treatments

Protein aggregates are often varying extensively in their morphological characteristics, which may lead to various biological outcomes, such as increased immunogenicity risk. However, isolation of aggregates with a specific morphology within an ensemble is often challenging. To gain vital knowledge on the effects of aggregate characteristics, samples containing a single morphology must be produced by direct control of the aggregation process. Moreover, the formed aggregates need to be in an aqueous solution suitable for biological assays, while keeping their morphology intact. Here we evaluated the dependence of morphology and integrity of amyloid-like fibrils and spherulites on preparation conditions and post-treatment methods. Samples containing either amyloid-like fibrils or spherulites produced from human insulin in acetic acid solutions are dependent on the presence of salt (NaCl). Moreover, mechanical shaking (600 rpm) inhibits spherulite formation, while only affecting the length of the formed fibrils compared to quiescent conditions. Besides shaking, the initial protein concentration in the formulation was found to control fibril length. Surprisingly, exchanging the solution used for aggregate formation to a physiologically relevant buffer, had a striking effect on the morphological integrity of the fibril and spherulite samples. Especially the secondary structure of one of our spherulite samples presented dramatic changes of the aggregated β-sheet content after exchanging the solution, emphasizing the importance of the aggregate stability. These results and considerations have profound implications on the data interpretation and should be implemented in the workflow for both fundamental characterization of aggregates as well as assays for evaluation of their corresponding biological effects.

Magnetoliposomes Containing Multicore Nanoparticles and a New Antitumor Thienopyridine Compound with Potential Application in Chemo/Thermotherapy

Multicore magnetic nanoparticles of manganese ferrite were prepared using carboxymethyl dextran as an agglutinating compound or by an innovative method using melamine as a cross-coupling agent. The nanoparticles prepared using melamine exhibited a flower-shape structure, a saturation magnetization of 6.16 emu/g and good capabilities for magnetic hyperthermia, with a specific absorption rate (SAR) of 0.14 W/g. Magnetoliposome-like structures containing the multicore nanoparticles were prepared, and their bilayer structure was confirmed by FRET (Förster Resonance Energy Transfer) assays. The nanosystems exhibited sizes in the range of 250–400 nm and a low polydispersity index. A new antitumor thienopyridine derivative, 7-[4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl]thieno[3,2-b]pyridine, active against HeLa (cervical carcinoma), MCF-7 (breast adenocarcinoma), NCI-H460 (non-small-cell lung carcinoma) and HepG2 (hepatocellular carcinoma) cell lines, was loaded in these nanocarriers, obtaining a high encapsulation efficiency of 98 ± 2.6%. The results indicate that the new magnetoliposomes can be suitable for dual cancer therapy (combined magnetic hyperthermia and chemotherapy).

Development of Thermo- and pH-Sensitive Liposomal Magnetic Carriers for New Potential Antitumor Thienopyridine Derivatives

The development of stimuli-sensitive drug delivery systems is a very attractive area of current research in cancer therapy. The deep knowledge on the microenvironment of tumors has supported the progress of nanosystems' ability for controlled and local fusion as well as drug release. Temperature and pH are two of the most promising triggers in the development of sensitive formulations to improve the efficacy of anticancer agents. Herein, magnetic liposomes with fusogenic sensitivity to pH and temperature were developed aiming at dual cancer therapy (by chemotherapy and magnetic hyperthermia). Magnetic nanoparticles of mixed calcium/manganese ferrite were synthesized by co-precipitation with citrate and by sol-gel method, and characterized by X-ray diffraction (XRD), scanning electron microscopy in transmission mode (STEM), and superconducting quantum interference device (SQUID). The citrate-stabilized nanoparticles showed a small-sized population (around 8 nm, determined by XRD) and suitable magnetic properties, with a low coercivity and high saturation magnetization (~54 emu/g). The nanoparticles were incorporated into liposomes of dipalmitoylphosphatidylcholine/cholesteryl hemisuccinate (DPPC:CHEMS) and of the same components with a PEGylated lipid (DPPC:CHEMS:DSPE-PEG), resulting in magnetoliposomes with sizes around 100 nm. Dynamic light scattering (DLS) and electrophoretic light scattering (ELS) measurements were performed to investigate the pH-sensitivity of the magnetoliposomes' fusogenic ability. Two new antitumor thienopyridine derivatives were efficiently encapsulated in the magnetic liposomes and the drug delivery capability of the loaded nanosystems was evaluated, under different pH and temperature conditions.

Zinc determines dynamical properties and aggregation kinetics of human insulin

Protein aggregation is a widespread process leading to deleterious consequences in the organism, with amyloid aggregates being important not only in biology but also for drug design and biomaterial production. Insulin is a protein largely used in diabetes treatment, and its amyloid aggregation is at the basis of the so-called insulin-derived amyloidosis. Here, we uncover the major role of zinc in both insulin dynamics and aggregation kinetics at low pH, in which the formation of different amyloid superstructures (fibrils and spherulites) can be thermally induced. Amyloid aggregation is accompanied by zinc release and the suppression of water-sustained insulin dynamics, as shown by particle-induced x-ray emission and x-ray absorption spectroscopy and by neutron spectroscopy, respectively. Our study shows that zinc binding stabilizes the native form of insulin by facilitating hydration of this hydrophobic protein and suggests that introducing new binding sites for zinc can improve insulin stability and tune its aggregation propensity.

Impact of Citrate and Lipid-Functionalized Magnetic Nanoparticles in Dehydropeptide Supramolecular Magnetogels: Properties, Design and Drug Release

Currently, the nanoparticle functionalization effect on supramolecular peptide-based hydrogels remains undescribed, but is expected to affect the hydrogels' self-assembly and final magnetic gel properties. Herein, two different functionalized nanoparticles: citrate-stabilized (14.4 ± 2.6 nm) and lipid-coated (8.9 ± 2.1 nm) magnetic nanoparticles, were used for the formation of dehydropeptide-based supramolecular magnetogels consisting of the ultra-short hydrogelator Cbz-L-Met-Z-ΔPhe-OH, with an assessment of their effect over gel properties. The lipid-coated nanoparticles were distributed along the hydrogel fibers, while citrate-stabilized nanoparticles were aggregated upon gelation, which resulted into a heating efficiency improvement and decrease, respectively. Further, the lipid-coated nanoparticles did not affect drug encapsulation and displayed improved drug release reproducibility compared to citrate-stabilized nanoparticles, despite the latter attaining a stronger AMF-trigger. This report points out that adsorption of nanoparticles to hydrogel fibers, which display domains that improve or do not affect drug encapsulation, can be explored as a means to optimize the development of supramolecular magnetogels to advance theranostic applications.

Tuning the aqueous self-assembly process of insulin by a hydrophobic additive

The presence of a fluorescent hydrophobic oligothiophene (6T) dramatically influences the process where insulin self-assembles into spherulites, resulting in large (up to 1.4 mm) fluorescent spherulites.

Electrostatics controls the formation of amyloid superstructures in protein aggregation

The possibility for proteins to aggregate in different superstructures, i.e. large-scale polymorphism, has been widely observed, but an understanding of the physicochemical mechanisms behind it is still out of reach. Here we present a theoretical model for the description of a generic aggregate formed from an ensemble of charged proteins. The model predicts the formation of multifractal structures with the geometry of the growth determined by the electrostatic interactions between single proteins. The model predictions are successfully verified in comparison with experimental curves for aggregate growth allowing us to reveal the mechanism of formation of such complex structures. The model is general and is able to predict aggregate morphologies occurring both in vivo and in vitro. Our findings provide a framework where the physical interactions between single proteins, the aggregate morphology, and the growth kinetics are connected into a single model in agreement with the experimental data.

Unlocked concanavalin A forms amyloid-like fibrils from coagulation of long-lived "crinkled" intermediates

Understanding the early events during amyloid aggregation processes is crucial to single out the involved molecular mechanisms and for designing ad hoc strategies to prevent and reverse amyloidogenic disorders. Here, we show that, in conditions in which the protein is positively charged and its conformational flexibility is enhanced, Concanavalin A leads to fibril formation via a non-conventional aggregation pathway. Using a combination of light scattering, circular dichroism, small angle X-ray scattering, intrinsic (Tryptophan) and extrinsic (ANS) fluorescence and confocal and 2-photon fluorescence microscopy we characterize the aggregation process as a function of the temperature. We highlight a multi-step pathway with the formation of an on-pathway long-lived intermediate and a subsequent coagulation of such “crinkled” precursors into amyloid-like fibrils. The process results in a temperature-dependent aggregation-coagulation pathway, with the late phase of coagulation determined by the interplay between hydrophobic and electrostatic forces. Our data provide evidence for the complex aggregation pathway for a protein with a highly flexible native conformation. We demonstrate the possibility to generate a long-lived intermediate whose proportion and occurrence are easily tunable by experimental parameters (i.e. temperature). As a consequence, in the case of aggregation processes developing through well-defined energy barriers, our results can open the way to new strategies to induce more stable in vitro on-pathway intermediate species through a minute change in the initial conformational flexibility of the protein. This will allow isolating and experimentally studying such transient species, often indicated as relevant in neurodegenerative diseases, both in terms of structural and cytotoxic properties.

Computational modeling of the relationship between amyloid and disease

Amyloid is a title conferred upon a special type of linear protein aggregate that exhibits a common set of structural features and dye binding capabilities. The formation of amyloid is associated with over twenty-seven distinct human diseases which are collectively referred to as the amyloidoses. Although there is great diversity amongst the amyloidoses with regard to the polypeptide monomeric precursor, targeted tissues and the nature and time course of disease development, the common underlying link of a structurally similar amyloid aggregate has prompted the search for a unified theory of disease progression in which amyloid production is the central element. Computational modeling has allowed the formulation and testing of scientific hypotheses for exploring this relationship. However, the majority of computational studies on amyloid aggregation are pitched at the atomistic level of description, in simple ideal solution environments, with simulation time scales of the order of microseconds and system sizes limited to a hundred monomers (or less). The experimental reality is that disease related amyloid aggregation processes occur in extremely complex reaction environments (i.e. the human body), over time-scales of months to years with monitoring of the reaction achieved using extremely coarse or indirect experimental markers that yield little or no atomistic insight. Clearly a substantial gap exists between computational and experimental communities with a deficit of 'useful' computational methodology that can be directly related to available markers of disease progression. This Review will place its focus on the development of these latter types of computational models and discuss them in relation to disease onset and progression.