Polysorbate 80 controls Morphology, structure and stability of human insulin Amyloid-Like spherulites.

Amyloid protein aggregates are not only associated with neurodegenerative diseases and may also occur as unwanted by-products in protein-based therapeutics. Surfactants are often employed to stabilize protein formulations and reduce the risk of aggregation. However, surfactants alter protein-protein interactions and may thus modulate the physicochemical characteristics of any aggregates formed. Human insulin aggregation was induced at low pH in the presence of varying concentrations of the surfactant polysorbate 80. Various spectroscopic and imaging methods were used to study the aggregation kinetics, as well as structure and morphology of the formed aggregates. Molecular dynamics simulations were employed to investigate the initial interaction between the surfactant and insulin. Addition of polysorbate 80 slowed down, but did not prevent, aggregation of insulin. Amyloid spherulites formed under all conditions, with a higher content of intermolecular beta-sheets in the presence of the surfactant above its critical micelle concentration. In addition, a denser packing was observed, leading to a more stable aggregate. Molecular dynamics simulations suggested a tendency for insulin to form dimers in the presence of the surfactant, indicating a change in protein-protein interactions. It is thus shown that surfactants not only alter aggregation kinetics, but also affect physicochemical properties of any aggregates formed.

Conformational Transitions upon Maturation Rule Surface and pH-Responsiveness of α-Lactalbumin Microparticulates.

De novo designed protein supramolecular structures are nowadays attracting much interest as highly performing biomaterials. While a clear advantage is provided by the intrinsic biocompatibility and biodegradability of protein and peptide building blocks, developing sustainable and green bottom up approaches for finely tuning the material properties still remains a challenge. Here, we present an experimental study on the formation of protein microparticles in the form of particulates from the protein α-lactalbumin using bulk mixing in water solution and high temperature. Once formed, the structure and stability of these spherical protein condensates change upon further thermal incubation while the size of aggregates does not significantly increase. Combining advanced microscopy and spectroscopy methods, we prove that this process, named maturation, is characterized by a gradual increase of amyloid-like structure in protein particulates, an enhancement in surface roughness and in molecular compactness, providing a higher stability and resistance of the structure in acidic environments. When dissolved at pH 2, early stage particulates disassemble into a homogeneous population of small oligomers, while the late stage particulates remain unaffected. Particulates at the intermediate stage of maturation partially disassemble into a heterogeneous population of fragments. Importantly, differently matured microparticles show different features when loading a model lipophilic molecule. Our findings suggest conformational transitions localized at the interface as a key step in the maturation of amyloid protein condensates, promoting this phenomenon as an intrinsic knob to tailor the properties of protein microparticles formed via bulk mixing in aqueous solution. This provides a simple and sustainable platform for the design and realization of protein microparticles for tailored applications.

Probing ensemble polymorphism and single aggregate structural heterogeneity in insulin amyloid self-assembly.

Ensembles of protein aggregates are characterized by a nano- and micro-scale heterogeneity of the species. This diversity translates into a variety of effects that protein aggregates may have in biological systems, both in connection to neurodegenerative diseases and immunogenic risk of protein drug products. Moreover, this naturally occurring variety offers unique opportunities in the field of protein-based biomaterials. In the above-mentioned fields, the isolation and structural analysis of the different amyloid types within the same ensemble remain a priority, still representing a significant experimental challenge. Here we address such complexity in the case of insulin for its relevance as biopharmaceutical and its involvement in insulin-derived amyloidosis. By combining Fourier Transform Infrared Microscopy (micro-FTIR) and fluorescence lifetime imaging microscopy (FLIM) we show the occurrence, within the same ensemble of insulin protein aggregates, of a variable ß-structure architecture and content not only dependent on the species analyzed (spherulites or fibrils), but also on the position within a single spherulite at submicron scale. We unambiguously reveal that the surface of the spherulites are characterized by ß-structures with an enhanced H-bond coupling compared to the core. This information, inaccessible via bulk methods, allows us to relate the aggregate structure at molecular level to the overall morphology of the aggregates. Our findings robustly solve the problem of probing the ensemble and single particle heterogeneity of amyloid samples. Furthermore, we offer a unique, scalable and ready-to-use screening methodology for in-depth characterization of self-assembled structures, being this translatable to material sciences, drug quality control and clinical imaging of amyloid-affected tissues.