Interactions of neuropeptide functional amyloids with biomolecules: insights into biological relevance and potential as biomaterials

Amyloids are fibrous peptide/protein nanostructures characterized by self-assembled cross-ß-sheet molecular networks. Amyloid formation is typically associated with protein misfolding pathologies, including Alzheimer's and Parkinson's diseases. However, in recent years, growing evidence supported amyloid formation under normal physiological conditions, where these nanostructures perform host native functions.<br><br>According to recent findings, a range of human neuropeptides and peptide/protein hormones are stored as amyloid-like nanostructures within intracellular secretory granules of brain cells. Upon appropriate stimulation, these neuropeptides are released into the extracellular matrix, where functional amyloids dissociate into monomeric peptides to perform biological functions.<br><br>This thesis focuses on three functional amyloid-forming human neuropeptides: substance P, luteinizing hormone-releasing hormone (LHRH) and somatostatin-14. After providing a review of known structures and properties of amyloids (Chapter 1), biophysical techniques and cell experiments (Chapter 2) were used to characterise unreported nanostructures formed by substance P and LHRH under conditions relevant to secretory granules (Chapter 3). Substance P is shown to self-assemble into nanotubes with a diameter of 6 nm, while LHRH forms nanofibrils arranged in a hexagonal network. Both these features are novel.<br><br>The influence of glycosaminoglycans, thought to be protein/peptide aggregation helpers, was investigated on the self-assembly mechanisms of all three model neuropeptides (Chapter 4). Glycosaminoglycans are shown to not only change self-assembling kinetics but also to alter the structure of the assemblies formed. This result is directly relevant to research methodology, indicating a potential danger of using such aggregation modifiers blindly in fundamental amyloid research.<br>For the three model neuropeptides, the cytotoxicity of the amyloid structures and soluble species was assessed towards brain cell lines (Chapter 5). Concentration- and structure-dependent cytotoxicity was found. Peptide soluble species and liquid crystalline arrays of neuropeptide nanostructures are non-toxic. However, aggregates/precipitates formed at high neuropeptide concentration in cell culture media were correlated to cytotoxicity to neuroblastoma cells and microglia.<br><br>The last results chapter is dedicated to exploring the use of the neuropeptide nanostructures as biotemplates to fabricate inorganic nanomaterials (Chapter 6). The neuropeptides are shown to catalyse the synthesis of silica nanotubes, gold nanoparticles, gold crystals and silver nanostructures. These materials offer novel perspectives of sol-gel synthesis pathways for inorganic nanostructures under soft conditions.<br><br>Overall, the results presented in this thesis contribute to enhancing the current understanding of functional amyloid formation. The bio-nanomaterials developed in this thesis could be developed into nanotechnology applications by further research, including nanoneedles for drug delivery and inorganic nanomaterials for diagnostic tools or nanoelectronics.