Heterotypic droplet formation by pro-inflammatory S100A9 and neurodegenerative disease-related alpha-synuclein

Liquid-liquid phase separation (LLPS) of proteins and nucleic acids is a rapidly emerging field of study, aimed at understanding the process of biomolecular condensate formation and its role in cellular functions. LLPS has been shown to be responsible for the generation of promyelocytic leukemia protein bodies, stress granules, and intrinsically disordered protein condensates. Recently, it has been discovered that different neurodegenerative disease-related proteins, such as alpha-synuclein (related to Parkinsons disease) and amyloid-beta (Alzheimers disease) are capable of forming heterotypic droplets. Other reports have also shown non-LLPS cross-interactions between various amyloidogenic proteins and the resulting influence on their amyloid fibril formation. This includes the new discovery of pro-inflammatory S100A9 affecting the aggregation of both amyloid-beta, as well as alpha-synuclein. Combined, these observations suggest that protein interactions during LLPS and heterotypic droplet formation may be a critical step in the onset of neurodegenerative diseases. In this study, we explore the formation of heterotypic droplets by S100A9 and alpha-synuclein using a range of different spectroscopic and microscopic techniques. We show that the protein mixture is capable of assembling into both homotypic, as well as heterotypic condensates and that this cross-interaction alters the aggregation mechanism of alpha-synuclein. In addition, it also stabilizes a specific fibril conformation, which has a higher propensity for self-replication. These results provide insight into the influence of S100A9 on the process of neurodegenerative disease-related protein LLPS and aggregation, bringing us one step closer to developing a potential cure or treatment modality.