Unravelling the Role of S100A9 in the Development of Neurodegenerative Disease

Abstract

S100A9 is a pro-inflammatory calcium binding protein released by neutrophils in response to neuroinflammatory events. These events can be triggered by a range of conditions including traumatic brain injury and bacterial infection but are also observed in a range of neurodegenerative conditions including Alzheimer's Disease and Parkinson's Disease. S100A9 is intrinsically amyloidogenic self-aggregating to form organised fibrils both in vitro and in vivo. These S100A9 fibrils congregate with amyloid-β to seed and accelerate the growth of amyloid-β fibrils, as observed in in vitro studies. S100A9 possesses two Ca2+ binding sites which when vacant increase its propensity to aggregate into an ordered fibrillar state. Inversely, increasing calcium concentration inhibits S100A9 fibril formation as verified by Thioflavin-T assay. Our solution-state NMR studies indicate that in the absence of calcium the protein is largely unfolded adopts a number of interconverting states which become more structured upon calcium binding. Interestingly complementary synchrotron radiation circular dichroism measurements suggest a high alpha-helical content irrespective of calcium levels, suggesting that these interconverting species are rich in short-lived helical states. To determine the structural transition that occur upon fibril formation, solid-state NMR investigations have been utilised. These studies demonstrate that S100A9 fibrils formed in vitro have a single well-ordered structure with mobile regions limited to 15-25 residues in size. Analysis of the chemical shifts indicate the protein exists as a single polymorph with a rigid protein core that is predominantly β-strand in structure. Employing paramagnetic relaxation assisted data collection (PAC) progress is being made towards a complete assignment permitting a detailed analysis of the conformation of S100A9 within amyloid fibres.