The Stabilization of S100A9 Structure by Calcium Inhibits the Formation of Amyloid Fibrils.

The calcium-binding protein S100A9 is recognized as an important component of the brain neuroinflammatory response to the onset and development of neurodegenerative disease. S100A9 is intrinsically amyloidogenic and in vivo co-aggregates with amyloid-ß peptide and α-synuclein in Alzheimer's and Parkinson's diseases, respectively. It is widely accepted that calcium dyshomeostasis plays an important role in the onset and development of these diseases, and studies have shown that elevated levels of calcium limit the potential for S100A9 to adopt a fibrillar structure. The exact mechanism by which calcium exerts its influence on the aggregation process remains unclear. Here we demonstrate that despite S100A9 exhibiting α-helical secondary structure in the absence of calcium, the protein exhibits significant plasticity with interconversion between different conformational states occurring on the micro- to milli-second timescale. This plasticity allows the population of conformational states that favour the onset of fibril formation. Magic-angle spinning solid-state NMR studies of the resulting S100A9 fibrils reveal that the S100A9 adopts a single structurally well-defined rigid fibrillar core surrounded by a shell of approximately 15-20 mobile residues, a structure that persists even when fibrils are produced in the presence of calcium ions. These studies highlight how the dysregulation of metal ion concentrations can influence the conformational equilibria of this important neuroinflammatory protein to influence the rate and nature of the amyloid deposits formed.

Interactions between S100A9 and Alpha-Synuclein: Insight from NMR Spectroscopy.

S100A9 is a pro-inflammatory protein that co-aggregates with other proteins in amyloid fibril plaques. S100A9 can influence the aggregation kinetics and amyloid fibril structure of alpha-synuclein (α-syn), which is involved in Parkinson's disease. Currently, there are limited data regarding their cross-interaction and how it influences the aggregation process. In this work, we analyzed this interaction using solution 19F and 2D 15N-1H HSQC NMR spectroscopy and studied the aggregation properties of these two proteins. Here, we show that α-syn interacts with S100A9 at specific regions, which are also essential in the first step of aggregation. We also demonstrate that the 4-fluorophenylalanine label in alpha-synuclein is a sensitive probe to study interaction and aggregation using 19F NMR spectroscopy.

Formation of Calprotectin Inhibits Amyloid Aggregation of S100A8 and S100A9 Proteins.

Calcium-binding S100A8 and S100A9 proteins play a significant role in various disorders due to their pro-inflammatory functions. Substantially, they are also relevant in neurodegenerative disorders via the delivery of signals for the immune response. However, at the same time, they can aggregate and accelerate the progression of diseases. Natively, S100A8 and S100A9 exist as homo- and heterodimers, but upon aggregation, they form amyloid-like oligomers, fibrils, or amorphous aggregates. In this study, we aimed to elucidate the aggregation propensities of S100A8, S100A9, and their heterodimer calprotectin by investigating aggregation kinetics, secondary structures, and morphologies of the aggregates. For the first time, we followed the in vitro aggregation of S100A8, which formed spherical aggregates, unlike the fibrillar structures of S100A9 under the same conditions. The aggregates were sensitive to amyloid-specific ThT and ThS dyes and had a secondary structure composed of ß-sheets. Similarly to S100A9, S100A8 protein was stabilized by calcium ions, resulting in aggregation inhibition. Finally, the formation of S100A8 and S100A9 heterodimers stabilized the proteins in the absence of calcium ions and prevented their aggregation.

The Surprising Nonlinear Effects of S100A9 Proteins in the Retina.

Age-related macular degeneration (AMD) is a complex disease in which inflammation is implicated as a key factor but the precise molecular mechanisms are poorly understood. AMD lesions contain an excess of the pro-inflammatory S100A9 protein, but its retinal significance was yet unexplored. S100A9 was shown to be intrinsically amyloidogenic in vitro and in vivo. Here, we hypothesized that the retinal effects of S100A9 are related to its supramolecular conformation. ARPE-19 cultures were treated with native dimeric and fibrillar S100A9 preparations, and cell viability was determined. Wild-type rats were treated intravitreally with the S100A9 solutions in the right eye and with the vehicle in the left. Retinal function was assessed longitudinally by electroretinography (ERG), comparing the amplitudes and configurations for each intervention. Native S100A9 had no impact on cellular viability in vitro or on the retinal function in vivo. Despite dispersed intracellular uptake, fibrillar S100A9 did not decrease ARPE-19 cell viability. In contrast, S100A9 fibrils impaired retinal function in vivo following intravitreal injection in rats. Intriguingly, low-dose fibrillar S100A9 induced contrasting in vivo effects, significantly increasing the ERG responses, particularly over 14 days postinjection. The retinal effects of S100A9 were further characterized by glial and microglial cell activation. We provide the first indication for the retinal effects of S100A9, showing that its fibrils inflicted retinal dysfunction and glial activation in vivo, while low dose of the same assemblies resulted in an unpredicted enhancement of the ERG amplitudes. These nonlinear responses highlight the consequences of self-assembly of S100A9 and provide insight into its pathophysiological and possibly physiological roles in the retina.