C9-ALS-Associated Proline-Arginine Dipeptide Repeat Protein Induces Activation of NLRP3 Inflammasome of HMC3 Microglia Cells by Binding of Complement Component 1 Q Subcomponent-Binding Protein (C1QBP), and Syringin Prevents This Effect

Amyotrophic lateral sclerosis (ALS) is a fatal disease in which motor neurons gradually degenerate. The mutation of the C9orf72 gene is the main genetic cause of ALS (C9-ALS). One of its specific pathological features is the production of proline-arginine (PR) dipeptide repeat protein (DPR). In this study, we developed a PR-DPR (PR(50))-expressing human HMC3 microglial cell model. We found that PR(50) mainly aggregates into spots in the nucleus and induces significant NLRP3 inflammasome activity. Moreover, mouse NSC-34 motor neuron cells treated with a conditional medium of PR(50)-expressing HMC3 cells (PR-CM) caused cell damage and apoptosis activity. However, R(50)-expressing HMC cells treated with MCC950 (an NLRP3 inhibitor) reversed this result. Furthermore, we identified complement component 1 q subcomponent-binding protein (C1QBP) as one of the interaction partners of PR(50). The downregulation of C1QBP in HMC3 cells induces NLRP3 inflammasome activity similar to PR(50) expression. Finally, we found that syringin can block the interaction between PR(50) and C1QBP, and effectively reduce the PR(50)-induced NLRP3 inflammasome activity in HMC3 cells. This improves the apoptosis of NSC-34 cells caused by PR-CM. This study is the first to link PR(50), C1QBP, and NLRP3 inflammasome activity in microglia and develop potential therapeutic strategies for syringin intervention in C9-ALS.