Application of polystyrene sulfonate (PSS) for inhibiting toxicity of ALS/FTD-linked dipeptide repeats

The expansion of GGGGCC (G4C2) repeats in the noncoding region of C9orf72 is the most common genetic cause of familial and sporadic amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The repeat region is translated into five different dipeptide repeats (DPRs), of which the arginine-rich DPRs (R-DPRs) poly-GR (GRn) and poly-PR (PRn) are highly neurotoxic and are probably primarily responsible for the disease. Here, we characterized the protective effect against R-DPR toxicity of polystyrene sulfonate (PSS), an FDA-approved drug applied in hyperkalemia, in biochemical, cellular, and animal models of ALS/FTD. We found that PSS, in a length-dependent manner, interacts very tightly with R-DPRs, and releases their bound RNA in R-DPR - RNA mixtures. PSS significantly influences the liquid-liquid phase separation (LLPS) of R-DPRs elicited by RNA and reduces their ensuing cell toxicity in Neuro2a cells. PSS is cell penetrable, and it is also effective in countering the toxicity of R-DPRs in zebrafish embryos. Except for the longest (n = 340) variant, PSS is toxic neither to cells nor to mice upon intracerebroventricular injection up to 1 mM concentration. Our results suggest that its polymeric nature endows PSS with an advantageous effect in C9-ALS/FTD and offers a possible remedy against this debilitating neurodegenerative disease.