Poly(ADP-ribose) Promotes Liquid-Liquid Phase Separation of C9orf72 Arginine Containing Dipeptide Repeats
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C9orf72 hexanucleotide repeat expansion is the most common cause of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The dipeptide repeats (DPRs) translated from C9orf72 hexanucleotide expansions are neurotoxic and can impair number of biological processes. The propensity of DPRs towards liquid-liquid phase separation may account for cellular abnormalities observed in neurodegenerative diseases. The pathological size of DPRs can go up to hundreds of repeats, while heathy individuals carry 2-8 repeats. Arginine containing DPRs longer than 20 units can undergo liquid-liquid phase separation (LLPS) in in vitro and in vivo models, but what promotes DPRs propensity of phase separation is not completely understood. Poly (ADP-ribose) is a biopolymer, known to mediate LLPS of many arginine rich proteins. We developed LLPS in vitro assay and showed that PAR was very potent at promoting phase separation for 20 repeats of arginine-containing (R-)DPRs, with sub-stoichiometric amount of PAR (1: 5000 of PAR:DPRs) being sufficient to foster R-DPRs phase separation. Interestingly, 20 and 10 repeats of R-DPRs underwent phase separation with PAR, but not 5 repeats. Moreover, our statistical analysis for the DPR-PAR droplets suggested homotypic or heterotypic interactions might be dependent on the stoichiometry in the reaction.