PARylation modulates stress granule assembly, phase separation, and neurotoxicity of ALS-related RNA-binding proteins
Mutations in RNA-binding proteins localized in ribonucleoprotein (RNP) granules, such as hnRNP A1 and TDP-43, promote aberrant protein aggregations, which are pathological hallmarks in neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Protein posttranslational modifications (PTMs) are known to regulate RNP granules. In this study, we investigate the function of PARylation, an important PTM involved in DNA damage repair and cell death, in RNP-related neurodegeneration. We reveal that PARylation levels are a major regulator of the dynamic assembly-disassembly of RNP granules, and the disease-related RNPs such as hnRNP A1 and TDP-43 can both be PARylated and bind to PARylated proteins. We further identify the PARylation site of hnRNP A1 at K298, which controls the cytoplasmic translocation of hnRNP A1 in response to stress, as well as the PAR-binding motif (PBM) of hnRNP A1, which is required for the delivery and association of hnRNP A1 to stress granules. Moreover, we show that PAR not only dramatically enhances the liquid-liquid phase separation of hnRNP A1, but also promotes the co-phase separation of hnRNP A1 and TDP-43 in vitro and their interaction in vivo. Finally, we establish that both genetic and pharmacological inhibition of PARP mitigates hnRNP A1 and TDP-43-mediated neurotoxicity in cell and Drosophila models of ALS. Together, our findings indicate a novel and crucial role of PARylation in regulating the assembly and the dynamics of RNP granules, and dysregulation of PARylation may contribute to ALS disease pathogenesis.