Gain-of-function profilin 1 mutations linked to familial amyotrophic lateral sclerosis cause seed-dependent intracellular TDP-43 aggregation.

Profilin 1 (PFN1) is an actin monomer-binding protein essential for regulating cytoskeletal dynamics in all cell types. Recently, mutations in the PFN1 gene have been identified as a cause of familial amyotrophic lateral sclerosis (ALS). The co-aggregation of PFN1 bearing mutations that cause ALS with TDP-43 (a key molecule in both sporadic and some familial forms of ALS), together with the classical TDP-43 pathology detected in post-mortem tissues of patients with autosomal dominant PFN1 mutation, imply that gain-of-toxic-function of PFN1 mutants is associated with the onset of ALS. However, it remains unknown how PFN1 mutants cause ALS. We found mutant PFN1 that causes ALS formed cytoplasmic aggregates positive for ubiquitin and p62, and these aggregates sequestered endogenous TDP-43. In cells harboring PFN1 aggregates, formation of aggresome-like structures was inhibited in the presence of proteasome inhibitor, and conversion of LC3-I to LC3-II was suppressed in the presence of lysosome inhibitor. Further, insoluble TDP-43 was increased in both cases. Co-expression of ALS-linked mutant PFN1 and TDP-43 increased insoluble and phosphorylated TDP-43 levels. The C-terminal region of TDP-43, essential for aggregation of TDP-43, was also indispensable for the interaction with PFN1. Interestingly, insoluble fractions prepared from cells expressing ALS-linked mutant PFN1 functioned as a seed to induce accumulation and phosphorylation of TDP-43, indicating that TDP-43 accumulated in the presence of the PFN1 mutants is converted to prion-like species. These findings provide new insight into the mechanisms of neurodegeneration in ALS, suggesting that gain-of-toxic-function PFN1 gene mutation leads to conformational change of TDP-43.