Deciphering the Inhibitory Mechanism of ALS-Associated N352S and S352p Variants against TDP-43 Aggregation and Its Destabilization Effect on TDP-43 Protofibrils.

Amyotrophic lateral sclerosis (ALS) is closely related to ubiquitin-positive inclusions formed by transactive response deoxyribonucleic acid (DNA) binding protein of 43 kDa (TDP-43). Previous experiments identified that the ALS-linked familial variant, N352S (asparagine substituted by serine), and subsequent phosphorylation of S352 (S352p) are associated with the aggregation of TDP-43. However, the underlying molecular mechanisms are still not fully understood. By performing all-atom explicit-solvent replica exchange molecular dynamics (REMD) simulations with a total simulation time of 100.8 μs, we scrutinized the impact of the N352S mutation and its phosphorylation variant S352p on the conformational ensembles of the TDP-43 dimer. Our simulation results show that both the N352S and S352p variants could promote the formation of unstructured conformation and impede the formation of β-structure and helix content, and the inhibitive effect of S352P is more obvious. Further analyses suggest that the H-bonding and hydrophobic interaction among TDP-43 peptides, as well as the R361-E362 salt bridge, are attenuated by N352S and S352p variants. Additional MD simulations show that N352S and S352p variants reduce the structural stability of the hydrophobic region and lower the number of H-bonds and contacts of two hydrophobic clusters, thus possessing a destabilization effect on the TDP-43 protofibrils. Our results unmask the molecular mechanism of the N352S mutation and its phosphorylation variant S352p toward the inhibition of TDP-43 aggregation and prove the protofibril-destabilizing effects of these two variants, which may be helpful for designing drugs for the treatment of ALS.