Site-Specific Aspartic Acid d-Isomerization in Tau R2 and R3 Peptide Seeds Attenuates Seed-Induced Fibril Formation of Full-Length Tau

The aggregation of tau protein is a central pathological event in Alzheimer's disease, and this pathology is hypothesized to spread via a prion-like mechanism driven by tau "seeds". While aggregated tau from Alzheimer's disease brains is known to contain age-related d-isomerized aspartic acid (d-Asp) residues, it remains unknown how this modification affects the seeding activity that drives disease propagation. Here, we investigated the impact of site-specific d-isomerization within R2 and R3 tau repeat-domain peptides, which form the core of tau fibrils. We demonstrate that the stereochemical integrity of these peptides is critical for their seeding function. d-isomerization at Asp314 within the R3 peptide seed severely impaired its ability to template the fibrillization of full-length tau in vitro. This finding was validated in a cellular model, where R3 seeds containing d-Asp314 were significantly less potent at inducing the formation of phosphorylated tau aggregates compared to wild-type seeds. Our results establish that Asp d-isomerization within tau seeds acts as a potent attenuator of their pathological seeding activity, suggesting this spontaneous modification may intrinsically modulate the progression of Alzheimer's disease.