Modulation of retroviral capsid assembly halts ARC-mediated TDP-43 intercellular spreading

Intercellular propagation of pathological TDP-43 drives the progression of ALS and FTD, yet the mechanisms enabling transmission remain elusive. Here, we demonstrate that Activity-regulated cytoskeleton associated protein (ARC/Arg3.1) is pathologically subverted to act as a retroviral-like capsid vehicle for TDP-43 spreading. In a Drosophila model, we reveal that massive Arc1 upregulation drives glia-to-neuron TDP-43 seeding, while its genetic ablation halts pathological transfer, improves motor function, and extends survival. In parallel human cellular models, stress-induced ARC colocalizes with TDP-43 to orchestrate its intercellular transmission. Guided by ARCs capsid architecture, we reproposed Lenacapavir, an FDA-approved HIV-1 capsid modulator, as a stable binder of ARC/Arg3.1 capsid interfaces. Lenacapavir treatment effectively blocks TDP-43 propagation in vitro and rescues disease phenotypes in vivo. Our findings establish ARC as a conserved vehicle for pathological protein transmission and deliver an immediately translatable pharmacological strategy to arrest disease progression in ALS and FTD.