(A) Under physiological conditions, nuclear TDP-43 is highly bound to GU-rich nuclear RNAs, limiting the pool of free TDP-43 capable of exiting nuclei by free diffusion through NPCs.
(B) Perturbations that deplete nuclear GU-rich RNAs or impair TDP-43 RNA binding (e.g., transcriptional blockade, RNase, RRM mutations, or putative disruption of GU-RNA homeostasis in disease) cause an initial increase in nuclear free TDP-43, accelerating its passive export until reaching steady state at a lower N/C ratio.
Insets depict the cellular GU-RNA N/C gradients that dictate TDP-43 localization.