Persistent export bias of TDP-43 under native autoregulation links insoluble accumulation to nuclear dysfunction

Nuclear depletion and cytoplasmic mislocalization of TDP-43 are central pathological features of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. TDP-43 protein levels are normally maintained by autoregulation through its native 3' untranslated region (3' UTR), but whether this feedback remains protective during chronic cytoplasmic bias is unclear. To address this, we engineered full-length human TDP-43 carrying an N-terminal nuclear export signal (NES) while retaining the native 3' UTR autoregulatory module. In HEK293T cells, NES insertion imposed cytoplasmic bias and promoted detergent-insoluble TDP-43 species. In differentiated SH-SY5Y cells, nuclear splicing defects and autoregulatory changes scaled with export-biased load; detergent-insoluble accumulation was already detectable within a low-load range, defined by whole-cell RIPA-soluble exogenous TDP-43 ≤ 30% of endogenous levels. Human iPSC-derived neurons showed a comparable cytoplasmic shift, discrete TDP-43-immunoreactive foci, and TDP-43-dependent splicing defects. Endogenous TARDBP depletion provided a functional rescue test: nuclear-competent WT-TDP-43-3' UTR restored TDP-43-dependent nuclear readouts, whereas NES-TDP-43-3' UTR did not. In the NES condition, weakened autorepression increased transgene-derived TARDBP transcripts, but the added output failed to expand the soluble, splice-competent pool and instead partitioned into insoluble fractions. Increasing soluble NES-TDP-43 to endogenous-equivalent levels likewise did not normalize splicing, indicating that abundance alone is insufficient when output remains export-biased. These findings support a model in which persistent export bias converts native TARDBP autoregulation into maladaptive feedback: compensatory output is uncoupled from productive nuclear recovery and diverted toward cytoplasmic insoluble/fragmented species.