Hypothesis

The age‑dependent decline of TFEB nuclear localization is not merely a consequence of mTORC1 hyperactivity but is primarily driven by age‑related loss of functional importin‑β1 (KPNB1)/IPO7 complexes that gate TFEB entry into the nucleus. Restoring this nuclear import pathway rescues TFEB‑dependent autophagy and consequently mitigates mitochondrial dysfunction, proteostatic failure, and inflammaging—suggesting a single upstream controller that coordinates multiple hallmarks.

Mechanistic Basis

TFEB shuttles between cytoplasm and nucleus via recognition by importin‑β1/KPNB1 and the isoform‑specific adaptor IPO7 through the Ran‑GTP gradient. Recent data show that mTORC1 phosphorylates TFEB at S138/S142, retaining it cytoplasmically, yet TFEB can still enter the nucleus when phosphatases are active. We propose that, with age, oxidative modification of KPNB1 (e.g., cysteine sulfenylation) reduces its affinity for TFEB‑containing cargo, while simultaneous decline in IPO7 expression diminishes the formation of the high‑affinity import complex. This dual hit creates a bottleneck that outweighs the effect of mTORC1 signaling, making nuclear import the rate‑limiting step for TFEB activity.

Predictions

1. Rescue experiment – Forced expression of oxidation‑resistant KPNB1 (Cys→Ser) or IPO7 in aged mouse kidneys will increase nuclear TFEB, restore ATG5/ATG7 transcription, and reduce mitochondrial ROS and lipofuscin accumulation.
2. Phenocopy experiment – Acute knock‑down of KPNB1 or IPO7 in young adult mice will reproduce the aging TFEB cytoplasmic shift, decrease autophagy flux, and induce hallmarks such as mitochondrial dysfunction and inflammatory cytokine elevation within 4 weeks.
3. Biomarker correlation – In human peripheral blood mononuclear cells, the ratio of nuclear to cytosolic TFEB will inversely correlate with plasma markers of oxidative protein damage (e.g., carbonyls) and with expression levels of KPNB1/IPO7 across the lifespan.

Experimental Design

• Animal models: Use CRISPR‑edited knock‑in mice expressing oxidation‑resistant KPNB1 (KPNB1^CR) and a separate line with inducible IPO7 overexpression in proximal tubule cells. Measure TFEB localization (immunofluorescence), autophagy flux (LC3‑II turnover with bafilomycin), mitochondrial membrane potential (JC‑1), and aging phenotypes (grip strength, frailty index) at 6, 12, and 18 months.
• Pharmacological test: Treat aged wild‑type mice with a cell‑permeable importin‑β1 activator (e.g., importazole‑derived small molecule) and assess the same endpoints.
• Human validation: Obtain PBMCs from donors aged 20–80, quantify nuclear TFEB by imaging flow cytometry, KPNB1/IPO7 mRNA by RT‑qPCR, and oxidative damage by ELISA for protein carbonyls. Perform regression analysis.

Potential Pitfalls & Alternatives

If nuclear import restoration fails to improve autophagy, the hypothesis would be falsified, indicating that TFEB regulation is dominated by phosphorylation/dephosphorylation dynamics rather than import capacity. Conversely, if import manipulation alters TFEB but does not affect downstream hallmarks, it would suggest that TFEB‑driven autophagy is necessary but not sufficient for systemic aging, pointing to parallel pathways.

Significance

Positioning the nuclear import machinery as a master upstream controller reframes the hallmarks of aging as downstream manifestations of a transport defect, offering a single nodal target for interventions that could simultaneously attenuate multiple age‑related pathologies.