Hypothesis

Aging reduces O‑GlcNAcylation of importin7 at a regulatory serine (Ser‑617), increasing its affinity for the ERK1/2 nuclear translocation sequence (NTS: Ser‑244/Pro‑245/Ser‑246). This shift enhances ERK nuclear import, prolongs nuclear dwell time, and sustains ERK‑driven selective protein degradation (SAPD) of c‑MYC and STAT3, thereby locking cells into a senescent state. Restoring importin7 O‑GlcNAcylation—or blocking its interaction with ERK—should reroute ERK to the cytoplasm, diminish SAPD of proliferative factors, and bypass senescence without globally inhibiting MAPK signaling, thus limiting oncogenic risk.

Mechanistic Rationale

• O‑GlcNAcylation as a nutrient‑sensing switch: The hexosamine biosynthesis pathway (HBP) flux declines with age, lowering cellular UDP‑GlcNAc levels. Importin7 contains a conserved serine residue adjacent to its ERK‑binding domain that is known to be O‑GlcNAcylated in vitro. Loss of this modification removes a steric/charge hindrance, allowing tighter docking of importin7 to phosphorylated ERK.

• Impact on shuttling kinetics: We predict a decrease in the nuclear export rate (CRM1‑dependent) unchanged, while the import rate (k_import) rises due to higher importin7‑ERK binding affinity. Nuclear dwell time (τ_nuclear = 1/k_export) therefore increases, extending the window for ERK to phosphorylate nuclear substrates that recruit E3 ligases (e.g., FBW7) responsible for SAPD of c‑MYC and STAT3.

• SAPD specificity: Nuclear‑retained ERK preferentially phosphorylates substrates that create phosphodegrons recognized by nuclear E3 ligases. Cytoplasmic ERK, by contrast, phosphorylates cytosolic targets that promote proliferation. Thus, compartmentalization, not total ERK activity, dictates the senescence versus growth outcome.

• Therapeutic window: Modulating importin7 O‑GlcNAcylation (e.g., with glucosamine supplementation or O‑GlcNAc transferase (OGT) activators) or using a peptide that mimics the O‑GlcNAc‑modified importin7 ERK‑binding site should reduce nuclear import without affecting ERK’s catalytic activity. This avoids the oncogenic concern of global MAPK inhibition while preserving ERK‑dependent survival signals in non‑senescent cells.

Testable Predictions

1. Biochemical: Importin7 from aged fibroblasts shows reduced O‑GlcNAcylation (measured by lectin blot or click‑chemistry) and increased binding affinity for phosphorylated ERK1/2 (SPR or ITC) compared with young cells.
2. Imaging: Live‑cell FRAP or FLIP of ERK‑KTR reporters will reveal a higher nuclear import rate (k_import) and longer nuclear residence in senescent cells; rescuing O‑GlcNAcylation will normalize these parameters to youthful levels.
3. Functional: OGT overexpression or glucosamine treatment in aged human fibroblasts will decrease nuclear p‑ERK, reduce SAPD of c‑MYC and STAT3 (western blot for ubiquitinated forms), lower SA‑β‑gal positivity, and increase proliferation (EdU incorporation) without triggering anchorage‑independent growth in soft‑agar assays.
4. In vivo: Mice with a knock‑in importin7 S617A (non‑O‑GlcNAcylatable) mutant will exhibit accelerated accumulation of senescent markers in liver and skin, whereas S617D (O‑GlcNAc mimic) mice will show delayed aging phenotypes and extended healthspan.

Falsifiability

If O‑GlcNAcylation of importin7 does not change with age, or if manipulating this modification fails to alter ERK nuclear shuttling kinetics and senescence markers, the hypothesis is refuted. Likewise, if global ERK inhibition—not just nuclear import modulation—is required to bypass senescence, the mechanistic link between importin7 O‑GlcNAcylation and senescence would be insufficient.

Keywords: ERK nuclear import, importin7, O‑GlcNAcylation, selective protein degradation, senescence, shuttling kinetics

• MAPK/ERK dual role in senescence
• Importin7 mediates ERK nuclear translocation
• Aged cells show altered ERK compartmentalization
• Genetic RAS/MAPK inhibition extends lifespan