Hypothesis

Latent Nrf2‑mediated hormesis in spiral ganglion neurons (SGNs) is suppressed by chronic NF‑κB‑driven KEAP1 expression.

Rationale

• Cochlear SGNs show no detectable HSP70, Nrf2, or mitohormetic activation after subthreshold oxidative or metabolic stress 1.
• In contrast, systemic interventions (caloric restriction, exercise) delay age‑related hearing loss, implying that cochlear cells can benefit from hormetic signals when they reach the organ 2.
• Persistent inflammation in the stria vascularis releases TNF‑α and IL‑1β that activate NF‑κB in neighboring SGNs 3.
• NF‑κB directly upregulates KEAP1 transcription, promoting ubiquitination and proteasomal degradation of Nrf2, thereby silencing the antioxidant response element (ARE) program 4.

Mechanistic Insight

We propose that the cochlea possesses a latent Nrf2 hormetic pathway that is kept "off" by a feed‑forward loop: inflammatory cytokines → NF‑κB → KEAP1 ↑ → Nrf2 ↓ → ARE genes ↓ → ROS accumulates → further inflammation. When NF‑κB signaling is attenuated, KEAP1 levels fall, allowing Nrf2 to stabilize, translocate to the nucleus, and drive expression of HO‑1, NQO1, GCLC, and mitochondrial antioxidant enzymes upon even mild oxidative challenge. This would convert a purely reactive damage response into a protective hormetic one.

Testable Predictions

1. Baseline state – In SGNs from aged mice, KEAP1 mRNA and protein are elevated, Nrf2 is predominantly cytosolic, and ARE‑target expression is low compared with young animals.
2. NF‑κB inhibition – Treating aged SGN explants with a selective IKKβ inhibitor (e.g., BAY 11‑7082) for 24 h reduces KEAP1 levels, increases nuclear Nrf2, and augments ARE‑reporter activity even without exogenous stress.
3. Hormetic challenge – Subsequent exposure to a low, subtoxic dose of H₂O₂ (50 µM, 1 h) in the presence of NF‑κB inhibitor triggers a robust Nrf2‑dependent transcriptional response (HO‑1, NQO1 ↑ > 3‑fold) and reduces mitochondrial ROS and caspase‑3 activation relative to H₂O₂ alone.
4. In vivo validation – Local delivery of an NF‑κB‑blocking peptide to the cochlea of middle‑aged mice, followed by weekly mild noise exposure (85 dB SPL, 1 h), preserves SGN counts and auditory brain‑stem response thresholds after 8 weeks, whereas noise alone or peptide alone does not.
5. Falsification – If NF‑κB inhibition fails to increase nuclear Nrf2 or to confer protection against low‑dose oxidative stress, the hypothesis is refuted, indicating that other mechanisms (e.g., epigenetic silencing of Nrf2 promoters) dominate.

Experimental Outline

• Animals: Young (3 mo) and aged (18‑mo) C57BL/6J mice.
• Ex vivo: Dissect cochleae, culture SGN explants; treat with vehicle, IKKβ inhibitor, low H₂O₂, or combination.
• Readouts: Western blot for KEAP1, Nrf2 (total & nuclear); qPCR for HO‑1, NQO1, GCLC; ROS (MitoSOX); apoptosis (cleaved caspase‑3 immunohistochemistry).
• In vivo: Micro‑pump delivery of IKKβ inhibitor via round‑window; noise exposure schedule; ABR testing and SGN survival counting.

Implications

Confirming this model would redefine cochlear aging not as an irreversible lack of hormetic capacity but as a suppressed one, opening therapeutic avenues that target neuroinflammatory signaling to unlock intrinsic antioxidant hormesis. It would also explain why systemic hormetic interventions (which lower circulating cytokines) benefit hearing, while direct cochlear preconditioning attempts have failed—because the local inflammatory tone must first be quelled.