Age‑Dependent Switch from Hormetic Protection to Maladaptive UPR Activation in the Cochlea

Hypothesis

In aged cochlear tissue, hormetic stressors such as intermittent hypoxia or low‑dose oxidative challenge convert from protective to maladaptive because declining Nrf2 activity releases repression of the PERK‑eIF2α‑ATF4 and IRE1α‑XBP1 branches of the unfolded protein response (UPR). This shift drives CHOP‑mediated apoptosis in low‑spontaneous‑rate spiral ganglion neurons (SGNs), accelerating age‑related hearing loss.

Mechanistic Basis

Nrf2 normally antioxidant response elements (ARE) also interact with KEAP1‑dependent inhibition of PERK and IRE1α stress sensors [1]. When Nrf2 declines with age, this brake is lifted, allowing basal ER stress to elevate UPR markers even before any external insult [2]. A hormetic pulse then produces an overshoot of UPR signaling: PERK‑eIF2α‑ATF4 drives CHOP transcription, while hyperactive IRE1α increases RIDD activity, degrading SGN‑specific mRNAs required for synaptic transmission [3]. The resulting proteotoxic overload overwhelms the already compromised capacity of aged SGNs, pushing them toward apoptosis rather than adaptive remodeling.

Predictions & Experimental Design

1. Young vs. aged mice (2 mo vs. 24 mo) will receive a preconditioning regimen of intermittent hypoxia (6 % O₂, 3 h/day, 5 days) or low‑dose tert‑butylhydroquinone (tBHQ, 10 µM) to activate Nrf2 pathways.
2. Readouts: Nrf2 nuclear translocation (Western blot), UPR activation (BiP, phospho‑PERK, phospho‑eIF2α, ATF4, CHOP, spliced XBP1), RIDD substrates (e.g., Synaptophysin mRNA), SGN survival (counts of Neurofilament‑positive cells), and auditory brainstem response (ABR) thresholds.
3. Expected outcomes: Young mice will show increased Nrf2 activity, modest UPR induction, preserved SGN numbers, and improved ABR thresholds post‑preconditioning. Aged mice will exhibit blunted Nrf2 response, exaggerated CHOP and RIDD markers, significant SGN loss, and worsened ABR thresholds relative to untreated aged controls.
4. Rescue test: Genetic or pharmacological upregulation of Nrf2 (e.g., Keap1‑conditional knockout or sulforaphane pretreatment) in aged mice should restore the protective hormetic profile, reducing CHOP expression and SGN loss.

Potential Outcomes & Falsifiability

• Support: A clear age‑by‑treatment interaction where hormetic preconditioning benefits young but harms aged cochleae, accompanied by the molecular switch described, validates the hypothesis.
• Refute: If aged mice show Nrf2‑independent improvements (e.g., reduced oxidative markers, increased SGN survival, better ABR) after hormetic stress, the proposed maladaptive UPR switch is insufficient or incorrect, falsifying the core claim.

This framework directly tests whether the age‑related collapse of hormetic capacity reflects a shift from adaptive signaling to maladaptive ER stress, providing a mechanistic bridge between Nrf2 decline, UPR dysregulation, and selective SGN vulnerability in cochlear aging.