Hypothesis

REST protein in spiral ganglion neurons (SGNs) acts as a bidirectional activity sensor, evicting both hypo- and hyperactive neurons when cochlear energy budgets fall below critical thresholds. This extends CNS longevity mechanisms to auditory aging, proposing that stria vascularis decline reduces endocochlear potential, forcing SGNs into a survival contest where REST-mediated efficiency screening determines fate.

Mechanistic Rationale

In the CNS, REST suppresses overactivity to extend lifespan, while hypoactive neurons fail to maintain BDNF/NT-3 autocrine signaling and succumb to apoptosis. Cochlear aging shows a paradox: low-spontaneous-rate SGNs die first via caspase-3/Bax activation, not hyperactive ones as in metabolic exhaustion. This suggests a Goldilocks zone for SGN activity—too low, and they're pruned; too high, and they'd be suppressed. But why would hypoactive SGNs be evicted? The answer lies in energy economics. Stria vascularis decline drops ATP and ion gradients, raising the metabolic cost of maintaining quiescent neurons. REST, known to repress genes linked to neuronal hyperactivity in lifespan studies, might also downregulate survival pathways in hypoactive SGNs when energy is scarce.

Novel insight: REST could function as a metabolic rheostat—integrating signals from stria vascularis health (e.g., ATP levels, oxidative stress) to set dynamic activity thresholds. Under high energy, REST tolerance widens, preserving SGNs across activity ranges. Under energy decline, thresholds tighten, evicting SGNs that don't generate sufficient synaptic return on investment. This explains the early loss of low-spontaneous-rate neurons: they're metabolically "expensive" to keep alive when every ATP counts.

Testable Predictions

1. REST expression inversely correlates with SGN survival in aged cochleas. Immunohistochemistry should show elevated REST in surviving SGNs during presbycusis, particularly in high-spontaneous-rate populations. If REST is absent or uniform, the hypothesis fails.
2. REST knockout mice resist low-activity SGN loss under metabolic stress. Using conditional SGN-specific REST deletion and cochlear energy stress models (e.g., strial ischemia), we predict preserved low-spontaneous-rate neurons. If loss persists, REST isn't the primary driver.
3. Pharmacological REST activation accelerates SGN pruning. Administering REST agonists in young, healthy cochleas should selectively eliminate hypoactive SGNs, mimicking age-related patterns. No effect would challenge REST's role.
4. BDNF/NT-3 supplementation rescues hypoactive SGNs only when REST is inhibited. This tests the interaction: if REST blocks neurotrophin signaling in low-activity neurons, dual intervention should show synergy.

Why This Synthesizes and Extends Prior Work

• Links CNS and cochlear aging via shared REST machinery, addressing the unexplored REST-SGN connection.
• Reframes "use-it-or-lose-it" as energy-dependent eviction: neurons aren't just lost due to disuse, but because REST reprograms them for apoptosis when metabolic stress makes maintenance inefficient.
• Predicts dual vulnerability: hyperactive SGNs might be spared in early presbycusis because REST suppresses them, but under chronic stress, even high-activity neurons could succumb if REST thresholds shift—potentially explaining late-stage widespread SGN loss.

Implications for Intervention

If correct, therapies should modulate REST or energy supply, not just broadly support neuronal survival. For example, enhancing stria vascularis function might widen REST thresholds, while targeted REST inhibitors could protect low-activity SGNs in early disease. This moves beyond "protect all neurons" to strategic conservation of circuit efficiency.