Hypothesis

Core proposition: Age‑related excitotoxic vulnerability in basal forebrain cholinergic neurons (BFCN) stems not from loss of cytosolic calcium buffering per se, but from a mismatch where enhanced rapid cytosolic buffering outpaces mitochondrial calcium uptake capacity. Restoring mitochondrial calcium handling—specifically by boosting the mitochondrial calcium uniporter (MCU) or its regulators—will normalize calcium dynamics during physiological challenges and prevent neurodegeneration, even if calbindin levels remain low.

Mechanistic rationale

Recent work shows that aged BFCN display:

• 50‑100 % increase in rapid cytosolic calcium buffering (likely via upregulated plasma‑membrane pumps or buffers) [3]
• Simultaneously reduced mitochondrial membrane potential and MCU activity, limiting mitochondrial calcium uptake during high‑load bursts [3]
• Calbindin loss correlates with selective degeneration and resistance to neurofibrillary tangles when present [1]

The paradox—greater cytosolic buffering yet higher vulnerability—suggests that the neuron can mop up small Ca²⁺ flashes but cannot sequester the large, sustained Ca²⁺ loads that arise during physiological firing or amyloid‑β‑induced excitotoxicity. The excess cytosolic Ca²⁺ then activates calpains, phosphatases, and mitochondrial permeability transition, driving degeneration.

Testable predictions

1. Mitochondrial calcium uptake is the limiting step.

   • Experiment: Use AAV‑mediated overexpression of MCU (or the activator MICU1) specifically in BFCN of aged wild‑type mice. Measure mitochondrial Ca²⁺ uptake with Rhod‑2 AM imaging during high‑frequency electrical stimulation (20 Hz, 1 s) in acute brain slices.
   • Prediction: MCU‑overexpressing neurons will show ≥30 % faster mitochondrial Ca²⁺ clearance and reduced peak cytosolic Ca²⁺ ([Ca²⁺]i) compared with GFP controls.

2. Rescuing mitochondrial uptake prevents excitotoxic degeneration independent of calbindin.

   • Experiment: In aged APP/PS1 mice, combine MCU overexpression with calbindin‑shRNA (to keep calbindin low). Assess BFCN survival (ChAT⁺ cell count) and phospho‑tau burden after 3 months.
   • Prediction: Despite low calbindin, MCU‑overexpressing BFCN will exhibit survival comparable to young controls (<15 % loss) and reduced tau pathology, whereas calbindin‑shRNA alone will not protect.

3. Combined calbindin restoration + mitochondrial enhancement yields additive protection.

   • Experiment: Four groups: (a) control AAV‑GFP, (b) calbindin overexpression, (c) MCU overexpression, (d) both calbindin + MCU.
   • Prediction: Group (d) will show the greatest preservation of BFCN density and cognitive performance in Morris water maze, exceeding the sum of individual effects (synergy index >1).

Falsifiability

If mitochondrial Ca²⁺ uptake enhancement fails to改善 cytosolic Ca²⁺ handling during high‑load challenges (no reduction in peak [Ca²⁺]i) or does not improve BFCN survival/excitotoxic resilience under any condition, the hypothesis that mitochondrial uptake is the rate‑limiting step is falsified. Likewise, if calbindin overexpression alone fully rescues the phenotype despite unchanged mitochondrial function, the central role of mitochondrial buffering would be questioned.

Therapeutic implication

The hypothesis prioritizes mitochondrial Ca²⁺ transport as a druggable target. Small‑molecule MCU activators (e.g., synthetic peptides that relieve MICU1 inhibition) or gene‑therapy approaches could be tested in aged rodents, with translational potential for early‑stage Alzheimer’s disease where BFCN loss precedes cortical atrophy.