Hypothesis: Region-specific calcium-binding protein loss is governed by a mitochondrial calcium-immune axis that differentially regulates CB transcription via NF-κB

Core proposition In aging brain, the decline of calbindin‑D28k (CB) in vulnerable populations such as basal forebrain cholinergic neurons results from sustained mitochondrial calcium overload that activates the cGAS‑STING‑NF‑κB pathway in neighboring tissue‑resident macrophages, leading to cytokine‑mediated suppression of the Calb1 gene. Resistant populations like the locus coeruleus avoid this cascade because they express high levels of mitochondrial antioxidant peroxiredoxin‑3 (PRDX3), which limits ROS‑driven mtDNA release and thus blocks inflammatory signaling, preserving CB expression.

Mechanistic chain

1. Age‑related decline in CB elevates basal intracellular Ca²⁺ ([3]), increasing mitochondrial Ca²⁺ uptake via the mitochondrial calcium uniporter (MCU).
2. Excess mitochondrial Ca²⁺ triggers opening of the permeability transition pore, causing release of oxidized mtDNA into the cytosol of neurons and nearby microglia/macrophages.
3. Cytosolic mtDNA activates cGAS, producing cGAMP, which stimulates STING and downstream TBK1‑IRF3 and NF‑κB signaling ([5]).
4. NF‑κB translocates to the nucleus and binds repressive elements in the Calb1 promoter, reducing CB transcription ([1][2]).
5. In loci with high PRDX3 (e.g., locus coeruleus), ROS scavenging prevents mtDNA oxidation, limiting cGAS activation despite similar calcium load, thereby maintaining CB expression.

Novel insight The hypothesis posits that the decisive factor is not calcium influx per se, but the coupling of calcium‑induced mitochondrial stress to innate immune signaling in a cell‑type‑specific manner, dictated by the local antioxidant capacity.

Testable predictions

• Pharmacological: Acute MCU inhibition (e.g., with Ru360) in aged mice will preserve CB immunoreactivity in basal forebrain and reduce phospho‑tau accumulation, whereas the same treatment will have minimal effect in locus coeruleus because CB is already stable.
• Genetic: Viral overexpression of PRDX3 in basal forebrain cholinergic neurons will attenuate age‑dependent CB loss and protect against tangle formation, while PRDX3 knockdown in locus coeruleus will reproduce CB decline and increase vulnerability to tau pathology.
• Biomarker: Spatial transcriptomics of aged human brain will show an inverse correlation between Calb1 expression and macrophage‑specific cGAS/STING signaling scores, with the strongest negative correlation in basal forebrain and weakest in locus coeruleus.
• Calcium imaging: Neurons lacking CB but treated with a STING antagonist will exhibit normalized basal Ca²⁺ levels despite ongoing MCU activity, indicating that inflammation feeds back to exacerbate calcium dysregulation.

Falsifiability If MCU inhibition fails to rescue CB levels or if PRDX3 manipulation does not alter CB expression in the predicted directions, the mitochondrial‑immune axis model would be refuted. Similarly, absence of a correlative link between macrophage cGAS‑STING activation and regional CB loss across multiple aging models would challenge the hypothesis.

Implications Targeting the mitochondrial calcium‑immune interface—either by modulating MCU activity, boosting mitochondrial antioxidants, or inhibiting cGAS‑STING signaling—could preserve calcium‑buffering capacity and delay or prevent the onset of Alzheimer‑type pathology in vulnerable circuits.