Hypothesis

We propose that age‑associated microglial phagoptosis of stressed basal forebrain cholinergic neurons (BFCNs) is initiated by complement C1q tagging of neurons that exhibit reduced cholinergic anti‑inflammatory output. This tagging creates a feed‑forward loop: loss of BFCN‑derived acetylcholine diminishes the cholinergic anti‑inflammatory pathway, leading to heightened microglial activation and further C1q deposition, ultimately driving BFCN loss independent of amyloid pathology.

Mechanistic Rationale

1. Stress‑induced neuronal signature – Aging BFCNs show mitochondrial dysfunction and increased surface expression of 'eat‑me' signals such as phosphatidylserine and complement C1q ligands when cholinergic tone falls (4).
2. Complement‑dependent microglial phagoptosis – Microglia express complement receptor CR3 (CD11b/CD18) that binds C1q‑opsonized targets, triggering phagoptosis rather than classic phagocytosis (5).
3. Loss of cholinergic anti‑inflammatory feedback – BFCNs normally release acetylcholine that activates α7‑nicotinic receptors on microglia, suppressing NF‑κB and complement production (3). As BFCNs degenerate, this brake weakens, amplifying microglial C1q release.
4. Independence from Alzheimer’s pathology – Experimental models where amyloid load is low but complement activation is high still show BFCN atrophy, supporting the idea that immune‑driven phagoptosis can operate alone (2).

Testable Predictions

• Prediction 1: In aged mice, pharmacological inhibition of C1q (e.g., anti‑C1q antibodies) will preserve BFCN numbers and cholinergic marker ChAT expression despite normal aging.
• Prediction 2: Conditional knockout of microglial CR3 will block the age‑related decline in BFCN size and restore retrograde NGF transport.
• Prediction 3: Enhancing cholinergic signaling via an α7‑nicotinic agonist will reduce microglial C1q deposition and slow BFCN loss, even when complement is intact.

Falsifiability

If blocking complement or microglial CR3 fails to ameliorate BFCN degeneration in aged animals, or if boosting cholinergic tone does not lower microglial C1q, the hypothesis would be refuted, indicating that alternative immune mechanisms dominate.

Broader Implications

Targeting the complement‑microglial phagoptosis axis could break the self‑amplifying immune‑neuronal loop, potentially slowing basal forebrain cholinergic decline and associated cognitive aging without requiring amyloid‑focused interventions.