Hypothesis

Age‑related synaptic pruning is not merely a response to extrinsic complement tags; it is driven by an intrinsic neuronal stress pathway whereby declining mitochondrial ATP production activates the ATF4‑CHOP transcription factor axis, which directly upregulates neuronal C1q expression. This creates a feedback loop in which metabolically stressed neurons label themselves for microglial removal, coupling bioenergetic failure to synaptic eviction.

Mechanistic Basis

• Mitochondrial dysfunction in aged neurons reduces ATP levels, leading to AMPK activation and a downstream increase in the integrated stress response (ISR) transcription factor ATF4 4.
• ATF4, often partnered with CHOP, binds to promoter regions of the C1q gene, enhancing its transcription independent of inflammatory cytokines 3.
• Elevated neuronal C1q tags synapses for complement activation (C3 deposition) and subsequent microglial phagocytosis via CD36/Crq 6 and 2.
• This mechanism explains why neurons with high basal firing rates—those most vulnerable to ATP depletion—are preferentially eliminated, aligning with observed neuronal hyperactivity as a pruning signal 4.

Predictions & Experimental Design

1. ATP rescue reduces neuronal C1q: Treating aged mouse hippocampal slices with a mitochondria‑targeted ATP booster (e.g., SS‑31) will lower ATF4/CHOP nuclear translocation and decrease C1q mRNA levels measured by qPCR 1.
2. ATF4 neuronal knockout attenuates pruning: Conditional deletion of ATf4 in forebrain excitatory neurons (Camk2a‑Cre; ATF4^fl/fl) will preserve synaptic density (synaptophysin staining) and reduce microglial phagocytic cups (Iba1/C3 co‑localization) in 24‑month‑old mice without altering global microglial activation state.
3. CHOP inhibition mimics ATP rescue: Pharmacological inhibition of CHOP (using ISRIB) in aged mice will decrease C1b deposition and improve LTP, linking the ISR directly to functional outcomes.
4. Metabolic labeling correlates with pruning risk: Neurons imaged with a mitochondrial ATP sensor (ATeam) showing chronic low ATP will exhibit higher surface C1q (detected via live‑cell antibody labeling) and be preferentially engulfed by microglia in two‑photon imaging sessions.

Potential Implications

If validated, this hypothesis redefines age‑related cognitive decline as a maladaptive extension of a homeostatic quality‑control system that becomes overactive when bioenergetic capacity falls below a threshold. Therapeutic strategies aimed at bolstering neuronal mitochondrial output or blocking the ATF4‑CHOP–C1q axis could preserve synapses without broadly suppressing microglial surveillance, offering a precision approach to mitigate synaptic loss in neurodegenerative aging.