The aging retina may actively eliminate photoreceptors not because of irreversible damage but as a microglial‑mediated pruning process that removes energetically inefficient neurons, mirroring the activity‑dependent synaptic pruning described in the aging brain. We hypothesize that declining opsin‑driven calcium influx in aged photoreceptors reduces activation of the CaMKIV‑MEF2 survival pathway, leading to decreased MEF2‑dependent transcription of anti‑apoptotic genes and increased exposure of phosphatidylserine (PS) on the outer segment membrane. This “eat‑me” signal, combined with age‑related upregulation of complement component C1q, tags underactive photoreceptors for microglial phagocytosis via the CR3 receptor. Importantly, we propose that the microglial phagocytic receptor Crq (CED‑1 homolog) acts as a checkpoint that couples PS recognition to complement‑dependent engulfment, such that loss of Crq uncouples PS exposure from actual phagocytosis unless complement activation is also present.

Key mechanistic steps

1. Energetic decline → reduced mitochondrial ATP and NAD+ levels → impaired SERCA/PMCA activity → lowered basal cytosolic Ca2+ in photoreceptors.
2. Low Ca2+ → insufficient CaMKIV activation → diminished MEF2 nuclear translocation → reduced transcription of survival genes (e.g., Bcl2, Bdnf) and increased PS scramblase activity (via Xkr8) → PS exposure.
3. Complement upregulation (C1q, C3) in the aging retinal microenvironment tags PS‑exposed photoreceptors.
4. Microglial Crq binds PS and cooperates with complement receptors (CR3) to trigger phagocytic synapse formation; blocking Crq prevents phagocytosis even when PS and C1q are present.
5. Activity rescue (optogenetic stimulation or NAD+ supplementation) restores Ca2+ transients, reactivates MEF2, suppresses PS exposure, and reduces complement tagging, thereby sparing photoreceptors despite age.

Testable predictions

• Prediction 1: In aged mice, NAD+ repletion (e.g., NR supplementation) will increase photoreceptor cytosolic Ca2+ fluxes (measured by in vivo GCaMP imaging), elevate nuclear p-MEF2 levels, decrease PS exposure (annexin V labeling), and reduce C1q deposition on outer segments, resulting in preserved ERG amplitudes and ONL thickness compared with vehicle controls.
• Prediction 2: Microglia‑specific Crq knockout (Cx3cr1‑CreER; Crq^fl/fl) will prevent PSD‑positive photoreceptor loss only in low‑activity conditions; when photoreceptor activity is elevated via chronic ChrimsonR optogenetic stimulation, Crq knockout will not provide additional neuroprotection beyond that afforded by activity restoration alone.
• Prediction 3: Complement blockade (anti‑C1q antibody) will rescue photoreceptor numbers and function in low‑activity, aged retinas but will show no significant benefit in retinas where optogenetic activity restoration has already normalized MEF2 signaling and PS exposure.

Falsifiability If NAD+ supplementation fails to increase calcium transients or MEF2 activity, or if PS exposure and complement deposition remain unchanged despite metabolic rescue, the first pillar of the hypothesis is refuted. If Crq deletion does not reduce phagocytosis of PS‑positive, C1q‑tagged photoreceptors under low‑activity conditions, the proposed microglial checkpoint mechanism is invalid. Finally, if complement blockade rescues photoreceptors equally well in both low‑activity and optogenetically hyperactive aged retinas, the activity‑dependence of complement-mediated pruning would be challenged.

This hypothesis links mitochondrial energetics, calcium‑dependent transcriptional regulation, lipid‑based “eat‑me” signaling, and complement‑microglial phagocytosis into a coherent, experimentally tractable framework for age‑related photoreceptor loss, suggesting that preserving neuronal activity—or its downstream metabolic consequences—may be a more logical therapeutic strategy than merely blocking degenerative pathways.