Hypothesis

Neuronal SMO/GLI activity functions as a metabolic fitness signal that suppresses microglial Crq-dependent phagocytosis. With age, declining SMO signaling lowers expression of the 'don't‑eat‑me' molecule CD47 on neurons, exposing them to glial pruning and contributing to selective loss of inefficient circuits.

Mechanistic Rationale

• SMO translocation to the primary cilium drives GLI-mediated transcription of antioxidant genes (SOD, GSH‑Px) and of CD47, a known inhibitor of phagocytosis (Shh/SMO/GLI promotes neuronal survival).
• Aging reduces ciliary SMO localization, decreasing GLI activity and CD47 transcription (Shh pathway activity declines with age).
• Low CD47 engages the microglial SIRPα receptor, disinhibiting Crq‑dependent synapse and soma elimination (Crq regulates synapse elimination during aging).
• Thus, cell‑intrinsic SMO/GLI loss creates a permissive environment for glial quality‑control mechanisms to remove metabolically costly, weakly connected neurons.

Predictions

1. In aged mice, neuronal CD47 levels will correlate positively with SMO/GLI target gene expression and inversely with microglial phagocytic markers.
2. Neuron‑specific SMO knockout will reduce CD47, increase Crq‑mediated engulfment of neurons, and accelerate cognitive decline compared with controls.
3. Restoring CD47 expression in SMO‑deficient neurons will rescue them from pruning without altering SMO signaling.
4. Pharmacological activation of SMO (e.g., SAG) in aged animals will upregulate CD47, decrease microglial Crq activity, and preserve synaptic density.

Experimental Approach

• Generate Camk2a‑Cre;Smo^fl/fl mice and assess CD47 immunohistochemistry, GLI1 mRNA, and Iba1+Crq+ phagocytic cups in hippocampus and cortex at 3, 12, and 24 months.
• Quantify engulfed neuronal fragments (NeuN+ debris inside Iba1+ cells) by confocal microscopy and flow cytometry.
• Perform behavioral batteries (Morris water maze, novel object recognition) to link pruning intensity with cognitive performance.
• Rescue experiments: AAV‑mediated CD47 overexpression in Camk2a‑Cre;Smo^fl/fl neurons; evaluate whether phagocytosis and behavior normalize.
• SMO agonist treatment: chronic SAG delivery via osmotic pumps in wild‑aged mice; measure CD47, microglial activation, and pruning indices.

These experiments directly test whether declining SMO/GLI signaling licenses microglial Crq‑mediated removal of neurons, transforming the observation of age‑related neuronal loss from passive damage to an active, signal‑driven selection process.