Hypothesis

The competitive hierarchy of autophagy receptors (NBR1 > p62/SQSTM1 for FIP200 binding) and their non‑overlapping microdomains on cargo create a spatial allocation system that limits autophagosome access to specific substrates. When this spatial segregation is disrupted—by alterations in extracellular matrix proteoglycans or by receptor mutations that impair microdomain formation—high‑affinity receptors monopolize the autophagy machinery, starving lower‑affinity receptors of degradation capacity. This imbalance shifts the neuron‑intrinsic autophagic pruning of synapses toward either excessive removal of postsynaptic proteins or insufficient clearance of damaged organelles, thereby tipping the synapse elimination scale toward microglial complement‑dependent phagocytosis or toward maladaptive autophagy‑driven loss. We predict that restoring receptor microdomain integrity will normalize the autophagy‑complement pincer attack and rescue synaptic density in aging cerebellum.

Mechanistic Reasoning

1. Receptor Competition as a Molecular Landlord

   • NBR1’s higher affinity for the FIP200 initiation complex establishes a biochemical pecking order that preferentially nucleates autophagosomes around NBR1‑bound cargo.[1]
   • p62 and NDP52 occupy distinct microdomains on the same cargo, recruiting separate effector complexes (e.g., p62‑linked ubiquitin chains vs. NDP52‑linked TBK1).[2]
   • These microdomains act like territorial boundaries that ensure each receptor class accesses a finite pool of phagophores without direct competition.

2. ECM Proteoglycans as Spatial Modulators

   • Heparan sulfate proteoglycans (HSPGs) in the cerebellar extracellular matrix can bind the ubiquitin‑associated (UBA) domain of p62, altering its conformation and reducing its ability to form stable microdomains.[5]
   • Loss or over‑expression of specific HSPGs (e.g., agrin, perlecan) therefore changes the effective surface area available for p62/NDP52 microdomain formation, biasing receptor occupancy toward NBR1.

3. Consequences for Synaptic Pruning

   • When p62/NDP52 microdomains shrink, ubiquitinated synaptic proteins (e.g., PSD‑95, GluA2) are less efficiently captured, while damaged mitochondria—high‑affinity NBR1 substrates—continue to be engulfed.[3]
   • The resulting selective autophagy flux preferentially removes mitochondria, triggering AMPK hyper‑activation and further autophagic synaptic protein loss independent of microglia.[3]
   • Simultaneously, reduced p62‑mediated ubiquitination diminishes the “eat‑me” signal that normally competes with C3b/iC3b opsonization, allowing microglial CR3‑mediated phagocytosis to dominate.[4]
   • The net effect is a bimodal increase in synapse loss: one arm driven by neuron‑intrinsic autophagy, the other by microglial complement phagocytosis.

4. Testable Predictions

   • Prediction 1: In young adult mice, conditional knockout of the HSPG core protein perlecan in cerebellar Purkinje cells will increase p62 microdomain size (measured by super‑resolution imaging) and reduce NBR1‑FIP200 co‑immunoprecipitation.[1,2]
   • Prediction 2: These mice will show preserved synaptic density despite elevated C3 levels, whereas wild‑type littermates exhibit the inverse correlation (high C3, low C3‑tagged synapses) seen in aging.[5]
   • Prediction 3: Rescue experiments overexpressing a p62 mutant that forces microdomain formation (e.g., fused to a dimerizing leucine zipper) will normalize autophagy flux (LC3‑II turnover) and reduce both autophagic synaptic protein loss and microglial phagocytosis (measured by pHrodo‑labeled synaptosomes).
   • Prediction 4: Behavioral assays (rotarod, eyeblink conditioning) will correlate with synaptic density metrics, establishing functional relevance.

Falsifiability

If disrupting HSPG–p62 interactions fails to alter receptor microdomain organization, does not shift the NBR1/p62 effector balance, and does not rescue synaptic loss in aged cerebellum, the hypothesis would be falsified. Conversely, observing the predicted changes would support the model that spatial segregation of autophagy receptors is a decisive checkpoint governing the neuron‑intrinsic versus microglial arms of synapse elimination.