Current consensus models of Alzheimer's disease (AD) view aberrant synaptic pruning primarily as a downstream, destructive consequence of amyloid and tau pathology. However, analyzing the recent literature reveals a striking, unresolved mechanistic gap between microglial/oligodendrocyte progenitor cell (OPC) pruning and tau propagation. I propose that synaptic pruning is not merely a symptom of neurodegeneration, but the active mechanical vector for trans-synaptic tau spread.

The Hypothesis

I hypothesize that when microglia (via the complement C1q/C3 pathway) and OPCs (via the GAS6-AXL pathway) engulf synapses burdened with pathogenic tau, they inadvertently facilitate its spread. Specifically, tau oligomers modified by K63-linked ubiquitination resist degradation within the glial lysosomal compartment. Instead of neutralizing the threat, the engulfment of these synapses shunts K63-tau into the multivesicular body (MVB) pathway. Glia then secrete these intact, highly neurotoxic tau strains via exosomes, efficiently seeding adjacent, healthy neural networks.

Mechanistic Breakdown

1. Peripheral Priming: Physiologically, peripheral organs clear brain-derived amyloid-beta. As peripheral clearance saturates or declines with age, central $A\beta$ accumulates. This accumulation does not act alone; it serves as an epigenetic primer for glia, potentially mimicking the persistent inflammatory states seen in histone mutations like H3.3G34R/V, shifting them into an aggressive pruning state.
2. Engulfment & Lysosomal Evasion: During this hyper-pruning phase, glia ingest synapses containing early tau oligomers. While K48-ubiquitination typically targets proteins for proteasomal degradation, K63-ubiquitination acts as an endosomal sorting signal. My mechanistic addition is that K63-ubiquitinated tau inside OPCs and microglia derails normal lysosomal fusion.
3. p53-Mediated Exosomal Shunting: Once inside the glial endosomal system, this tau may interact with internalized p53 aggregates, further inhibiting DNA damage responses and stress degradation pathways within the glia. The cell's survival mechanism is to expel the un-degradable K63-tau/p53 complexes via exosomes.
4. Amplification via Cross-Seeding: Once expelled, these glial-derived exosomes are exceptionally efficient at cross-seeding further tau pathology, similar to the highly neurotoxic strains generated by IAPP cross-seeding in Type 2 Diabetes.

Falsifiability and Testing

This hypothesis is strictly testable. If correct, blocking synaptic pruning should proportionally halt the extracellular spread of K63-ubiquitinated tau.

• In vivo: Treat tau-seeding mouse models with C3 antagonists (PMX205) or GAS6-AXL inhibitors. We should observe not only the preservation of synapses but a precipitous drop in exosome-encapsulated K63-tau in interstitial fluid (ISF) and cerebrospinal fluid (CSF).
• In vitro: Co-culture wild-type OPCs with neurons expressing fluorescently-tagged K63-tau. Using live-cell imaging and flow cytometry, we can trace the uptake of synaptic tau via AXL receptors and quantify its subsequent release in OPC-derived exosomes.

By reframing OPCs and microglia as active, exosomal distribution hubs for K63-tau, we explain why simply clearing amyloid (even at the impressive 92% rate of next-gen antibodies like trontinemab) may fail to stop tau-driven cognitive decline if the glial "prune-and-spread" cycle has already been initiated.