This hypothesis proposes that the CREB-BDNF-TrkB activity-dependent signaling cascade directly controls the spatial positioning and expression levels of complement regulators CD55 and CD46 on synaptic membranes, creating an activity-based tagging system for synaptic elimination. High-frequency neural activity triggers calcium influx and CaMKIV/PKA-mediated CREB1 phosphorylation at serine 133, which transcriptionally upregulates CD55 and CD46 expression while simultaneously promoting their trafficking to active synapses through BDNF-TrkB signaling. The TrkB-activated PI3K/Akt pathway enhances surface insertion of CD55/CD46 at frequently stimulated synapses by phosphorylating trafficking proteins and stabilizing regulator clustering, while the Ras/MAPK cascade reinforces this protective phenotype through sustained CREB activation. Conversely, synapses with low activity levels exhibit reduced CREB-mediated transcription, leading to diminished CD55 and CD46 surface expression and creating microdomains of complement vulnerability.

This hypothesis proposes that the CREB-BDNF-TrkB activity-dependent signaling cascade directly controls the spatial positioning and expression levels of complement regulators CD55 and CD46 on synaptic membranes, creating an activity-based tagging system for synaptic elimination. High-frequency neural activity triggers calcium influx and CaMKIV/PKA-mediated CREB1 phosphorylation at serine 133, which transcriptionally upregulates CD55 and CD46 expression while simultaneously promoting their trafficking to active synapses through BDNF-TrkB signaling. The TrkB-activated PI3K/Akt pathway enhances surface insertion of CD55/CD46 at frequently stimulated synapses by phosphorylating trafficking proteins and stabilizing regulator clustering, while the Ras/MAPK cascade reinforces this protective phenotype through sustained CREB activation. Conversely, synapses with low activity levels exhibit reduced CREB-mediated transcription, leading to diminished CD55 and CD46 surface expression and creating microdomains of complement vulnerability. This activity-dependent complement regulator positioning enables precise targeting of weak or silent synapses for complement-mediated pruning while protecting active, functional connections. The differential CD55/CD46 expression creates distinct complement convertase decay rates across synaptic populations—active synapses rapidly dissociate C3 and C5 convertases through high CD55 levels and efficiently cleave complement components via CD46-factor I interactions, while inactive synapses become susceptible to complement deposition and membrane attack complex formation. This mechanism provides a molecular explanation for experience-dependent synaptic refinement during critical periods and may be dysregulated in neurodevelopmental disorders characterized by aberrant pruning.