Activity-Dependent Synaptic Tagging via CREB-IGF1-IGF1R Signaling

The activity-dependent synaptic tagging mechanism operates through coordinated interplay of CREB1, IGF1 (insulin-like growth factor 1), and IGF1R (insulin-like growth factor 1 receptor) signaling cascades. Neural activity-induced calcium influx activates CaMKIV and PKA, which phosphorylate CREB1 at serine 133, enabling binding to CRE sites in target gene promoters. Phosphorylated CREB1 initiates transcription of IGF1, which contains multiple CRE-responsive elements in its promoter region. IGF1 synthesis produces a precursor protein that undergoes post-translational processing to generate mature IGF1, packaged into secretory vesicles and released through activity-dependent mechanisms. The secreted IGF1 binds to IGF1R on synaptic membranes, triggering receptor autophosphorylation at tyrosine residues and recruitment of insulin receptor substrate proteins (IRS1/2). IGF1R activation initiates the PI3K/Akt/mTOR pathway, which promotes protein synthesis essential for synaptic maintenance and enhances CREB phosphorylation through mTORC1-dependent S6K1 activation. This creates a sustained transcriptional program that upregulates complement inhibitory proteins CD46 and CD55 while maintaining proper phosphatidylserine asymmetry through enhanced ATP synthesis and flippase activity. The IGF1-IGF1R axis provides more prolonged synaptic protection compared to BDNF-TrkB signaling due to IGF1’s longer half-life and its ability to activate both survival and metabolic pathways simultaneously. Additionally, IGF1R signaling enhances local protein synthesis at synapses through eIF4E-BP1 phosphorylation, enabling rapid deployment of protective factors without requiring somatic gene transcription.