Dynamic recruitment of CaMKII into SHANK3 phase-separated condensates tunes postsynaptic density remodeling during long-term potentiation

Activity-dependent postsynaptic density (PSD) remodeling orchestrates α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking during long-term potentiation (LTP), yet the underlying mechanisms remain poorly understood. Here, we demonstrate that SHANK3 critically mediates the rapid recruitment of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) to PSD and the ensuing PSD remodeling through liquid‒liquid phase separation (LLPS). We found that deletion of the large intrinsically disordered region (IDR) of SHANK3 impairs its LLPS and clustering at dendritic spines of mouse hippocampal neurons, leading to reduction in PSD size and AMPAR expression. Activation of photoactivatable CaMKII promotes its rapid recruitment to SHANK3 condensates, facilitating condensate fusion and PSD remodeling. A human autism-related SHANK3 mutation (InsG3680) truncates part of the large IDR, disrupts SHANK3 LLPS, and causes deficits in PSD remodeling and AMPAR recruitment. Overall, we propose that activity-dependent modulation of SHANK3 condensate dynamics via CaMKII recruitment is a mechanism underlying PSD reorganization during LTP. In this study, the authors find that SHANK3’s large intrinsically disordered region mediates phase separation to support postsynaptic density remodeling during long-term potentiation, providing an insight into how autism spectrum disorder-linked SHANK3 mutations disrupt synaptic plasticity.