TrkB phosphorylation by Cdk5 is required for activity-dependent structural plasticity and spatial memory

Using a knock-in strategy to ablate a Cdk5-targeted serine phosphorylation site on residue 478 of the TrkB receptor, the authors demonstrate the role of this phosphorylation in activity-dependent functional and structural plasticity, as well as in learning and memory. They further show that TIAM1 and Rac1 act downstream of TrkB S478 phosphorylation during spine remodeling. The neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor TrkB participate in diverse neuronal functions, including activity-dependent synaptic plasticity that is crucial for learning and memory. On binding to BDNF, TrkB is not only autophosphorylated at tyrosine residues but also undergoes serine phosphorylation at S478 by the serine/threonine kinase cyclin-dependent kinase 5 (Cdk5). However, the in vivo function of this serine phosphorylation remains unknown. We generated knock-in mice lacking this serine phosphorylation ( Trkb S478A/S478A mice) and found that the TrkB phosphorylation–deficient mice displayed impaired spatial memory and compromised hippocampal long-term potentiation (LTP). S478 phosphorylation of TrkB regulates its interaction with the Rac1-specific guanine nucleotide exchange factor TIAM1, leading to activation of Rac1 and phosphorylation of S6 ribosomal protein during activity-dependent dendritic spine remodeling. These findings reveal the importance of Cdk5-mediated S478 phosphorylation of TrkB in activity-dependent structural plasticity, which is crucial for LTP and spatial memory formation.