The neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor TrkB, a receptor tyrosine kinase, are important for synaptic plasticity. In addition to phosphorylation at tyrosine residues, TrkB is also phosphorylated on Ser478 by the proline-directed kinase Cdk5, and studies with cultured neurons have indicated that this is important for neurite outgrowth. Lai et al. engineered Ser478 phosphorylation site–deficient mice (TrkBS478A/S478A) to examine the function of this phosphorylation event in vivo. The mice developed normally, and morphological analysis indicated that dendritic arborization and spines in the hippocampus were not significantly different from those in the wild-type control animals. Biochemical fractionation of mouse brains showed that TrkBS478A was localized to the postsynaptic density, and immunofluorescence analysis and biotinylation assays with cultured neurons showed that TrkBS478A was present at synapses and targeted to the plasma membrane. Thus, phosphorylation at this residue was not necessary for neuronal development or localization of the receptor. Application of BDNF or uncaging of glutamate to mimic synaptic activity failed to promote dendritic spine remodeling (increase in area and density) in hippocampal neurons from TrkBS478A/S478A mice, suggesting that this phosphorylation event is important for connecting TrkB signaling to the cytoskeleton. TIAM1, a Rac1 guanine nucleotide exchange factor (GEF), coimmunoprecipitated with TrkB in brain lysates from wild-type mice but not from TrkBS478A/S478A mice, and BDNF failed to stimulate TIAM1 tyrosine phosphorylation or Rac1 activity in cortical neurons cultured from the TrkBS478A/S478A mice. Compared with the response of cortical neurons from wild-type mice, stimulation of cortical neurons from the mutant mice with glutamate produced a reduced phosphorylation and activation of the kinase PAK and a reduced phosphorylation of the ribosomal protein S6, a target of the mTOR (mammalian target of rapamycin) pathway, suggesting that Ser478 phosphorylation is important for coupling TrkB to the cytoskeleton and to protein synthesis. Hippocampal slices from the mutant mice showed reduced long-term potentiation using stimulation conditions known to induce BDNF-dependent synaptic plasticity. The mutant mice exhibited compromised spatial and object memory formation but not contextual fear memory formation, consistent with an important role for Cdk5-mediated TrkB phosphorylation in specific forms of hippocampal memory. Thus, Ser478 phosphorylation of TrkB appears to function in adult mice to couple synaptic stimulation with the cytoskeleton and protein synthesis to promote synaptic remodeling and memory.
K.-O. Lai, A. S. L. Wong, M.-C. Cheung, P. Xu, Z. Liang, K.-C. Lok, H. Xie, M. E. Palko, W.-H. Yung, L. Tessarollo, Z. H. Cheung, N. Y. Ip, TrkB phosphorylation by Ckd5 is required for activity-dependent structural plasticity and spatial memory. Nat. Neurosci. 15, 1506–1515 (2012). [PubMed]