Abstract
MET, a pleiotropic receptor tyrosine kinase implicated in autism risk, influences multiple neurodevelopmental processes. There is a knowledge gap, however, in the molecular mechanism through which MET mediates developmental events related to disorder risk. In the neocortex, MET is expressed transiently during periods of peak dendritic outgrowth and synaptogenesis, with expression enriched at developing synapses, consistent with demonstrated roles in dendritic morphogenesis, modulation of spine volume, and excitatory synapse development. In a recent coimmunoprecipitation/mass spectrometry screen, β-catenin was identified as part of the MET interactome in developing neocortical synaptosomes. Here, we investigated the influence of the MET/β-catenin complex in mouse neocortical synaptogenesis. Western blot analysis confirms that MET and β-catenin coimmunoprecipitate, but N-cadherin is not associated with the MET complex. Following stimulation with hepatocyte growth factor (HGF), β-catenin is phosphorylated at tyrosine(142) (Y142) and dissociates from MET, accompanied by an increase in β-catenin/N-cadherin and MET/synapsin 1 protein complexes. In neocortical neurons in vitro, proximity ligation assays confirmed the close proximity of these proteins. Moreover, in neurons transfected with synaptophysin-GFP, HGF stimulation increases the density of synaptophysin/bassoon (a presynaptic marker) and synaptophysin/PSD-95 (a postsynaptic marker) clusters. Mutation of β-catenin at Y142 disrupts the dissociation of the MET/β-catenin complex and prevents the increase in clusters in response to HGF. The data demonstrate a new mechanism for the modulation of synapse formation, whereby MET activation induces an alignment of presynaptic and postsynaptic elements that are necessary for assembly and formation of functional synapses by subsets of neocortical neurons that express MET/β-catenin complex.