Generation of TrkA/TrkB chimeric receptor constructs reveals molecular mechanisms underlying BDNF-induced dendritic outgrowth in hippocampal neurons.

Neurotrophins (NTs) regulate neuronal survival, differentiation, and synaptic plasticity through tropomyosin receptor kinases (Trks). The molecular mechanisms underlying these functions, however, have remained incompletely understood. In the present study, we first showed that brain-derived neurotrophic factor (BDNF) increased both the number of primary dendrites and dendritic complexity in cultured hippocampal neurons. Since hippocampal neurons predominantly express the BDNF receptor TrkB, but not the nerve growth factor (NGF) receptor Trk, we generated DNA constructs encoding the extracellular domain of TrkA fused with the transmembrane and intracellular domain of TrkB and introduced these constructs into cultured hippocampal neurons. To visualize the dendrites, the TrkA/TrkB fusion proteins were bicistronically expressed with green fluorescence protein (GFP). Interestingly, the GFP-labeled neurons grew dendrites and activated the TrkA/TrkB receptors in response to NGF, but not BDNF. We next generated a series of TrkA/TrkB receptors with mutations at tyrosine residues in the TrkB kinase domain, and sought to identify the signaling pathway required for NT-induced dendrite outgrowth. Sholl analyses demonstrated that TrkB signaling through Shc, but not through PLC-γ, plays a crucial role in NT-elicited dendritic outgrowth in hippocampal neurons.