Abstract
Endocytosis of Trk (tropomyosin-related kinase) receptors is critical for neurotrophin signal transduction and biological functions. However, the mechanism governing endocytosis of TrkB (tropomyosin-related kinase B) and the specific contributions of TrkB endocytosis to downstream signaling are unknown. In this study, we report that blocking clathrin, dynamin, or AP2 in cultured neurons of the central nervous system inhibited brain-derived neurotrophic factor (BDNF)-induced activation of Akt but not ERK. Treating neurons with the clathrin inhibitor monodansylcadaverine or a peptide that blocks dynamin function specifically abrogated Akt pathway activation in response to BDNF but did not affect the response of other downstream effectors or the up-regulation of immediate early genes neuropeptide Y and activity-regulated cytoskeleton-associated protein. Similar effects were found in neurons expressing small interfering RNA to silence AP2 or a dominant negative form of dynamin that inhibits clathrin-mediated endocytosis. In PC12 cells, ERK but not Akt activation required TrkA endocytosis following stimulation with nerve growth factor, whereas the opposite was true when TrkA-expressing neurons were stimulated with nerve growth factor in the central nervous system. Thus, the specific effects of internalized Trk receptors probably depend on the presence of cell type-specific modulators of neurotrophin signaling and not on differences inherent to Trk receptors themselves. Endocytosis-dependent activation of Akt in neurons was found to be critical for BDNF-supported survival and dendrite outgrowth. Together, these results demonstrate the functional requirement of clathrin- and dynamin-dependent endocytosis in generating the full intracellular response of neurons to BDNF in the central nervous system.
Highlights
Formation of the neurotrophin-Trk complex initiates its endocytosis [5], and endocytosis is required for some biological functions of neurotrophins [6]
We show that TrkB endocytosis is required for brain-derived neurotrophic factor (BDNF)-induced neuronal protection and dendritic growth but not the up-regulation of neuropeptide Y (NPY) and activityregulated cytoskeleton-associated protein (ARC)
Together our results demonstrate that activation of Akt signaling downstream of TrkB relies on ligand-receptor endocytosis for its initiation, and BDNF-TrkB endocytosis is a required step in the regulation of biological functions
Summary
Formation of the neurotrophin-Trk complex initiates its endocytosis [5], and endocytosis is required for some biological functions of neurotrophins [6]. We report that BDNF-induced TrkB endocytosis in cultured neurons of the central nervous system depends on clathrin, dynamin, and AP2, and blocking endocytosis prevents BDNFinduced activation of Akt but not ERK.
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