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The Journal of Neuroscience, November 1, 2006, 26(44):11487-11500; doi:10.1523/JNEUROSCI.2364-06.2006
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Development/Plasticity/Repair
The Dynamic Distribution of TrkB Receptors before, during, and after Synapse Formation between Cortical Neurons
Raquel A. Gomes, Cara Hampton, Faten El-Sabeawy, Shasta L. Sabo, andA. Kimberley McAllister Center for Neuroscience, University of California at Davis, Davis, California 95616
Correspondence should be addressed to Dr. A. Kimberley McAllister, Associate Professor, Center for Neuroscience, University of California at Davis, 1544 Newton Court, Davis, CA 95616. Email: kmcallister{at}ucdavis.edu
Although brain-derived neurotrophic factor (BDNF) potently regulates neuronal connectivity in the developing CNS, the mechanism by which BDNF influences the formation and/or maintenance of glutamatergic synapses remains unknown. Details about the subcellular localization of the BDNF receptor, TrkB, relative to synaptic and nonsynaptic proteins on excitatory neurons should provide insight into how BDNF might exert its effects during synapse formation. Here, we investigated the subcellular localization of tyrosine kinase receptor B (TrkB) relative to synaptic vesicle-associated proteins and NMDA receptors using immunocytochemistry, confocal microscopy, and time-lapse imaging in dissociated cultures of cortical neurons before, during, and after the peak of synapse formation. We find that TrkB is present in puncta on the surface and intracellularly in both dendrites and axons throughout development. Before synapse formation, some TrkB puncta in dendrites colocalize with NMDA receptors, and almost all TrkB puncta in axons colocalize with synaptic vesicle proteins. Clusters of TrkB fused to the enhanced green fluorescent protein (TrkB-EGFP) are highly mobile in both axons and dendrites. In axons, TrkB-EGFP dynamics are almost identical to vesicle-associated protein (VAMP2-EGFP), and these proteins are often transported together. Finally, surface TrkB is found in structures that actively participate in synapse formation: axonal growth cones and dendritic filopodia. Over time, surface TrkB becomes enriched at glutamatergic synapses, which contain both catalytic and truncated TrkB. These results suggest that TrkB is in the right place at the right time to play a direct role in the formation of glutamatergic synapses between cortical neurons.
Key words: synaptic vesicle transport; visual cortex; synaptogenesis; neurotrophin; BDNF; TrkB-EGFP
Received June 4, 2006; revised Aug. 17, 2006; accepted Sept. 23, 2006.
Correspondence should be addressed to Dr. A. Kimberley McAllister, Associate Professor, Center for Neuroscience, University of California at Davis, 1544 Newton Court, Davis, CA 95616. Email: kmcallister{at}ucdavis.edu
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