The Dynamic Distribution of TrkB Receptors before, during, and after Synapse Formation between Cortical Neurons

Abstract

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.

Nanoparticle-mediated signaling endosome localization regulates growth cone motility and neurite growth

“…First, MNPs functionalized with a TrkB receptor agonist that stimulates neurite growth by activating TrkBdependent signaling pathways are endocytosed into RGC and DRG signaling endosomes and transported in nascent neurites between the soma and growth cones. These fMNP signaling endosomes are similar in size, transported at rates similar to Trk signaling endosomes in PC12 cells (30) and cortical neurons (31), and their transport did not affect growth cone motility or neurite growth rate on their own. Because signaling endosome dysfunction is linked to numerous nervous system diseases (19)(20)(21), fMNP signaling endosome studies may provide insight into targeting nanotherapeutics by identifying cargo-or domain-specific adaptor and motor proteins (32), and by identifying the effects nanoconjugation have on the endocytosis and trafficking of antireceptor agonist antibodies (33) that may impact nanotherapeutic efficacy.…”

GRIP1 interlinks N-cadherin and AMPA receptors at vesicles to promote combined cargo transport into dendrites

“…Previous studies reported that axonal trafficking organelles in general could contain multiple cargoes (25)(26)(27). TrkB, for instance, is transported with synaptic vesicle precursors to function in synapse formation (28). Other reports suggest that linker proteins connect cell adhesion molecules on the plasma membrane to intracellular transport vesicles to allow for a direct supply to nascent synapses after initial contact formation (29,30).…”

Neurotrophin-mediated dendrite-to-nucleus signaling revealed by microfluidic compartmentalization of dendrites

“…Additionally, BDNF induces genes known to regulate synapse function, such as activity-regulated cytoskeletal (Arc) protein (10)(11)(12). Because the effects of BDNF on transcription are frequently explored by bath application, it is not clear if BDNF-mediated transcription is because of activation of TrkB in dendrites, axons, or the cell body (13). If dendritically localized TrkB receptors can regulate gene expression, mechanisms to convey the signal from dendrites to the nucleus, a distance that can be several hundred micrometers, would be required.…”