SNAREs Controlling Vesicular Release of BDNF and Development of Callosal Axons

Abstract

SUMMARY At presynaptic active zones, exocytosis of neurotransmitter vesicles (SVs) is driven by SNARE complexes that recruit Syb2 and SNAP25. However, it remains unknown which SNAREs promote the secretion of neuronal proteins, including those essential for circuit development and experience-dependent plasticity. Here, we demonstrate that Syb2 and SNAP25 mediate the vesicular release of BDNF in axons and dendrites of cortical neurons, suggesting these SNAREs act in multiple spatially-segregated secretory pathways. Remarkably, axonal secretion of BDNF is also strongly regulated by SNAP47 which interacts with SNAP25 but appears to be dispensable for exocytosis of SVs. Cell-autonomous ablation of SNAP47 disrupts the layer-specific branching of callosal axons of projection cortical neurons in vivo, and this phenotype is recapitulated by ablation of BDNF or its receptor, TrkB. Our results provide insights into the molecular mechanisms of protein secretion and define the functions of SNAREs in BDNF signaling and regulation of neuronal connectivity.

Differential Maturation of the Two Regulated Secretory Pathways in Human iPSC-Derived Neurons

“…Similar principles are expected to operate for DCVs (de Wit et al., 2009, Shimojo et al., 2015). These proteins are abundantly expressed in glutamatergic and GABAergic human iPSC-derived neurons (Figures 2A and S4A–S4P).…”

“…Such a (low) secretion efficiency is similar to that previously reported in mouse neurons (Farina et al., 2015, van de Bospoort et al., 2012), and is consistent with the generally accepted notion that DCV secretion requires intense stimulation (for a review, see Bartfai et al., 1988). The SNARE proteins synaptobrevin-2, SNAP25, and syntaxin1 were abundantly expressed in our micro-networks, and TeNT abolished DCV secretion, indicating the SNARE dependence of DCV secretion in human neurons, as shown before in rodents (de Wit et al., 2009, Shimojo et al., 2015). Finally, DCV secretion was, as in rodent neurons (de Wit et al., 2009, Farina et al., 2015, van de Bospoort et al., 2012), strictly Ca 2+ dependent.…”

Membrane Trafficking in Neuronal Development: Ins and Outs of Neural Connectivity

“…Additionally, genetic deletion of SNAP25 did not impede the ability of thalamocortical axons to be guided to the cortex, as assessed using both in vitro explant assays and in vivo DiI tracing from dorsal thalamus to the cortex (Blakey et al, 2012; Molnar et al, 2002). Both SNAP25 and SNAP47 have been shown to contribute to the secretion of BDNF in callosal neurons (Shimojo et al, 2015). Since genetic deletion of SNAP25 does not impair neurite outgrowth and axon guidance, but acute cleavage of SNAP25 with BoNTA does, this suggests another t-SNARE compensates for long-term loss of SNAP25 function.…”

“…VAMP2, SNAP25 and SNAP47 mediate the vesicular release of the neurotrophic factor BDNF from the axon. Moreover, ablation of the function of SNAP47, BDNF or the BDNF receptor TrkB disrupts callosal axon branching both in vitro and in vivo, suggesting that SNAP47 plays a role in both exocytosis and axon branching (Shimojo et al, 2015). Thus SNAP47 may have both redundant and unique functions neurologically, although more study regarding its various roles is necessary given the paucity of information on this protein.…”

Contralateral targeting of the corpus callosum

“…• Vesicle-associated membrane protein 2 (Vamp2), Synaptosome associated protein (Snap)25 and Snap47 are Snap receptors (SNAREs) that mediate the vesicular exocytosis of Brain derived neurotrophic factor (Bdnf), which binds to Tropomyosin receptor kinase (Trk) B. Disruption of components of this pathway has been shown to decrease branching in contralateral projections of S1 electroporated L2/3 callosal neurons (Shimojo et al, 2015).…”