Synaptic vesicle traffic is supported by transient actin filaments and regulated by PKA and NO

Abstract

Synaptic vesicles (SVs) can be pooled across multiple synapses, prompting questions about their dynamic allocation for neurotransmission and plasticity. We find that the axonal traffic of recycling vesicles is not supported by ubiquitous microtubule-based motility but relies on actin instead. Vesicles freed from synaptic clusters undergo ~1 µm bouts of active transport, initiated by nearby elongation of actin filaments. Long distance translocation arises when successive bouts of active transport were linked by periods of free diffusion. The availability of SVs for active transport can be promptly increased by protein kinase A, a key player in neuromodulation. Vesicle motion is in turn impeded by shutting off axonal actin polymerization, mediated by nitric oxide-cyclic GMP signaling leading to inhibition of RhoA. These findings provide a potential framework for coordinating post-and pre-synaptic strength, using retrograde regulation of axonal actin dynamics to mobilize and recruit presynaptic SV resources.

Axonal CB1 receptors mediate inhibitory bouton formation via cAMP increase

“…Presynaptic PKA activity is involved in CB1-mediated synaptic plasticity (Chevaleyre et al, 2007;Cui et al, 2016) and also is therefore a strong candidate for regulating presynaptic bouton formation. PKA may for instance alter local clustering and inter-bouton exchange of synaptic vesicles (Chenouard et al, 2020;Patzke et al, 2019). PKA resides close to the plasma membrane and preferably phosphorylates membrane proteins in its close proximity (Tillo et al, 2017).…”

“…cAMP signaling via PKA, Epac2 or Rho GTPases may affect the axonal cytoskeleton. Actin is important in the formation, stabilization and maintenance of presynaptic terminals (Bednarek and Caroni, 2011;Chenouard et al, 2020;Chia et al, 2014Chia et al, , 2013Frias et al, 2019;Pielage et al, 2011) and cAMP fluctuations may drive local modifications in the actin cytoskeleton (Bernier et al, 2019) underlying structural presynaptic changes. Our findings suggest that axonal CB1 receptors serve an important role in local, on demand synapse formation.…”

Cytoskeletal regulation guides neuronal trafficking to effectively supply the synapse

“…Synaptic vesicles (SVs) SVs cluster at presynapses where they are exocytosed to release neurotransmitter into the synaptic cleft to stimulate the postsynapse. Upon neuronal stimulation, SVs undergo bidirectional interbouton translocation, which is important for appropriate neurotransmitter release [100][101][102][103] . SV clusters therefore can be shared amongst multiple presynaptic boutons, primarily via transport initiated by actin filament elongation independent of microtubule transport 102 (Figure 2B).…”

“…Whether KIF1A is released from DCVs upon liprin-a and TANC2 capture has yet to be determined, but, as Ca 2+ concentrations are generally higher at postsynaptic spines, the Ca 2+ -CaM-mediated KIF1A-cargo interaction may be maintained. is facilitated by actin elongation 102 , myosin-V-driven movement along actin filaments 104 , and activity-dependent transport on augmin-g-tubulin-nucleated microtubules 103 . (C) Dense core vesicles (DCVs; blue) can be axonally trafficked by KIF1C, once autoinhibition of this motor has been relieved by Hook3 or PTPN21 106 , or by KIF1A bound to synaptotagmin-4 (Syt4) 105 .…”

Bassoon controls synaptic vesicle pools via regulation of presynaptic phosphorylation and cAMP homeostasis

“…Syn phosphorylation by cAMP/PKA signalling increases the SV mobilisation. It allows the transition of vesicles from RP to the RRP, but also promotes an increased exchange of SVs between adjacent synaptic varicosities along axons (Menegon et al, 2006;Valente et al, 2012;Patzke et al, 2019;Chenouard et al, 2020).…”