Neuromodulator Signaling Bidirectionally Controls Vesicle Numbers in Human Synapses

Abstract

Neuromodulators bind to pre-and postsynaptic G protein-coupled receptors (GPCRs), are able to quickly change intracellular cyclic AMP (cAMP) and Ca 2+ levels, and are thought to play important roles in neuropsychiatric and neurodegenerative diseases. Here, we discovered in human neurons an unanticipated presynaptic mechanism that acutely changes synaptic ultrastructure and regulates synaptic communication. Activation of neuromodulator receptors bidirectionally controlled synaptic vesicle numbers within nerve terminals. This control correlated with changes in the levels of cAMP-dependent protein kinase A-mediated phosphorylation of synapsin-1. Using a conditional deletion approach, we reveal that the neuromodulator-induced control of synaptic vesicle numbers was largely dependent on synapsin-1. We propose a mechanism whereby non-phosphorylated synapsin-1 ''latches'' synaptic vesicles to presynaptic clusters at the active zone. cAMP-dependent phosphorylation of synapsin-1 then removes the vesicles. cAMP-independent dephosphorylation of synapsin-1 in turn recruits vesicles. Synapsin-1 thereby bidirectionally regulates synaptic vesicle numbers and modifies presynaptic neurotransmitter release as an effector of neuromodulator signaling in human neurons.

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

“…Disengagement of SVs from synaptic clusters 8 , 14 , 55 – 58 might provide a regulatory mechanism to control SV relocation by feeding extrasynaptic AT, thereby linking SV traffic to synaptic plasticity. We tested this hypothesis by activation of PKA, a well-studied intermediary of multiple neuromodulators, dopamine 59 , serotonin 60 , and brain-derived neurotrophic factor 61 , which influence synaptic plasticity.…”

“…We tested this hypothesis by activation of PKA, a well-studied intermediary of multiple neuromodulators, dopamine 59 , serotonin 60 , and brain-derived neurotrophic factor 61 , which influence synaptic plasticity. Among many downstream actions, PKA can phosphorylate the vesicular adhesion molecule synapsin 1 62 – 64 , leading to dissociation of SVs with synapsin 1 from synaptic clusters and subsequent dispersion 58 , 65 – 67 , possibly by altering synapsin 1 liquid phases 57 . The deletion of synapsins greatly increases SV interbouton mobility seen as accelerated FRAP of synaptophysin-I:EGFP 68 .…”

Synapsin is required for dense core vesicle capture and cAMP-dependent neuropeptide release

“…The intrinsic upstream trigger for cAMP signaling at cholinergic synapses remains to be identified. Recently, neuromodulators were shown to affect synapsin phosphorylation after 30 min through cAMP pathways, and to alter SV numbers in murine and human neurons (Patzke et al, 2019). PKA inhibition increased, and cAMP upregulation decreased SV numbers.…”

Increased and synchronous recruitment of release sites underlies hippocampal mossy fiber presynaptic potentiation

“…In fact, vesicle clustering at the presynaptic terminal is known to be mediated by the synapsin family of proteins (Milovanovic et al, 2018;Pechstein et al, 2020) and synapsins contain a conserved PKA phosphorylation site (Serine9) (Czernik et al, 1987). PKA and synapsin mediated modulation of vesicle availability has been observed also in cultured human neurons (Patzke et al, 2019).…”

“…Forskolin-driven increase in cAMP concentration and the subsequent activation of PKA have recently been shown to act on synapsin to modulate short-term plasticity (Cheng et al, 2018), multivesicular release (Vaden et al, 2019), and vesicle availability (Patzke et al, 2019). We analyzed SV 3Ddistribution in the presynaptic mossy fiber bouton and compared the number and localization of SVs under forskolin and control conditions.…”