Ca2+ and cAMP open differentially dilating synaptic fusion pores.

Neuronal dense-core vesicles (DCVs) contain neuropeptides and much larger proteins that affect synaptic growth and plasticity. Rather than using full collapse exocytosis that commonly mediates peptide hormone release by endocrine cells, DCVs at the Drosophila neuromuscular junction release their contents via fusion pores formed by kiss and run exocytosis. Here fluorogen activating protein (FAP) imaging reveals the permeability range of synaptic DCV fusion pores and then shows that this constraint is circumvented by cAMP-induced extra fusions with dilating pores that result in DCV emptying. These Ca2+-independent full fusions require PKA-R2, a PKA phosphorylation site on complexin and the acute presynaptic function of Rugose/Neurobeachin, a PKA-R2 anchor implicated in learning and autism. Therefore, localized Ca2+-independent cAMP signaling opens dilating fusion pores to release large cargos that cannot pass through the narrower fusion pores that mediate spontaneous and activity dependent neuropeptide release. These results imply that the fusion pore is a variable filter that differentially sets the composition of proteins released at the synapse by independent exocytosis triggers responsible for routine peptidergic transmission (Ca2+) and synaptic development (cAMP).