Journal of Neuroscience vol:29 issue:1 pages:23-37
Neurons secrete many diffusible signals from synaptic and other secretory vesicles. We characterized secretion of guidance cues, neuropeptides,
neurotrophins, and proteases from single secretory vesicles using pHluorin-tagged cargo in cortical neurons. Stimulation
triggered transient and persistent fusion events. Transient events represented full release followed by cargo diffusion or incomplete
release followed by vesicle retrieval, as previously observed in neuroendocrine cells. Unexpectedly, we also observed that certain cargo,
such as Semaphorin 3A (Sema3A), was delivered at the cell surface as stable deposits. Stable deposits and transient events were observed
for single cargo and both were SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) and calcium dependent.
The ratio between stable and transient events did not depend on cargo size, subcellular localization (synaptic vs extrasynaptic secretion),
or the presence of the extracellular matrix. Instead, the ratio is cargo specific and depends on an interaction with the vesicle matrix
through a basic domain in the cargo protein. Inhibition of this interaction through deletion of the basic domain in Sema3A abolished
stable deposits and rendered all events transient. Strikingly, cargo favoring transient release was stably deposited after corelease with
cargo favoring stable deposit. These data argue against cargo diffusion after exocytosis as a general principle. Instead, the vesicle matrix
retains secreted signals, probably for focal signaling at the cell surface.