Mobilization of synaptic vesicles depends on synaptic vesicle endocytosis

The synapsins are a multi-member family of phosphoproteins that interact with synaptic vesicles as well as with
components of the cytoskeleton. They control the number
and distribution of synaptic vesicles within the presynaptic
terminal and influence synaptic vesicle dynamics. Consequently, they play an important role in the regulation of
basic neurotransmission properties and of certain forms of
short term synaptic plasticity. However, how they modulate
synaptic vesicle clustering, and conversely, mobilization of
reserve vesicles, remains unclear.
At rest the synapsins are clearly localized to synaptic
vesicle clusters. However, it was shown that strong
stimulation disperses the synapsins from the synapses into
the axon, in parallel to the mobilization of reserve vesicles
from the vesicle clusters. Because phosphorylation changes
the affinity of the synapsins to the cytoskeleton, it has been
assumed that this serves as a switch for vesicle mobilization. However, we now show that when neurons are
rendered incapable of exocytosis by either Tetanus Toxin
or by deletion of Munc13, the synapsins did not disperse
from the presynaptic puncta during strong stimulation, even
though they were phosphorylated. This indicates that
phosphorylation of the synapsins is insufficient to induce
vesicle mobilization, even though it does modulate the
affinity of the synapsins to the cytoskeleton and to the
vesicles. Furthermore, we find that blockage of endocytosis, without affecting exocytosis, also blocks synapsin
redistribution, but not its phosphorylation. This suggests a
vital role for endocytosis in synaptic vesicle mobilization
process. We propose that endocytosis activates a signaling
feedback route to the mobilization of the reserve pool.