Muncl 8-1 redistributes in nerve terminals in an activity- and PKC-dependent manner

Muncl 8-1 is a soluble protein essential for synaptic transmission. To investigate the dynamics of endogenous Muncl 8-1 in neurons, we created a mouse model expressing fluorescently tagged Muncl 8-1 from the endogenous munc18-1 locus. We show using fluorescence recovery after photobleaching in hippocampal neurons that the majority of Muncl 8-1 trafficked through axons and targeted to synapses via lateral diffusion together with syntaxin-1. Muncl 8-1 was strongly expressed at presynaptic terminals, with individual synapses showing a large variation in expression. Axon synapse exchange rates of Muncl 8-1 were high: during stimulation, Muncl 8-1 rapidly dispersed from synapses and reclustered within minutes. Muncl 8-1 reclustering was independent of syntaxin-1, but required calcium influx and protein kinase C (PKC) activity. Importantly, a PKC-insensitive Muncl 8-1 mutant did not recluster. We show that synaptic Muncl 8-1 levels correlate with synaptic strength, and that synapses that recruit more Muncl 8-1 after stimulation have a larger releasable vesicle pool. Hence, PKC-dependent dynamic control of Muncl 8-1 levels enables individual synapses to tune their output during periods of activity.