Synaptic Activity Regulates the Abundance and Binding of Complexin

Abstract

Nervous system function relies on precise chemical communication between neurons at specialized junctions known as synapses. Complexin (CPX) is one of a small number of cytoplasmic proteins that are indispensable in controlling neurotransmitter release through SNARE and synaptic vesicle interactions. However, the mechanisms that recruit and stabilize CPX are poorly understood. The mobility of CPX tagged with photoactivatable green fluorescent protein (pGFP) was quantified in vivo using Caenorhabditis elegans. Although pGFP escaped the synapse within seconds, CPX-pGFP displayed both fast and slow decay components, requiring minutes for complete exchange of the synaptic pool. The longer synaptic residence time of CPX arose from both synaptic vesicle and SNARE interactions, and surprisingly, CPX mobility depended on synaptic activity. Moreover, mouse CPX-GFP reversibly dispersed out of hippocampal presynaptic terminals during stimulation, and blockade of vesicle fusion prevented CPX dispersion. Hence, synaptic CPX can rapidly redistribute and this exchange is influenced by neuronal activity, potentially contributing to use-dependent plasticity.

A minimalist model to measure interactions between proteins and synaptic vesicles

“…Additionally, calmodulin is found in different cellular compartments, and is involved in different transport mechanisms. Another example is complexin 1, for which a diffusion coefficient of 2 μm 2 /s, similar to our value, was measured in synapses of living C. elegans 12 . Additionally, we compared our in vitro data with our recent data measured by FRAP in living neurons 6 .…”

“…We employed fluorescence correlation spectroscopy (FCS) to quantify the mobility of eleven different synaptic proteins in this system, and we found a dramatic decrease of the protein mobility in the presence of the SVs, confirming an effective interaction between each of the proteins and the SVs. Importantly, we found a strong agreement of the in vitro diffusion coefficients and corresponding values measured in living cells 6,[11][12][13] . This result implies not only that such proteins interact with the vesicles, but also that their mobility is governed by this interaction, confirming the hypothesis that the SV cluster is a major factor in the organization of presynaptic proteins [7][8][9] .…”

Secretory vesicle trafficking in awake and anaesthetized mice: differential speeds in axons versus synapses

“…Molecular (or organelle‐) crowding could contribute to slowing of DCV transport through these structures, as indicated by diffusion models in boutons (Wragg et al . ). (2) DCVs are mainly transported via microtubule‐mediated transport mechanisms (Barkus et al .…”

“…(1) Axonal en-passant boutons are small compartments packed with proteins, mitochondria and synaptic vesicles. Molecular (or organelle-) crowding could contribute to slowing of DCV transport through these structures, as indicated by diffusion models in boutons (Wragg et al 2015).…”

Evolutionary Divergence of the C-terminal Domain of Complexin Accounts for Functional Disparities between Vertebrate and Invertebrate Complexins

“…Several recent studies have proposed that the amphipathic region of the CTD mediates a curvature-sensitive membrane binding interaction that directs both mammalian and nematode complexin to SVs ( Wragg et al, 2013 ; Snead et al, 2014 ; Gong et al, 2016 ). Without the CTD, the inhibitory function of complexin is impaired ( Xue et al, 2009 ; Kaeser-Woo et al, 2012 ; Wragg et al, 2013 ), as is complexin localization at the synapse ( Buhl et al, 2013 ; Iyer et al, 2013 ; Wragg et al, 2015 ). In addition to the amphipathic region, the CTD of all complexins terminates with either a second hydrophobic lipid-binding motif or a lipidated CAAX box motif, further emphasizing a potential membrane-interacting role for this region of complexin ( Reim et al, 2005 ; Cho et al, 2010 ; Buhl et al, 2013 ; Iyer et al, 2013 ).…”