Complexin Stabilizes Newly Primed Synaptic Vesicles and Prevents Their Premature Fusion at the Mouse Calyx of Held Synapse

Abstract

Complexins (Cplxs) are small synaptic proteins that cooperate with SNARE-complexes in the control of synaptic vesicle (SV) fusion. Studies involving genetic mutation, knock-down, or knock-out indicated two key functions of Cplx that are not mutually exclusive but cannot easily be reconciled, one in facilitating SV fusion, and one in "clamping" SVs to prevent premature fusion. Most studies on the role of Cplxs in mammalian synapse function have relied on cultured neurons, heterologous expression systems, or membrane fusion assays in vitro, whereas little is known about the function of Cplxs in native synapses. We therefore studied consequences of genetic ablation of Cplx1 in the mouse calyx of Held synapse, and discovered a developmentally exacerbating phenotype of reduced spontaneous and evoked transmission but excessive asynchronous release after stimulation, compatible with combined facilitating and clamping functions of Cplx1. Because action potential waveforms, Ca 2ϩ influx, readily releasable SV pool size, and quantal size were unaltered, the reduced synaptic strength in the absence of Cplx1 is most likely a consequence of a decreased release probability, which is caused, in part, by less tight coupling between Ca 2ϩ channels and docked SV. We found further that the excessive asynchronous release in Cplx1-deficient calyces triggered aberrant action potentials in their target neurons, and slowed-down the recovery of EPSCs after depleting stimuli. The augmented asynchronous release had a delayed onset and lasted hundreds of milliseconds, indicating that it predominantly represents fusion of newly recruited SVs, which remain unstable and prone to premature fusion in the absence of Cplx1.

Functional Roles of Complexin 3 and Complexin 4 at Mouse Photoreceptor Ribbon Synapses

“…Loss-of-function studies have indicated inhibitory and facilitatory activities of Cplxs in SNARE-mediated vesicle fusion and distinct Cplx domains were suggested to be responsible for the opposing functions (Xue et al, 2007;Maximov et al, 2009;Kaeser-Woo et al, 2012;Yang et al, 2013;Lai et al, 2014;Chang et al, 2015; for review see Trimbuch and Rosenmund, 2016). Although most of our knowledge about Cplx function stems from studies on Cplx1 and Cplx2 present at conventional brain synapses, little is known about Cplx function at sensory synapses.…”

“…In the mouse auditory system, Cplxs were not detected in cochlear hair cells, but Cplx1 was found in spiral ganglion neurons and at the end bulb of Held in the cochlear nucleus (Strenzke et al, 2009;Chang et al, 2015). In the mouse retina, all four Cplx isoforms were detected with Cplx3 and Cplx4 present at photoreceptor ribbon synapses (Reim et al, 2005).…”

Complexin 3 Increases the Fidelity of Signaling in a Retinal Circuit by Regulating Exocytosis at Ribbon Synapses

“…Cplxs appear to suppress spontaneous exocytosis and, thereby, preserve vesicles in a release-ready state (Chang et al, 2015; Rizo and Xu, 2015). Therefore, we recorded spontaneous, AMPAR-mediated EPSCs in AIIs in the presence of L-AP4, a mGluR6 agonist that hyperpolarizes presynaptic RBs and reduces spontaneous EPSC frequency; spontaneous EPSCs in this condition are presumed to be quantal, miniature EPSCs (mEPSCs) (Jarsky et al, 2010; Mehta et al, 2013; Singer et al, 2004).…”

Superpriming of synaptic vesicles as a common basis for intersynapse variability and modulation of synaptic strength

“…Several recent studies described perturbations, either pharmacological or genetic, which change the synaptic release probability in the absence of changes in Ca 2+ current, [Ca 2+ ] i dynamics, and SV pool sizes (8,(49)(50)(51). Whereas changes in the latter parameters are well understood and have been recognized as strong modulators of transmitter release, the mechanisms underlying differences in "intrinsic" release readiness have received less attention.…”