Mobility of Calcium Channels in the Presynaptic Membrane

Abstract

Unravelling principles underlying neurotransmitter release are key to understand neural signaling. Here, we describe how surface mobility of voltage-dependent calcium channels (VDCCs) modulates release probabilities (P(r)) of synaptic vesicles (SVs). Coupling distances of <10 to >100 nm have been reported for SVs and VDCCs in different synapses. Tracking individual VDCCs revealed that within hippocampal synapses, ∼60% of VDCCs are mobile while confined to presynaptic membrane compartments. Intracellular Ca(2+) chelation decreased VDCC mobility. Increasing VDCC surface populations by co-expression of the α2δ1 subunit did not alter channel mobility but led to enlarged active zones (AZs) rather than higher channel densities. VDCCs thus scale presynaptic scaffolds to maintain local mobility. We propose that dynamic coupling based on mobile VDCCs supports calcium domain cooperativity and tunes neurotransmitter release by equalizing Pr for docked SVs within AZs.

Surface dynamics of voltage-gated ion channels

“…38 However, the use of the intracellular calcium transients induced by the opening of one, or small clusters of VGCC called sparklet, 136 as readout for channel localization over time is difficult, since the kinetic of intracellular calcium signals and dynamics of membrane proteins are very different. Direct measurement of calcium channel surface dynamics in the cell membrane has been made possible by well-defined fluorescent probes attached directly to the pore-forming subunit 40,64,119 or associated a2d-subunits. 96,147 Calcium channel dynamics differs along the neuronal membrane, but the majority of channels are confined in clusters along the membrane, as described for Ca V 1.2.…”

“…40,64 In the axonal membrane, Ca V 2.1 and Ca V 2.2 channels are mobile, but confined in presynaptic-terminals. 119 Therefore, the contribution of channel mobility to calcium-mediated signaling is most likely a local variable within zones of confinement along dendrites and axons, 40,119 which are a few hundred nanometres in diameter. Within such membrane areas (illustrated in Fig.…”

“…1), as the presynaptic active zone or postsynaptic spines, small, but dynamic, populations of VGCCs can substantially improve the capacity of the calcium signal as suggested for the presynaptic vesicle release probability. 119 In the presynaptic active zone, the relatively low calcium sensitivity of the vesicular calcium sensor proteins demands a very close alignment of calcium channels and releasable vesicles within a radius below 100 nm. 20,46,71,118 Investigations of the release probability, channel distribution, as well as evoked calcium signals, point to a clustered distribution of presynaptic calcium channels.…”

“…In particular, the development of high-resolution imaging techniques demonstrated that dynamic activity-dependent redistribution of ionotropic receptors in the synaptic membrane as a complementary mechanism of shortterm synaptic plasticity 9,33,65,111 might also be relevant for voltage-gated ion channels. 41,119 Aspects of surface dynamics Molecular dynamics of ion channels has been related to either the structural dynamics of the pore-forming subunit, allowing the passage of ions through the membrane, 11 or to lateral diffusion in the cellular membrane caused by its sub-cellular heterogeneity. 102,112 The need of molecular motions for the opening and closing of ion channels is well defined and is in the range of a few Angstroms (or several hundred picometers).…”

“…84,109 Apart from this neuron-specific structure, most of the surface expressed voltage-gated ion channels are mobile within cellular membranes. 3,32,40,108,119,124 The existence of such surface mobility produces the potential to characterize molecular interactions of the observed molecule based merely on changes in its diffusion behavior. Given that kinetic properties of most ion channels depend not only on the pore-forming subunit, but to a large extent on the assembly and interaction with accessory subunits, one can assume that channel composition, and hence function, might be tuned by a dynamic assembly over time.…”