Abstract
The spatial arrangement of Ca2+ channels and vesicles remains unknown for most CNS synapses, despite of the crucial importance of this geometrical parameter for the Ca2+ control of transmitter release. At a large model synapse, the calyx of Held, transmitter release is controlled by several Ca2+ channels in a "domain overlap" mode, at least in young animals. To study the geometrical constraints of Ca2+ channel placement in domain overlap control of release, we used stochastic MCell modelling, at active zones for which the position of docked vesicles was derived from electron microscopy (EM). We found that random placement of Ca2+ channels was unable to produce high slope values between release and presynaptic Ca2+ entry, a hallmark of domain overlap, and yielded excessively large release probabilities. The simple assumption that Ca2+ channels can be located anywhere at active zones, except below a critical distance of ~ 30 nm away from docked vesicles ("exclusion zone"), rescued high slope values and low release probabilities. Alternatively, high slope values can also be obtained by placing all Ca2+ channels into a single supercluster, which however results in significantly higher heterogeneity of release probabilities. We also show experimentally that high slope values, and the sensitivity to the slow Ca2+ chelator EGTA-AM, are maintained with developmental maturation of the calyx synapse. Taken together, domain overlap control of release represents a highly organized active zone architecture in which Ca2+ channels must obey a certain distance to docked vesicles. Furthermore, domain overlap can be employed by near-mature, fast-releasing synapses.
Highlights
Transmitter release at CNS synapses happens at active zones of sub-micrometer dimensions, which harbor docked vesicles and vesicle fusion proteins, as well as presynaptic scaffold proteins and voltage-gated Ca2+ channels [1]
Geometric distribution of docked vesicles at the active zone of calyx synapses We wished to explore the possible placement of Ca2+ channels compatible with domain overlap control of release
To investigate whether such void spaces could arise randomly, we compared experimentally observed vesicle distributions with the same parameters derived from random x-y placements of vesicles at a given active zone (Fig 1C, 1D, 1E, 1F, 1G and 1H)
Summary
Ca2+ channels provide the rise in intracellular Ca2+ concentration necessary to initiate the membrane fusion of transmitter—filled vesicles at synapses. From Ca2+ channels, the distance between Ca2+ channels and vesicles on the range of tens of nanometers is a crucial determinant of the vesicle fusion probability. We show by computational modelling that the channels should be located at some distance to vesicles (~ 30 nm), to allow for release control by several channels, a release mechanism found at many synapses. In realistic synapses with a high density of docked vesicles, this translates into a likely localization of Ca2+ channels at membrane sites not occupied by docked vesicles. We present a computational model of how Ca2+ channels can be localized in an active zone with several docked vesicles, to enable control of release by several Ca2+ channels
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
