title: An alien divalent ion reveals a major role for Ca²⁺ buffering in controlling slow transmitter release.
abstract: Ca(2+)-dependent transmitter release occurs in a fast and in a slow phase, but the differential roles of Ca(2+) buffers and Ca(2+) sensors in shaping release kinetics are still controversial. Replacing extracellular Ca(2+) by Sr(2+) causes decreased fast release but enhanced slow release at many synapses. Here, we established presynaptic Sr(2+) uncaging and made quantitative Sr(2+)- and Ca(2+)-imaging experiments at the mouse calyx of Held synapse, to reveal the interplay between Ca(2+) sensors and Ca(2+) buffers in the control of fast and slow release. We show that Sr(2+) activates the fast, Synaptotagmin-2 (Syt2) sensor for vesicle fusion with sixfold lower affinity but unchanged high cooperativity. Surprisingly, Sr(2+) also activates the slow sensor that remains in Syt2 knock-out synapses with a lower efficiency, and Sr(2+) was less efficient than Ca(2+) in the limit of low concentrations in wild-type synapses. Quantitative imaging experiments show that the buffering capacity of the nerve terminal is markedly lower for Sr(2+) than for Ca(2+) (~5-fold). This, together with an enhanced Sr(2+) permeation through presynaptic Ca(2+) channels (~2-fold), admits a drastically higher spatially averaged Sr(2+) transient compared with Ca(2+). Together, despite the lower affinity of Sr(2+) at the fast and slow sensors, the massively higher amplitudes of spatially averaged Sr(2+) transients explain the enhanced late release. This also allows us to conclude that Ca(2+) buffering normally controls late release and prevents the activation of the fast release sensor by residual Ca(2+).
authors.affiliation: Laboratory of Synaptic Mechanisms, Brain Mind Institute, School of Life Science, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland, and.
authors.forename: Norbert
authors.initials: N
authors.lastname: Babai
authors.affiliation: Laboratory of Synaptic Mechanisms, Brain Mind Institute, School of Life Science, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland, and.
authors.forename: Olexiy
authors.identifier: http://orcid.org/0000-0002-8115-7733
authors.initials: O
authors.lastname: Kochubey
authors.affiliation: Blue Brain Project, Brain Mind Institute, School of Life Science, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
authors.forename: Daniel
authors.initials: D
authors.lastname: Keller
authors.affiliation: Laboratory of Synaptic Mechanisms, Brain Mind Institute, School of Life Science, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland, and ralf.schneggenburger@epfl.ch.
authors.forename: Ralf
authors.initials: R
authors.lastname: Schneggenburger
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D018345:Mice, Knockout; D009411:Nerve Endings / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D061848:Optical Imaging; D013324:Strontium / Q000378:metabolism*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D050861:Synaptotagmin II / Q000235:genetics / Q000378:metabolism
chemical_list: D002614:Chelating Agents; D050861:Synaptotagmin II; D002118:Calcium; D013324:Strontium
keywords: calcium buffering capacity; calcium sensor; endogenous fixed buffer; slow release sensor; strontium; synaptotagmin