Apparent calcium dependence of vesicle recruitment

Abstract

During sustained synaptic transmission, recruitment of new transmitter-filled vesicles to the release site counteracts vesicle depletion and thus synaptic depression. An elevated intracellular Ca concentration has been proposed to accelerate the rate of vesicle recruitment at many synapses. This conclusion is often based on the finding that increased intracellular Ca buffering slows the recovery from synaptic depression. However, the molecular mechanisms of the activity-dependent acceleration of vesicle recruitment have only been analysed at some synapses. Using physiological stimulation patterns in postsynaptic recordings and step depolarizations in presynaptic bouton recordings, we investigate vesicle recruitment at cerebellar mossy fibre boutons. We show that increased intracellular Ca buffering slows recovery from depression dramatically. However, pharmacological and genetic interference with calmodulin or the calmodulin-Munc13 pathway, which has been proposed to mediate Ca -dependence of vesicle recruitment, barely affects vesicle recovery from depression. Furthermore, we show that cerebellar mossy fibre boutons have two pools of vesicles: rapidly fusing vesicles that recover slowly and slowly fusing vesicles that recover rapidly. Finally, models adopting such two pools of vesicles with Ca -independent recruitment rates can explain the slowed recovery from depression upon increased Ca buffering. Our data do not rule out the involvement of the calmodulin-Munc13 pathway during stronger stimuli or other molecular pathways mediating Ca -dependent vesicle recruitment at cerebellar mossy fibre boutons. However, we show that well-established two-pool models predict an apparent Ca -dependence of vesicle recruitment. Thus, previous conclusions of Ca -dependent vesicle recruitment based solely on increased intracellular Ca buffering should be considered with caution.

Ca‐dependence of synaptic vesicle exocytosis and endocytosis at the hippocampal mossy fibre terminal

“…This is similar to the recent findings at cerebellar mossy fibre terminals (Ritzau‐Jost et al . ). The calyx of Held terminals have Ca‐sensitive and insensitive components of recovery (Wu & Borst, ; Sakaba & Neher, ; Lipstein et al .…”

“…For example, the modified serial model, which assumes that vesicles fuse from both the RRP and the RP (Wu & Borst, ; Ritzau‐Jost et al . ), could fit the data. Similarly, the parallel model, which assumes parallel vesicle recruitment to the RRP and the RP (and release from both pools), could fit the data (data not shown).…”

Blocking slow exocytosis with slow Ca2+ buffers slows recovery from depression

“…Soon after the discovery of Ca 2+ ‐dependent recovery, Wu & Borst () showed that prolonged stimulation at the calyx of Held results in the release of two distinct pools of vesicles with differing kinetics of both release and replenishment They proposed a model with striking similarities to that of Ritzau‐Jost et al . (). Importantly, Ritzau‐Jost et al .…”

“…In this issue of The Journal of Physiology Ritzau‐Jost et al . () provide an elegant analysis and a new perspective on Ca 2+ ‐dependent recovery from depression, and suggest that – for at least one synapse – the phenomenon that has been observed may have little to do with vesicle replenishment.…”

“…Ritzau‐Jost () and colleagues set out to evaluate the role of these replenishment mechanisms at cerebellar mossy fibre boutons (cMFBs). The cMFB synapse exhibits fast replenishment and can reliably transmit high‐frequency firing rates (Saviane & Silver, ), but the Ca 2+ dependence of recovery has yet to be directly studied at this synapse.…”