Calcium dependence of neurotransmitter release at a high fidelity synapse

Abstract

The Ca2+-dependence of the recruitment, priming, and fusion of synaptic vesicles are fundamental parameters controlling neurotransmitter release and synaptic plasticity. Despite intense efforts, these important steps in the synaptic vesicles cycle remain poorly understood because disentangling recruitment, priming, and fusion of vesicles is technically challenging. Here, we investigated the Ca2+-sensitivity of these steps at cerebellar mossy fiber synapses, which are characterized by fast vesicle recruitment mediating high-frequency signaling. We found that the basal free Ca2+ concentration (<200 nM) critically controls action potential-evoked release, indicating a high-affinity Ca2+ sensor for vesicle priming. Ca2+ uncaging experiments revealed a surprisingly shallow and non-saturating relationship between release rate and intracellular Ca2+ concentration up to 50 μM. Sustained vesicle recruitment was Ca2+-independent. Finally, quantitative mechanistic release schemes with five Ca2+ binding steps incorporating rapid vesicle recruitment via parallel or sequential vesicle pools could explain our data. We thus show that co-existing high and low-affinity Ca2+ sensors mediate recruitment, priming, and fusion of synaptic vesicles at a high-fidelity synapse.

Upregulated Ca2+ release from the endoplasmic reticulum leads to impaired presynaptic function in Alzheimer’s disease

“…This is consistent with observations of approximately hundred-fold decrease in total vesicle population in hippocampal boutons [2, 3]. In addition, our model mimics the Ca 2+ -dependent increase in peak release rate observed at high fidelity synapses of cerebellar mossy fibre boutons (cMFBs), which permit direct presynaptic recordings and are reported to have high structural similarities with their hippocampal counterparts ( Fig 2C ) [131, 132]. Moreover, in agreement with previous recordings of spontaneous release events from CA3-CA1 synapses, we affirmed that the spontaneous release rate elicited by resting level [Ca 2+ ] c of 100 nM is within the reported range of 10 −4 and 10 −5 per ms [4, 5, 6].…”

“…(D) [Ca 2+ ] c dependence of time-to-peak rate indicates exponentially decreasing but longer time delay to peak release when matched with data for the Calyx-of-Held. Experimental values shown for comparison in (C, D) are from [131]…”

L-type Ca2+channels mediate regulation of glutamate release by subthreshold potential changes

“…However, it is not well understood how changes in basal Ca 2+ , which are far smaller than AP-evoked local Ca 2+ increases, regulate transmitter release. Recently, the Ca 2+ -dependence of vesicle priming, fusion, and replenishment was studied in cerebellar mossy fiber boutons, revealing that the number of RRP strongly depends on basal Ca 2+ between 30 and 180 nM (43). As a potential signaling molecule that regulates Ca 2+ -dependent vesicle priming, the interaction of diacylglycerol/PLC or Ca 2+ /phospholipids with Munc13s (10, 16, 44–46) has been proposed (43).…”

“…Recently, the Ca 2+ -dependence of vesicle priming, fusion, and replenishment was studied in cerebellar mossy fiber boutons, revealing that the number of RRP strongly depends on basal Ca 2+ between 30 and 180 nM (43). As a potential signaling molecule that regulates Ca 2+ -dependent vesicle priming, the interaction of diacylglycerol/PLC or Ca 2+ /phospholipids with Munc13s (10, 16, 44–46) has been proposed (43). In the present study, we demonstrated that V m -induced changes in transmitter release are mediated by basal Ca 2+ changes and involve CaM and PLC activation (Fig.…”