Presynaptic nanodomains: a tale of two synapses

Abstract

Here we summarize the evidence from two “giant” presynaptic terminals—the squid giant synapse and the mammalian calyx of Held—supporting the involvement of nanodomain calcium signals in triggering of neurotransmitter release. At the squid synapse, there are three main lines of experimental evidence for nanodomain signaling. First, changing the size of the unitary calcium channel current by altering external calcium concentration causes a non-linear change in transmitter release, while changing the number of open channels by broadening the presynaptic action potential causes a linear change in release. Second, low-affinity calcium indicators, calcium chelators, and uncaging of calcium all suggest that presynaptic calcium concentrations are as high as hundreds of micromolar, which is more compatible with a nanodomain type of calcium signal. Finally, neurotransmitter release is much less affected by the slow calcium chelator, ethylene glycol tetraacetic acid (EGTA), in comparison to the rapid chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N’,N’-tetraacetic acid (BAPTA). Similarly, as the calyx of Held synapse matures, EGTA becomes less effective in attenuating transmitter release while the number of calcium channels required to trigger a single fusion event declines. This suggests a developmental transformation of microdomain to nanodomain coupling between calcium channels and transmitter release. Calcium imaging and uncaging experiments, in combination with simulations of calcium diffusion, indicate the peak calcium concentration seen by presynaptic calcium sensors reaches at least tens of micromolar at the calyx of Held. Taken together, data from these provide a compelling argument that nanodomain calcium signaling gates very rapid transmitter release.

Spindle function inXenopusoocytes involves possible nanodomain calcium signaling

“…The foregoing results—rapid depolymerization of spindle microtubules in the presence of BAPTA-based calcium buffers and yet no evidence of overall increase of calcium at the spindle compared with the surrounding cytoplasm—suggest the involvement of highly localized, or nanodomain, calcium signaling (Neher, 1998; Wang and Augustine, 2014). In a calcium nanodomain, the calcium channel and the calcium sensor are within tens of nanometers, often in the same molecular complex.…”

“…This different sensitivity is reminiscent of that found in another calcium-mediated process: neurotransmitter release in the presynaptic nerve terminals of squid giant synapse (Adler et al , 1991). A large body of literature has yielded detailed mechanistic understanding of this system and the different responses to BAPTA and EGTA (Neher, 1998; Wang and Augustine, 2014). The close proximity, and indeed the direct molecular contact, of the calcium source (voltage-gated calcium channels) and calcium sensor (the vesicular protein synaptotagmin) form a calcium signaling unit called a nanodomain (the calcium channel and the sensor are within 20 nm of each other), such that the high local calcium concentration directly, on a time scale of microseconds, drives synaptotagmin-mediated vesicle fusion (exocytosis) and neurotransmitter release.…”

“…In fact, it was its pH dependence and slow calcium-binding kinetics that prompted the design and synthesis of BAPTA and the various BAPTA derivatives that are calcium chelators (Tsien, 1980). Whereas BAPTA and its derivatives have the same calcium-binding motifs as EGTA (and hence the same Ca 2+ selectivity over other divalent cations), BAPTAs are fast calcium chelators with diffusion-limited on-rate and therefore are effective in buffering nanodomain calcium transients (Wang and Augustine, 2014). Our data strongly suggest that spindle formation, more specifically microtubule polymerization, requires spindle-based nanodomain calcium signaling.…”

“…Similarly, BAPTA buffers caused immediate microtubule depolymerization at all stages (Figures 4, A and B, and 5B). Given its complete insensitivity to EGTA, the calcium transients required for spindle function must be spindle based (Wang and Augustine, 2014). Third, calmodulin is a common calcium sensor with two calcium-binding domains, each capable of binding two Ca 2+ .…”

Release of chemical transmitters from cell bodies and dendrites of nerve cells

“…A great deal of modern research has been devoted to understanding transmitter release by presynaptic terminals. Most general knowledge about the mechanism of transmitter release has come from experiments on neuromuscular junctions [1] and the squid giant synapse ( [2]; for review, see [3,4]), and more recently from synapses formed by neurons in culture, first obtained between adult leech neurons [5] and afterwards between other neuron types. Yet, paradoxically, axon varicosities of parasympathetic nerve axons and chromaffin cells in the adrenal medulla are sites of extrasynaptic release at which chemically mediated synaptic transmission was postulated for the first time in 1921 by Loewi [6] and in 1904 by Elliot [7].…”