Nano-organization of synaptic calcium signaling

Abstract

Recent studies suggest an exquisite structural nano-organization within single synapses, where sites of evoked fusion - marked by clustering of synaptic vesicles, active zone proteins and voltage-gated calcium channels - are directly juxtaposed to postsynaptic receptor clusters within nanocolumns. This direct nanometer scale alignment between presynaptic fusion apparatus and postsynaptic receptors is thought to ensure the fidelity of synaptic signaling and possibly allow multiple distinct signals to occur without interference from each other within a single active zone. The functional specificity of this organization is made possible by the inherent nano-organization of calcium signals, where all the different calcium sources such as voltage-gated calcium channels, intracellular stores and store-operated calcium entry have dedicated local targets within their nanodomain to ensure precision of action. Here, we discuss synaptic nano-organization from the perspective of calcium signals, where some of the principal findings from early work in the 1980s continue to inspire current studies that exploit new genetic tools and super-resolution imaging technologies.

Keywords: calcium signaling; synapse; synaptic plasticity.

Figure 1.. Nano-organization of calcium signals in the excitatory synapse.

The arrival of an action potential to the presynaptic terminal triggers the opening of clustered Ca_V2 channels, producing evoked calcium transients (ECT) that induce synaptic vesicle fusion and evoked neurotransmitter release. In contrast, the stochastic opening of diffusely distributed Ca_V2 channels contributes to the baseline calcium (BC) signal that is partially responsible for spontaneous neurotransmitter release. Another calcium signal that influences spontaneous neurotransmission is the store-operated calcium entry (SOCE), where STIM2 proteins from the ER activate CRAC channels in the plasma membrane. Meanwhile, RyR in the presynaptic ER produce spontaneous calcium transients (SCT) that are independent of neurotransmitter release. In the postsynaptic density (PSD), evoked neurotransmitter release activates clustered NMDARs and Ca²⁺-permeable AMPARs to produce postsynaptic evoked calcium transients (ECT). Moreover, spontaneous release of glutamate activates NMDARs and this triggers RyR-mediated calcium-induced calcium release (CICR) from the ER that inhibits AMPAR insertion in the plasma membrane. Other forms of CICR in the postsynaptic compartment are triggered by Ca_V1 and Ca_V2 channels. Finally, GPCRs, like mGluR1/5, can trigger Ca²⁺ ER release through IP3R activation. All of these calcium signals are tightly controlled by the presence of Ca²⁺ buffers (mobile or immobile) and Ca²⁺ pumps (cell surface or ER membrane). AMPAR, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; BC, baseline calcium; BK, big potassium channel; CaMKII, calcium/calmodulin-stimulated protein kinase II; CP-AMPAR, Ca²⁺-permeable AMPAR; Ca_V1, voltage-gated calcium channel type 1; Ca_V2, voltage-gated calcium channel type 2; Ca_V3, voltage-gated calcium channel type 3; CICR, calcium-induced calcium release; CRAC, calcium release activated channel; CREB, cAMP response element-binding protein; ECT, evoked calcium transient; GPCR, G-protein coupled receptor; IP3R, inositol triphosphate receptor; mGluR1/5, metabotropic glutamate receptor type 1/5; nAChR, nicotinic acetylcholine receptor; NCX, sodium-calcium exchanger; NMDAR, N-methyl-d-aspartate receptor; PMCA, plasma membrane Ca²⁺-ATPase; PSD, postsynaptic density; RyR, ryanodine receptor; SCT, spontaneous calcium transient; SERCA, sarcoplasmic reticulum Ca²⁺-ATPase; SOCE, store-operated calcium entry; STIM1/2, stromal interaction molecule 1/2; TRPC, transient receptor potential canonical channel.

Figure 2.. Regulation of local calcium concentration in the nano- and microdomain of a presynaptic Ca_V channel.

The opening of presynaptic Ca_V2 channels causes a fast and transient calcium (Ca²⁺) influx. Calcium concentration rapidly increases up to 50–100 µM in the Ca_V2 channel's nanodomain, activating closely located targets like sytaptotagmin (Syt). The scaffolding proteins RIM and RIM-BP tether Ca_V2 channels to synaptic vesicles, granting that synaptotagmin molecules are at the nanodomain reach. Calcium ions bind to synaptotagmin and trigger vesicle fusion and neurotransmitter release. In the channel's nanodomain, calcium concentration is modulated by calmodulin (CaM), which binds calcium and activates specific downstream target proteins. Conversely, calcium oscillations in the micrometer range are buffered by other mobile calcium binding proteins (Ca-B) and calcium pumps, such as the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA).