Presynaptic Calcium Channels

Abstract

Presynaptic Ca²⁺ entry occurs through voltage-gated Ca²⁺ (Ca_V) channels which are activated by membrane depolarization. Depolarization accompanies neuronal firing and elevation of Ca²⁺ triggers neurotransmitter release from synaptic vesicles. For synchronization of efficient neurotransmitter release, synaptic vesicles are targeted by presynaptic Ca²⁺ channels forming a large signaling complex in the active zone. The presynaptic Ca_V2 channel gene family (comprising Ca_V2.1, Ca_V2.2, and Ca_V2.3 isoforms) encode the pore-forming α1 subunit. The cytoplasmic regions are responsible for channel modulation by interacting with regulatory proteins. This article overviews modulation of the activity of Ca_V2.1 and Ca_V2.2 channels in the control of synaptic strength and presynaptic plasticity.

Keywords: Ca2+ binding proteins; Ca2+ channels; G-proteins; synaptic proteins; synaptic transmission.

Figure 1

Ca²⁺ channel structure and organization. (a) The subunit composition and structure of high-voltage-activated Ca²⁺ channels. The cryo-EM structure of the rabbit voltage-gated Ca²⁺ channel Cav1.1 complex at a nominal resolution of 4.2 Å. The overall EM density map on the left is colored according to different subunits. The structure model on the right is color-coded for distinct subunits. Reproduced from [12]. (b) The α1 subunit consists of four homologous domains (I-IV), each consisting of six transmembrane segments (S1-S6). S1–S4 represents the voltage-sensing module. S5–S6 represents the pore-forming unit. The large intracellular loops linking the different domains of the α1 subunit serve as sites of interaction of different regulatory proteins important for channel regulation, including G-protein (Gβγ, Gα), RIM, SNARE proteins, and synaptotagmin at the synprint site (shown in green bar), calmodulin (CaM), and neuronal Ca²⁺ sensor proteins (nCaS) at the IQ-like motif, which begins with the sequence isoleucine-methionine (IM) instead of isoleucine-glutamine (IQ) and the nearby downstream CaM-binding domain (CBD), calmodulin kinase II (CaMKII), and protein kinase C (PKC). Adapted from [4].

Figure 2

Gβγ-mediated noradrenaline inhibition of transmitter release and N-terminal/I-II loop AID peptides of Ca_V2.2 α1-subunit. Noradrenaline (NA) induced Ba²⁺ current inhibition (upper traces) and transmitter release (lower graphs) were attenuated in the presence of Gβγ-interaction site of N-terminal peptide (Ca_V2.2^45-55, YKQSIAQRART) or AID peptide (Ca_V^377-393, RQQQIEREL NGYLEWIF) (See Figure 1b). Ba²⁺ currents were recorded from superior cervical ganglion (SCG) neurons acutely dissociated from 3- to 6-week-old Wistar rats, while the synaptic transmission was recorded from long-term cultured SCG neurons isolated from p7 rat. NA was bath-applied 30 min after injection of the peptide at 1 mM in the injection pipette. EPSPs were normalized to amplitude prior to NA application at time = 0 min. Bar graph summarizing NA effects, *p < 0.05 vs. NA effects in controls (Student’s t-test). Adapted from [98].

Figure 3

Spatial regulation of transmitter release by the I-II loop interaction with SNAREs. (a) The I-II loop interacts with t-SNAREs, resulting in inhibition of Ca2.2 channels opening. Once AP opens the channels, an increase in Ca²⁺ mediates interaction with SNAREs complex and induces transmitter release. Adapted from [4]. (b) Triple APs induces a large synchronous transmitter release from the first AP. In contrast, asynchronous transmitter release was observed in the presence of 130 μM synprint peptide (see Figure 1b). Adapted from [99].

Figure 4

Temporal regulation of Ca²⁺ channel activity by CaM and nCaS after AP(s) firing modulates synaptic transmission. (a) Regulation of transmitter release (lower trace) after an AP firing (upper trace). Dependent on the inter-stimulus interval the second AP induces paired-pulse depression (PPD) and facilitation (PPF). The PPD was prevented by ΔCBD, while PPF was prevented by IM-AA mutation of Ca_V2.1 channels. (b) Model for Ca²⁺/CaM-dependent inactivation and facilitation of Ca²⁺ channels and neurotransmitter release. (c) Biphasic synaptic transmission during 1-s train of APs at 30 Hz changed to synaptic depression by the IM-AA mutation or to synaptic facilitation by the ΔCBD. (d) Overexpression of CaBP1 (blue) blocks synaptic facilitation, while overexpression of VILIP-2 (red) blocks synaptic depression, during 1-s train of APs at 10 Hz. Adapted from [90] (a–c) and [113] (d).