| Voltage-gated calcium channel | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||||
| Symbol | VGCC | ||||||||||
| Pfam | PF08473 | ||||||||||
| OPM superfamily | 8 | ||||||||||
| OPM protein | 8epl | ||||||||||
| Membranome | 214 | ||||||||||
| |||||||||||
Voltage-gated calcium channels (VGCCs), also known asvoltage-dependent calcium channels (VDCCs), are a group ofvoltage-gated ion channels found in themembrane of excitable cells (e.g.muscle,glial cells,neurons) with apermeability to thecalcium ion Ca2+.[1][2] These channels are slightly permeable tosodium ions, so they are also called Ca2+–Na+ channels, but their permeability to calcium is about 1000-fold greater than to sodium under normal physiological conditions.[3]
Atphysiologic or restingmembrane potential, VGCCs are normally closed. They are activated (i.e.: opened) atdepolarized membrane potentials and this is the source of the "voltage-gated"epithet. The concentration ofcalcium (Ca2+ ions) is normally several thousand times higher outside the cell than inside. Activation of particular VGCCs allows a Ca2+ influx into the cell, which, depending on the cell type, results in activation of calcium-sensitivepotassium channels,muscular contraction,[4] excitation of neurons, up-regulation ofgene expression, or release ofhormones orneurotransmitters.
VGCCs have been immunolocalized in thezona glomerulosa of normal and hyperplastic humanadrenal, as well as inaldosterone-producingadenomas (APA), and in the latter T-type VGCCs correlated with plasma aldosterone levels of patients.[5] Excessive activation of VGCCs is a major component ofexcitotoxicity, as severely elevated levels of intracellular calcium activates enzymes which, at high enough levels, can degrade essential cellular structures.
Voltage-gated calcium channels are formed as a complex of several different subunits: α1, α2δ, β1-4, and γ. The α1 subunit forms the ion-conducting pore while the associated subunits have several functions including modulation of gating.[6]
There are several different kinds of high-voltage-gated calcium channels (HVGCCs). They are structurally homologous among varying types; they are all similar, but not structurally identical. In the laboratory, it is possible to tell them apart by studying their physiological roles and/or inhibition by specifictoxins. High-voltage-gated calcium channels include theneural N-type channel blocked by ω-conotoxin GVIA, the R-type channel (R stands forResistant to the other blockers and toxins, exceptSNX-482) involved in poorly defined processes in thebrain, the closely related P/Q-type channel blocked by ω-agatoxins, and the dihydropyridine-sensitive L-type channels responsible for excitation-contraction coupling ofskeletal,smooth, andcardiac muscle and for hormone secretion in endocrine cells.
| Current type | 1,4-dihydropyridine sensitivity (DHP) | ω-conotoxin sensitivity (ω-CTX) | ω-agatoxin sensitivity (ω-AGA) |
|---|---|---|---|
| L-type | blocks | resistant | resistant |
| N-type | resistant | blocks | resistant |
| P/Q-type | resistant | resistant | blocks |
| R-type | resistant | resistant | resistant |
Reference for the table can be found at Dunlap, Luebke and Turner (1995).[7]
The α1 subunit pore (~190 kDa in molecular mass) is the primary subunit necessary for channel functioning in the HVGCC, and consists of the characteristic four homologous I–IV domains containing six transmembrane α-helices each. The α1 subunit forms the Ca2+ selective pore, which contains voltage-sensing machinery and the drug/toxin-binding sites. A total of ten α1 subunits that have been identified in humans:[1] α1 subunit contains 4 homologous domains (labeled I–IV), each containing 6 transmembrane helices (S1–S6). This arrangement is analogous to a homo-tetramer formed by single-domain subunits of voltage-gated potassium channels (that also each contain 6 TM helices). The 4-domain architecture (and several key regulatory sites, such as the EF hand and IQ domain at the C-terminus) is also shared by the voltage gated sodium channels, which are thought to be evolutionarily related to VGCCs.[8] The transmembrane helices from the 4 domains line up to form the channel proper; S5 and S6 helices are thought to line the inner pore surface, while S1–4 helices have roles in gating and voltage sensing (S4 in particular).[9] VGCCs are subject to rapid inactivation, which is thought to consist of 2 components: voltage-gated (VGI) and calcium-gated (CGI).[10] These are distinguished by using either Ba2+ or Ca2+ as the charge carrier in the external recording solution (in vitro). The CGI component is attributed to the binding of the Ca2+-binding signaling proteincalmodulin (CaM) to at least 1 site on the channel, as Ca2+-null CaM mutants abolish CGI in L-type channels. Not all channels exhibit the same regulatory properties and the specific details of these mechanisms are still largely unknown.
| Type | Voltage | α1 subunit (gene name) | Associated subunits | Most often found in |
|---|---|---|---|---|
| L-type calcium channel ("Long-Lasting" AKA "DHP Receptor") | HVA (high voltage activated) | Cav1.1 (CACNA1S) Cav1.2 (CACNA1C)Cav1.3 (CACNA1D) Cav1.4 (CACNA1F) | α2δ, β, γ | Skeletal muscle, smooth muscle, bone (osteoblasts), ventricular myocytes** (responsible for prolonged action potential in cardiac cell; also termed DHP receptors), dendrites and dendritic spines of cortical neurones |
| P-type calcium channel ("Purkinje") /Q-type calcium channel | HVA (high voltage activated) | Cav2.1 (CACNA1A) | α2δ, β, possibly γ | Purkinje neurons in the cerebellum /Cerebellargranule cells |
| N-type calcium channel ("Neural"/"Non-L") | HVA (high voltage activated) | Cav2.2 (CACNA1B) | α2δ/β1, β3, β4, possibly γ | Throughout thebrain and peripheral nervous system. |
| R-type calcium channel ("Residual") | intermediate voltage activated | Cav2.3 (CACNA1E) | α2δ, β, possibly γ | Cerebellargranule cells, other neurons |
| T-type calcium channel ("Transient") | low voltage activated | Cav3.1 (CACNA1G) Cav3.2 (CACNA1H) Cav3.3 (CACNA1I) | neurons, cells that havepacemaker activity, bone (osteocytes) |
The α2δ gene forms two subunits: α2 and δ (which are both the product of the same gene). They are linked to each other via a disulfide bond and have a combined molecular weight of 170 kDa. The α2 is the extracellular glycosylated subunit that interacts the most with the α1 subunit. The δ subunit has a single transmembrane region with a short intracellular portion, which serves to anchor the protein in the plasma membrane. There are 4 α2δ genes:
Co-expression of the α2δ enhances the level of expression of the α1 subunit and causes an increase in current amplitude, faster activation and inactivation kinetics and a hyperpolarizing shift in the voltage dependence of inactivation. Some of these effects are observed in the absence of the beta subunit, whereas, in other cases, the co-expression of beta is required.
The α2δ-1 and α2δ-2 subunits are the binding site forgabapentinoids. This drug class includes two anticonvulsant drugs,gabapentin (Neurontin) andpregabalin (Lyrica), that also find use in treating chronic neuropathic pain. The α2δ subunit is also a binding site of the central depressant andanxiolyticphenibut, in addition to actions at other targets.[11]
The intracellular β subunit (55 kDa) is an intracellular MAGUK-like protein (Membrane-Associated Guanylate Kinase) containing a guanylate kinase (GK) domain and an SH3 (src homology 3) domain. The guanylate kinase domain of the β subunit binds to the α1 subunit I-II cytoplasmic loop and regulates HVGCC activity. There are four known genes for the β subunit:
It is hypothesized that the cytosolic β subunit has a major role in stabilizing the final α1 subunit conformation and delivering it to the cell membrane by its ability to mask anendoplasmic reticulum retention signal in the α1 subunit. The endoplasmic retention brake is contained in the I–II loop in the α1 subunit that becomes masked when the β subunit binds.[12] Therefore, the β subunit functions initially to regulate the current density by controlling the amount of α1 subunit expressed at the cell membrane.
In addition to this trafficking role, the β subunit has the added important functions of regulating the activation and inactivation kinetics, and hyperpolarizing the voltage-dependence for activation of the α1 subunit pore, so that more current passes for smallerdepolarizations. The β subunit has effects on the kinetics of the cardiac α1C inXenopus laevis oocytes co-expressed with β subunits. The β subunit acts as an important modulator of channel electrophysiological properties.
Until very recently, the interaction between a highly conserved 18-amino acid region on the α1 subunit intracellular linker between domains I and II (the Alpha Interaction Domain, AID) and a region on the GK domain of the β subunit (Alpha Interaction Domain Binding Pocket) was thought to be solely responsible for the regulatory effects by the β subunit. Recently, it has been discovered that the SH3 domain of the β subunit also gives added regulatory effects on channel function, opening the possibility of the β subunit having multiple regulatory interactions with the α1 subunit pore. Furthermore, the AID sequence does not appear to contain an endoplasmic reticulum retention signal, and this may be located in other regions of the I–II α1 subunit linker.
The γ1 subunit is known to be associated with skeletal muscle VGCC complexes, but the evidence is inconclusive regarding other subtypes of calcium channel. The γ1 subunit glycoprotein (33 kDa) is composed of four transmembrane spanning helices. The γ1 subunit does not affect trafficking, and, for the most part, is not required to regulate the channel complex. However, γ2, γ3, γ4 and γ8 are also associated with AMPA glutamate receptors.
There are 8 genes for gamma subunits:
When asmooth muscle cell is depolarized, it causes opening of the voltage-gated (L-type) calcium channels.[13][14] Depolarization may be brought about by stretching of the cell, agonist-binding its G protein-coupled receptor (GPCR), orautonomic nervous system stimulation. Opening of the L-type calcium channel causes influx of extracellular Ca2+, which then bindscalmodulin. The activated calmodulin molecule activatesmyosin light-chain kinase (MLCK), which phosphorylates themyosin inthick filaments. Phosphorylated myosin is able to formcrossbridges withactinthin filaments, and the smooth muscle fiber (i.e., cell) contracts via thesliding filament mechanism. (See reference[13] for an illustration of the signaling cascade involving L-type calcium channels in smooth muscle.)
L-type calcium channels are also enriched in thet-tubules ofstriated muscle cells, i.e., skeletal and cardiacmyofibers. When these cells are depolarized, the L-type calcium channels open as in smooth muscle. In skeletal muscle, the actual opening of the channel, which is mechanically gated to acalcium-release channel (a.k.a.ryanodine receptor, or RYR) in thesarcoplasmic reticulum (SR), causes opening of the RYR. Incardiac muscle, opening of the L-type calcium channel permits influx of calcium into the cell. The calcium binds to the calcium release channels (RYRs) in the SR, opening them; this phenomenon is calledcalcium-induced calcium release, or CICR. However the RYRs are opened, either through mechanical-gating or CICR, Ca2+ is released from the SR and is able to bind totroponin C on the actin filaments. The muscles then contract through the sliding filament mechanism, causing shortening ofsarcomeres and muscle contraction.
Early in development, there is a high amount of expression ofT-type calcium channels. During maturation of the nervous system, the expression ofN orL-type currents becomes more prominent.[15] As a result, mature neurons express more calcium channels that will only be activated when the cell is significantlydepolarized. The different expression levels of low-voltage activated (LVA) and high-voltage activated (HVA) channels can also play an important role inneuronal differentiation. In developingXenopusspinal neurons LVA calcium channels carry a spontaneous calcium transient that may be necessary for the neuron to adopt aGABAergic phenotype as well asprocess outgrowth.[16]
Voltage-gated calcium channels antibodies are associated withLambert-Eaton myasthenic syndrome and have also been implicated inparaneoplastic cerebellar degeneration.[17]
Voltage-gated calcium channels are also associated withmalignant hyperthermia[18],CACNA1C-related disorders[19][20][21] andTimothy syndrome.[22]
Mutations of theCACNA1C gene, with asingle-nucleotide polymorphism in the third intron of the Cav1.2 gene,[23] are associated with a variant oflong QT syndrome calledTimothy's syndrome[24] and also withCACNA1C-related disorders.[25][26] A growing number of rare variants in theCACNA1C gene have been implicated as the cause of syndromic presentations.[27][28][29][30][31] Large-scale genetic analyses have shown the possibility thatCACNA1C is associated withbipolar disorder[32] and subsequently also withschizophrenia.[33][34][35] Also, aCACNA1C risk allele has been associated to a disruption in brain connectivity in patients with bipolar disorder, while not or only to a minor degree, in their unaffected relatives or healthy controls.[36]