Voltage-gated calcium channel
Voltage-gated calcium channels (VGCCs), also known as voltage-dependent calcium channels (VDCCs), are a group of voltage-gated ion channels found in the membrane of excitable cells (e.g., muscle, glial cells, neurons, etc.) with a permeability to the calcium ion Ca2+. These channels are slightly permeable to sodium 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. At physiologic or resting membrane potential, VGCCs are normally closed. They are activated (i.e., opened) at depolarized membrane potentials and this is the source of the "voltage-gated" epithet. The concentration of calcium (Ca2+ ions) is normally several thousand times higher outside the cell than inside. Activation of particular VGCCs allows Ca2+ to rush into the cell, which, depending on the cell type, results in activation of calcium-sensitive potassium channels, muscular contraction, excitation of neurons, up-regulation of gene expression, or release of hormones or neurotransmitters. VGCCs have been immunolocalized in the zona glomerulosa of normal and hyperplastic human adrenal, as well as in aldosterone-producing adenomas (APA), and in the latter T-type VGCCs correlated with plasma aldosterone levels of patients. Excessive activation of VGCCs is a major component of excitotoxicity, 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.
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 specific toxins. High-voltage-gated calcium channels include the neural N-type channel blocked by ω-conotoxin GVIA, the R-type channel (R stands for Resistant to the other blockers and toxins, except SNX-482) involved in poorly defined processes in the brain, the closely related P/Q-type channel blocked by ω-agatoxins, and the dihydropyridine-sensitive L-type channels responsible for excitation-contraction coupling of skeletal, smooth, and cardiac 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)|
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: 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 evolutionary related to VGCCs. 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). VGCCs are subject to rapid inactivation, which is thought to consist of 2 components: voltage-gated (VGI) and calcium-gated (CGI). 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 protein calmodulin (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)
|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 / Cerebellar granule cells|
|N-type calcium channel ("Neural"/"Non-L")||HVA (high voltage activated)||Cav2.2 (CACNA1B)||α2δ/β1, β3, β4, possibly γ||Throughout the brain and peripheral nervous system.|
|R-type calcium channel ("Residual")||intermediate voltage activated||Cav2.3 (CACNA1E)||α2δ, β, possibly γ||Cerebellar granule cells, other neurons|
|T-type calcium channel ("Transient")||low voltage activated||Cav3.1 (CACNA1G)
|neurons, cells that have pacemaker 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 for gabapentinoids. This drug class includes two anticonvulsant drugs, gabapentin (Neurontin) and pregabalin (Lyrica), that also find use in treating chronic neuropathic pain. The α2δ subunit is also a binding site of the central depressant and anxiolytic drug phenibut, in addition to actions at other targets.
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 an endoplasmic 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. 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 smaller depolarizations. The β subunit has effects on the kinetics of the cardiac α1C in Xenopus 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 a smooth muscle cell is depolarized, it causes opening of the voltage-gated (L-type) calcium channels. Depolarization may be brought about by stretching of the cell, agonist-binding its G protein-coupled receptor (GPCR), or autonomic nervous system stimulation. Opening of the L-type calcium channel causes influx of extracellular Ca2+, which then binds calmodulin. The activated calmodulin molecule activates myosin light-chain kinase (MLCK), which phosphorylates the myosin in thick filaments. Phosphorylated myosin is able to form crossbridges with actin thin filaments, and the smooth muscle fiber (i.e., cell) contracts via the sliding filament mechanism. (See reference for an illustration of the signaling cascade involving L-type calcium channels in smooth muscle).
L-type calcium channels are also enriched in the t-tubules of striated muscle cells, i.e., skeletal and cardiac myofibers. 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 a calcium-release channel (a.k.a. ryanodine receptor, or RYR) in the sarcoplasmic reticulum (SR), causes opening of the RYR. In cardiac 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 called "calcium-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 to troponin C on the actin filaments. The muscles then contract through the sliding filament mechanism, causing shortening of sarcomeres and muscle contraction.
Changes in expression during development
Early in development, there is a high amount of expression of T-type calcium channels. During maturation of the nervous system, the expression of N or L-type currents becomes more prominent. As a result, mature neurons express more calcium channels that will only be activated when the cell is significantly depolarized. The different expression levels of low-voltage activated (LVA) and high-voltage activated (HVA) channels can also play an important role in neuronal differentiation. In developing Xenopus spinal neurons LVA calcium channels carry a spontaneous calcium transient that may be necessary for the neuron to adopt a GABAergic phenotype as well as process outgrowth.
- Catterall WA, Perez-Reyes E, Snutch TP, Striessnig J (December 2005). "International Union of Pharmacology. XLVIII. Nomenclature and structure-function relationships of voltage-gated calcium channels". Pharmacological Reviews. 57 (4): 411–25. doi:10.1124/pr.57.4.5. PMID 16382099.
- Yamakage M, Namiki A (February 2002). "Calcium channels--basic aspects of their structure, function and gene encoding; anesthetic action on the channels--a review" (PDF). Canadian Journal of Anaesthesia = Journal Canadien d'Anesthesie. 49 (2): 151–64. doi:10.1007/BF03020488. PMID 11823393.
- Hall JE (2011). Guyton and Hall Textbook of Medical Physiology with Student Consult Online Access (PDF) (12th ed.). Philadelphia: Elsevier Saunders. p. 64. ISBN 978-1-4160-4574-8. Retrieved 2011-03-22.
- Wilson DP, Susnjar M, Kiss E, Sutherland C, Walsh MP (August 2005). "Thromboxane A2-induced contraction of rat caudal arterial smooth muscle involves activation of Ca2+ entry and Ca2+ sensitization: Rho-associated kinase-mediated phosphorylation of MYPT1 at Thr-855, but not Thr-697". The Biochemical Journal. 389 (Pt 3): 763–74. doi:10.1042/BJ20050237. PMC 1180727. PMID 15823093.
- Felizola SJ, Maekawa T, Nakamura Y, Satoh F, Ono Y, Kikuchi K, Aritomi S, Ikeda K, Yoshimura M, Tojo K, Sasano H (October 2014). "Voltage-gated calcium channels in the human adrenal and primary aldosteronism". The Journal of Steroid Biochemistry and Molecular Biology. 144 Pt B (part B): 410–6. doi:10.1016/j.jsbmb.2014.08.012. PMID 25151951.
- Dolphin AC (January 2006). "A short history of voltage-gated calcium channels". British Journal of Pharmacology. 147 Suppl 1 (Suppl 1): S56–62. doi:10.1038/sj.bjp.0706442. PMC 1760727. PMID 16402121.
- Dunlap K, Luebke JI, Turner TJ (February 1995). "Exocytotic Ca2+ channels in mammalian central neurons". Trends in Neurosciences. 18 (2): 89–98. doi:10.1016/0166-2236(95)93882-X. PMID 7537420.
- Zakon HH (June 2012). "Adaptive evolution of voltage-gated sodium channels: the first 800 million years". Proceedings of the National Academy of Sciences of the United States of America. 109 Suppl 1: 10619–25. doi:10.1073/pnas.1201884109. PMC 3386883. PMID 22723361.
- Tombola F, Pathak MM, Isacoff EY (1 November 2006). "How does voltage open an ion channel?". Annual Review of Cell and Developmental Biology. 22 (1): 23–52. doi:10.1146/annurev.cellbio.21.020404.145837. PMID 16704338.
- Cens T, Rousset M, Leyris JP, Fesquet P, Charnet P (Jan–Apr 2006). "Voltage- and calcium-dependent inactivation in high voltage-gated Ca(2+) channels". Progress in Biophysics and Molecular Biology. 90 (1–3): 104–17. doi:10.1016/j.pbiomolbio.2005.05.013. PMID 16038964.
- Zvejniece L, Vavers E, Svalbe B, Veinberg G, Rizhanova K, Liepins V, Kalvinsh I, Dambrova M (October 2015). "R-phenibut binds to the α2-δ subunit of voltage-dependent calcium channels and exerts gabapentin-like anti-nociceptive effects". Pharmacology Biochemistry and Behavior. 137: 23–9. doi:10.1016/j.pbb.2015.07.014. PMID 26234470.
- Bichet D, Cornet V, Geib S, Carlier E, Volsen S, Hoshi T, Mori Y, De Waard M (January 2000). "The I-II loop of the Ca2+ channel alpha1 subunit contains an endoplasmic reticulum retention signal antagonized by the beta subunit". Neuron. 25 (1): 177–90. doi:10.1016/S0896-6273(00)80881-8. PMID 10707982.
- Webb RC (December 2003). "Smooth muscle contraction and relaxation". Advances in Physiology Education. 27 (1–4): 201–6. doi:10.1152/advan.00025.2003. PMID 14627618.
- Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2002). Molecular Biology of the Cell (4th ed.). New York, NY: Garland Science. p. 1616. ISBN 0-8153-3218-1.
- Sanes DH, Reh TA (2012). Development of the nervous system (Third ed.). Elsevier Academic Press. pp. 211–214. ISBN 9780080923208. OCLC 762720374.
- Rosenberg SS, Spitzer NC (October 2011). "Calcium signaling in neuronal development". Cold Spring Harbor Perspectives in Biology. 3 (10): a004259. doi:10.1101/cshperspect.a004259. PMC 3179332. PMID 21730044.
- Bekircan-Kurt CE, Derle Çiftçi E, Kurne AT, Anlar B (March 2015). "Voltage gated calcium channel antibody-related neurological diseases". World Journal of Clinical Cases. 3 (3): 293–300. doi:10.12998/wjcc.v3.i3.293. PMC 4360501. PMID 25789302.
- Andrea Welling: Voltage-Dependent Calcium Channels, BIOTREND Reviews No. 04, January 2009, 2009 BIOTREND Chemicals AG
- "Voltage-Gated Ion Channels". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology.
- Calcium+Channels at the US National Library of Medicine Medical Subject Headings (MeSH)