Two-pore channel | |||||||||
---|---|---|---|---|---|---|---|---|---|
Identifiers | |||||||||
Symbol | TPC | ||||||||
Pfam | PF08473 | ||||||||
OPM superfamily | 8 | ||||||||
OPM protein | 6c96 | ||||||||
Membranome | 214 | ||||||||
|
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) with a permeability to the calcium ion Ca2+. [1] [2] 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. [3]
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 a Ca2+ influx into the cell, which, depending on the cell type, results in activation of calcium-sensitive potassium channels, muscular contraction, [4] 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. [5] 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. [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 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) |
---|---|---|---|
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 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 ) 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 / 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 ) Cav3.2 ( CACNA1H ) Cav3.3 ( CACNA1I ) | 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 phenibut, 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 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. [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 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. [13] [14] 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 [13] 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.
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. [15] 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. [16]
Voltage-gated calcium channels antibodies are associated with Lambert-Eaton myasthenic syndrome and have also been implicated in paraneoplastic cerebellar degeneration. [17]
Voltage-gated calcium channels are also associated with malignant hyperthermia [18] and Timothy syndrome. [19]
Mutations of the CACNA1C gene, with a single-nucleotide polymorphism in the third intron of the Cav1.2 gene, [20] are associated with a variant of long QT syndrome called Timothy's syndrome [21] and also with Brugada syndrome. [22] Large-scale genetic analyses have shown the possibility that CACNA1C is associated with bipolar disorder [23] and subsequently also with schizophrenia. [24] [25] [26] Also, a CACNA1C 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. [27]
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ions across the cell membrane, controlling the flow of ions across secretory and epithelial cells, and regulating cell volume. Ion channels are present in the membranes of all cells. Ion channels are one of the two classes of ionophoric proteins, the other being ion transporters.
An acetylcholine receptor or a cholinergic receptor is an integral membrane protein that responds to the binding of acetylcholine, a neurotransmitter.
Calcium channel blockers (CCB), calcium channel antagonists or calcium antagonists are a group of medications that disrupt the movement of calcium through calcium channels. Calcium channel blockers are used as antihypertensive drugs, i.e., as medications to decrease blood pressure in patients with hypertension. CCBs are particularly effective against large vessel stiffness, one of the common causes of elevated systolic blood pressure in elderly patients. Calcium channel blockers are also frequently used to alter heart rate, to prevent peripheral and cerebral vasospasm, and to reduce chest pain caused by angina pectoris.
A neuromuscular junction is a chemical synapse between a motor neuron and a muscle fiber.
Unlike the action potential in skeletal muscle cells, the cardiac action potential is not initiated by nervous activity. Instead, it arises from a group of specialized cells known as pacemaker cells, that have automatic action potential generation capability. In healthy hearts, these cells form the cardiac pacemaker and are found in the sinoatrial node in the right atrium. They produce roughly 60–100 action potentials every minute. The action potential passes along the cell membrane causing the cell to contract, therefore the activity of the sinoatrial node results in a resting heart rate of roughly 60–100 beats per minute. All cardiac muscle cells are electrically linked to one another, by intercalated discs which allow the action potential to pass from one cell to the next. This means that all atrial cells can contract together, and then all ventricular cells.
Voltage-gated ion channels are a class of transmembrane proteins that form ion channels that are activated by changes in a cell's electrical membrane potential near the channel. The membrane potential alters the conformation of the channel proteins, regulating their opening and closing. Cell membranes are generally impermeable to ions, thus they must diffuse through the membrane through transmembrane protein channels.
Cyclic nucleotide–gated ion channels or CNG channels are ion channels that function in response to the binding of cyclic nucleotides. CNG channels are nonselective cation channels that are found in the membranes of various tissue and cell types, and are significant in sensory transduction as well as cellular development. Their function can be the result of a combination of the binding of cyclic nucleotides and either a depolarization or a hyperpolarization event. Initially discovered in the cells that make up the retina of the eye, CNG channels have been found in many different cell types across both the animal and the plant kingdoms. CNG channels have a very complex structure with various subunits and domains that play a critical role in their function. CNG channels are significant in the function of various sensory pathways including vision and olfaction, as well as in other key cellular functions such as hormone release and chemotaxis. CNG channels have also been found to exist in prokaryotes, including many spirochaeta, though their precise role in bacterial physiology remains unknown.
Ligand-gated ion channels (LICs, LGIC), also commonly referred to as ionotropic receptors, are a group of transmembrane ion-channel proteins which open to allow ions such as Na+, K+, Ca2+, and/or Cl− to pass through the membrane in response to the binding of a chemical messenger (i.e. a ligand), such as a neurotransmitter.
Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels.
Calcium channel, voltage-dependent, L type, alpha 1C subunit is a protein that in humans is encoded by the CACNA1C gene. Cav1.2 is a subunit of L-type voltage-dependent calcium channel.
T-type calcium channels are low voltage activated calcium channels that become inactivated during cell membrane hyperpolarization but then open to depolarization. The entry of calcium into various cells has many different physiological responses associated with it. Within cardiac muscle cell and smooth muscle cells voltage-gated calcium channel activation initiates contraction directly by allowing the cytosolic concentration to increase. Not only are T-type calcium channels known to be present within cardiac and smooth muscle, but they also are present in many neuronal cells within the central nervous system. Different experimental studies within the 1970s allowed for the distinction of T-type calcium channels from the already well-known L-type calcium channels. The new T-type channels were much different from the L-type calcium channels due to their ability to be activated by more negative membrane potentials, had small single channel conductance, and also were unresponsive to calcium antagonist drugs that were present. These distinct calcium channels are generally located within the brain, peripheral nervous system, heart, smooth muscle, bone, and endocrine system.
The P-type calcium channel is a type of voltage-dependent calcium channel. Similar to many other high-voltage-gated calcium channels, the α1 subunit determines most of the channel's properties. The 'P' signifies cerebellar Purkinje cells, referring to the channel's initial site of discovery. P-type calcium channels play a similar role to the N-type calcium channel in neurotransmitter release at the presynaptic terminal and in neuronal integration in many neuronal types.
N-type calcium channels, also called Cav2.2 channels, are voltage gated calcium channels that are localized primarily on the nerve terminals and dendrites as well as neuroendocrine cells. The calcium N-channel consists of several subunits: the primary subunit α1B and the auxiliary subunits α2δ and β. The α1B subunit forms the pore through which the calcium enters and helps to determine most of the channel's properties. These channels play an important role in the neurotransmission during development. In the adult nervous system, N-type calcium channels are critically involved in the release of neurotransmitters, and in pain pathways. N-type calcium channels are the target of ziconotide, the drug prescribed to relieve intractable cancer pain. There are many known N-type calcium channel blockers that function to inhibit channel activity, although the most notable blockers are ω-conotoxins.
The L-type calcium channel is part of the high-voltage activated family of voltage-dependent calcium channel. "L" stands for long-lasting referring to the length of activation. This channel has four isoforms: Cav1.1, Cav1.2, Cav1.3, and Cav1.4.
Calcium-activated potassium channel subunit alpha-1 also known as large conductance calcium-activated potassium channel, subfamily M, alpha member 1 (KCa1.1), or BK channel alpha subunit, is a voltage gated potassium channel encoded by the KCNMA1 gene and characterized by their large conductance of potassium ions (K+) through cell membranes.
Potassium voltage-gated channel, Shab-related subfamily, member 1, also known as KCNB1 or Kv2.1, is a protein that, in humans, is encoded by the KCNB1 gene.
Calcium channel, voltage-dependent, L type, alpha 1D subunit is a protein that in humans is encoded by the CACNA1D gene. Cav1.3 channels belong to the Cav1 family, which form L-type calcium currents and are sensitive to selective inhibition by dihydropyridines (DHP).
Calcium channel, voltage-dependent, T type, alpha 1H subunit, also known as CACNA1H, is a protein which in humans is encoded by the CACNA1H gene.
The voltage-dependent N-type calcium channel subunit alpha-1B is a protein that in humans is encoded by the CACNA1B gene. The α1B protein, together with β and α2δ subunits forms N-type calcium channel. It is a R-type calcium channel.
In electrophysiology, the term gating refers to the opening (activation) or closing of ion channels. This change in conformation is a response to changes in transmembrane voltage.
{{cite journal}}
: Cite journal requires |journal=
(help)[ clarification needed ]