Ankyrin-2, also known as Ankyrin-B, and Brain ankyrin, is a protein which in humans is encoded by the ANK2 gene. [2] [3] Ankyrin-2 is ubiquitously expressed, but shows high expression in cardiac muscle. Ankyrin-2 plays an essential role in the localization and membrane stabilization of ion transporters and ion channels in cardiomyocytes, as well as in costamere structures. Mutations in ANK2 cause a dominantly-inherited, cardiac arrhythmia syndrome known as long QT syndrome 4 [4] as well as sick sinus syndrome; mutations have also been associated to a lesser degree with hypertrophic cardiomyopathy. Alterations in ankyrin-2 expression levels are observed in human heart failure.
Ankyrin-B protein is around 220 kDa, with several isoforms. [5] The ANK2 gene is approximately 560 kb in size and consists of 53 exons on human chromosome 4; ANK2 is also transcriptionally regulated via over 30 alternative splicing events with variable expression of isoforms in cardiac muscle. [6] [7] [8] Ankyrin-B is a member of the ankyrin family of proteins, and is a modular protein which is composed of three structural domains: an N-terminal domain containing multiple ankyrin repeats; a central region with a highly conserved spectrin binding domain and death domain; and a C-terminal regulatory domain which is the least conserved and subject to variation, and determines ankyrin-B activity. [2] [9] [10] The membrane-binding region of ankyrin-B is composed of 24 consecutive ankyrin repeats, and it is the membrane-binding domain of ankyrins that confer functional differences among ankyrin isoforms. [10] Though ubiquitously expressed, ankyrin-B shows high expression levels in cardiac muscle, and is expressed 10-fold lower levels in skeletal muscle, suggesting that ankyrin-B plays a specifically adapted functional role in cardiac muscle. [11]
Ankyrin-B is a member of the ankyrin family of proteins. Ankyrin-1 has been shown to be essential in normal function of erythrocytes; [12] however, ankyrin-B and ankyrin-3 play essential roles in the localization and membrane stabilization of ion transporters and ion channels in cardiomyocytes. [11] [13]
Functional insights into ankyrin-B function have come from studies employing ankyrin-B chimeric proteins. One study showed that the death/C-terminal domain of ankyrin-B determines both the subcellular localization as well as activity in restoring normal inositol trisphosphate receptor and ryanodine receptor localization and cardiomyocyte contractility. [10] Further studies have shown that the beta-hairpin loops within the ankyrin repeat domain of ankyrin-B are required for the interaction with the inositol trisphosphate receptor, and a reduction of ankyrin-B in neonatal cardiomyocytes reduces the half-life of the inositol trisphosphate receptor by 3-fold and destabilizes its proper localization; all of these effects were rescued by reintroducing ankyrin-B. [14] Moreover, a specific sequence in ankyrin-B (absent in other ankyrin isoforms) folds as an amphipathic alpha helix is required for normal levels of sodium-calcium exchanger, sodium potassium ATPase and inositol triphosphate receptor in cardiomyocytes, and is regulated by HDJ1/HSP40 binding to this region. [15]
Additional insights into ankyrin-B function have come from studies employing ankyrin-B transgenic animals. Cardiomyocytes from ankyrin-B (-/+) mice exhibited irregular spatial patterns and periodicity of calcium release, as well as abnormal distribution of the sarcomplasmic reticular calcium ATPase, SERCA2, and ryanodine receptors; effects that were rescued by transfection of ankyrin-B. [16] Effects on ryanodine receptors specifically were also rescued by a potent Ca2+/calmodulin-dependent protein kinase II inhibitor, suggesting that inhibition of Ca2+/calmodulin-dependent protein kinase II may also be a potential treatment strategy. [17] [18] These mice also display several electrophysiological abnormalities, including bradycardia, variable heart rate, long QT intervals, catecholaminergic polymorphic ventricular tachycardia, syncope, and sudden cardiac death. [19] Mechanistic explanations underlying these effects were explained in a later study conducted in the ankyrin-B (-/+) mice, which showed that reduction of ankyrin-B alters the transport of sodium and calcium and enhances the coupled openings of ryanodine receptors, which results in a higher frequency of calcium sparks and waves of calcium. [20]
It is now becoming clear that ankyrin-B exists in a biomolecular complex with the sodium potassium ATPase, sodium calcium exchanger and inositol triphosphate receptor which is localized in T-tubules within discrete microdomains of cardiomyocytes that are distinct from dyads formed by dihydropyridine receptors complexed to ryanodine receptors. The human ankyrin-B arrhythmogenic mutation (Glu1425Gly) blocks the formation of this complex, which provides a mechanism behind cardiac arrhythmias in patients. [11] Studies from other labs have shed light on the requirement of ankyrin-B in the targeting and post-translational stability of the sodium calcium exchanger in cardiomyocytes, which is clinically important because elevated expression of the sodium calcium exchanger is a factor related to arrhythmia and heart failure. [21] Ankyrin-B forms a membrane complex with ATP-sensitive potassium channels, which is necessary for normal channel trafficking and targeting the channel to sarcolemmal membranes; this interaction is also important in the response of cardiomyocytes to cardiac ischemia and metabolic regulation. [22] [23]
Ankyrin-B has also been identified to associate at sarcomeric M-lines and costameres in cardiac muscle and skeletal muscle, respectively. Exon 43′ in ankyrin-B is specifically and predominantly expressed in cardiac muscle and harbors key residues for modulating the interaction between ankyrin-B and obscurin. This interaction is also key for targeting protein phosphatase 2A to cardiac M-lines to propagate phosphorylation signaling paradigms. [24] In skeletal muscle, ankyrin-B interacts with dynactin-4 and with β2-spectrin, which is required for proper localization and functioning of the dystrophin complex and costamere structures, as well as protection from exercise-induced injury. [25]
Mutations in the ANK2 gene have been associated with a dominantly-inherited, cardiac arrhythmia syndrome known as long QT syndrome , type 4, [4] also known as ankyrin-B syndrome which can be described as an atypical arrhythmia syndrome with bradycardia, atrial fibrillation, conduction block, arrhythmia and risk of sudden cardiac death. [26] [27] [28] Intense investigation has been carried out regarding the linking of ANK2 mutations to the range of severity of cardiac phenotypes, and initial evidence suggests that the varying degrees of loss of function of ankyrin-B may explain the effect of any particular mutation. [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
Initially, a Glu1425Gly mutation in ANK2 was found to cause dominantly-inherited long QT syndrome type 4, cardiac arrhythmia. The mechanistic underpinnings of this mutation include abnormal expression and targeting of the sodium pump, the sodium-calcium exchanger, and inositol-1,4,5-trisphosphate receptors to transverse tubules, as well as calcium handling resulting in extrasystoles. [39] Further analysis in ANK2 mutations localized in the regulatory domain of ankyrin-2, which is specific to the ankyrin-2 isoform, indicated that long QT syndrome was not a consistent clinical manifestation of ANK2 mutations; [40] however, the effect on Ca(2+) dynamics and localization/expression of the sodium calcium exchanger, sodium potassium ATPase and inositol triphosphate receptor in cardiomyocytes were consistent observations. This study demonstrated that common pathogenic features of all ANK2 mutations was the abnormal coordination of a panel of related ion channels and transporters. [41] Additional mechanistic studies have shown that atrial cardiomyocytes lacking ankyrin-B have shortened action potentials, which can be explained by decreased voltage-dependent calcium channel expression, specifically Ca(v)1.3, which is responsible for low voltage-activated L-type Ca(2+) currents. Ankyrin-B directly associates with and is required for targeting Ca(v)1.3 to membranes. [42]
ANK2 mutations have also been identified in patients with sinus node dysfunction. Mechanistic studies on effects of these mutations in mice showed severe bradycardia and variability in heart rate, as well as dysfunction in ankyrin-B-based trafficking pathways in primary and subsidiary pacemaker cells. [43] [44] [45] In a large genotype-phenotype study of 874 patients with hypertrophic cardiomyopathy, patients with ANK2 variants exhibited greater maximum left ventricular wall thickness. [46]
In patients with both ischemic and non-ischemic heart failure, ankyrin-B levels are altered. Further mechanistic study showed that reactive oxygen species, intracellular calcium and calpain regulate cardiac ankyrin-B levels, and ankyrin-B is required for normal cardioprotection following ischemia reperfusion injury. [47]
Brugada syndrome (BrS) is a genetic disorder in which the electrical activity of the heart is abnormal due to channelopathy. It increases the risk of abnormal heart rhythms and sudden cardiac death. Those affected may have episodes of syncope. The abnormal heart rhythms seen in those with Brugada syndrome often occur at rest. They may be triggered by a fever.
Long QT syndrome (LQTS) is a condition affecting repolarization (relaxing) of the heart after a heartbeat, giving rise to an abnormally lengthy QT interval. It results in an increased risk of an irregular heartbeat which can result in fainting, drowning, seizures, or sudden death. These episodes can be triggered by exercise or stress. Some rare forms of LQTS are associated with other symptoms and signs including deafness and periods of muscle weakness.
Romano–Ward syndrome is the most common form of congenital Long QT syndrome (LQTS), a genetic heart condition that affects the electrical properties of heart muscle cells. Those affected are at risk of abnormal heart rhythms which can lead to fainting, seizures, or sudden death. Romano–Ward syndrome can be distinguished clinically from other forms of inherited LQTS as it affects only the electrical properties of the heart, while other forms of LQTS can also affect other parts of the body.
Andersen–Tawil syndrome, also called Andersen syndrome and long QT syndrome 7, is a rare genetic disorder affecting several parts of the body. The three predominant features of Andersen–Tawil syndrome include disturbances of the electrical function of the heart characterised by an abnormality seen on an electrocardiogram and a tendency to abnormal heart rhythms, physical characteristics including low-set ears and a small lower jaw, and intermittent periods of muscle weakness known as hypokalaemic periodic paralysis.
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.
Ryanodine receptors form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons. There are three major isoforms of the ryanodine receptor, which are found in different tissues and participate in different signaling pathways involving calcium release from intracellular organelles. The RYR2 ryanodine receptor isoform is the major cellular mediator of calcium-induced calcium release (CICR) in animal cells.
T-tubules are extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells. With membranes that contain large concentrations of ion channels, transporters, and pumps, T-tubules permit rapid transmission of the action potential into the cell, and also play an important role in regulating cellular calcium concentration.
Spectrin is a cytoskeletal protein that lines the intracellular side of the plasma membrane in eukaryotic cells. Spectrin forms pentagonal or hexagonal arrangements, forming a scaffold and playing an important role in maintenance of plasma membrane integrity and cytoskeletal structure. The hexagonal arrangements are formed by tetramers of spectrin subunits associating with short actin filaments at either end of the tetramer. These short actin filaments act as junctional complexes allowing the formation of the hexagonal mesh. The protein is named spectrin since it was first isolated as a major protein component of human red blood cells which had been treated with mild detergents; the detergents lysed the cells and the hemoglobin and other cytoplasmic components were washed out. In the light microscope the basic shape of the red blood cell could still be seen as the spectrin-containing submembranous cytoskeleton preserved the shape of the cell in outline. This became known as a red blood cell "ghost" (spectre), and so the major protein of the ghost was named spectrin.
Caveolin-3 is a protein that in humans is encoded by the CAV3 gene. Alternative splicing has been identified for this locus, with inclusion or exclusion of a differentially spliced intron. In addition, transcripts utilize multiple polyA sites and contain two potential translation initiation sites.
Ankyrins are a family of proteins that mediate the attachment of integral membrane proteins to the spectrin-actin based membrane cytoskeleton. Ankyrins have binding sites for the beta subunit of spectrin and at least 12 families of integral membrane proteins. This linkage is required to maintain the integrity of the plasma membranes and to anchor specific ion channels, ion exchangers and ion transporters in the plasma membrane. The name is derived from the Greek word ἄγκυρα (ankyra) for "anchor".
Sodium channel protein type 5 subunit alpha, also known as NaV1.5 is an integral membrane protein and tetrodotoxin-resistant voltage-gated sodium channel subunit. NaV1.5 is found primarily in cardiac muscle, where it mediates the fast influx of Na+-ions (INa) across the cell membrane, resulting in the fast depolarization phase of the cardiac action potential. As such, it plays a major role in impulse propagation through the heart. A vast number of cardiac diseases is associated with mutations in NaV1.5 (see paragraph genetics). SCN5A is the gene that encodes the cardiac sodium channel NaV1.5.
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited genetic disorder that predisposes those affected to potentially life-threatening abnormal heart rhythms or arrhythmias. The arrhythmias seen in CPVT typically occur during exercise or at times of emotional stress, and classically take the form of bidirectional ventricular tachycardia or ventricular fibrillation. Those affected may be asymptomatic, but they may also experience blackouts or even sudden cardiac death.
TRPC is a family of transient receptor potential cation channels in animals.
Potassium voltage-gated channel subfamily E member 2 (KCNE2), also known as MinK-related peptide 1 (MiRP1), is a protein that in humans is encoded by the KCNE2 gene on chromosome 21. MiRP1 is a voltage-gated potassium channel accessory subunit associated with Long QT syndrome. It is ubiquitously expressed in many tissues and cell types. Because of this and its ability to regulate multiple different ion channels, KCNE2 exerts considerable influence on a number of cell types and tissues. Human KCNE2 is a member of the five-strong family of human KCNE genes. KCNE proteins contain a single membrane-spanning region, extracellular N-terminal and intracellular C-terminal. KCNE proteins have been widely studied for their roles in the heart and in genetic predisposition to inherited cardiac arrhythmias. The KCNE2 gene also contains one of 27 SNPs associated with increased risk of coronary artery disease. More recently, roles for KCNE proteins in a variety of non-cardiac tissues have also been explored.
Calmodulin 1 is a protein in humans that is encoded by the CALM1 gene.
Ryanodine receptor 2 (RYR2) is one of a class of ryanodine receptors and a protein found primarily in cardiac muscle. In humans, it is encoded by the RYR2 gene. In the process of cardiac calcium-induced calcium release, RYR2 is the major mediator for sarcoplasmic release of stored calcium ions.
Alpha II-spectrin, also known as Spectrin alpha chain, brain is a protein that in humans is encoded by the SPTAN1 gene. Alpha II-spectrin is expressed in a variety of tissues, and is highly expressed in cardiac muscle at Z-disc structures, costameres and at the sarcolemma membrane. Mutations in alpha II-spectrin have been associated with early infantile epileptic encephalopathy-5, and alpha II-spectrin may be a valuable biomarker for Guillain–Barré syndrome and infantile congenital heart disease.
Plakophilin-2 is a protein that in humans is encoded by the PKP2 gene. Plakophilin 2 is expressed in skin and cardiac muscle, where it functions to link cadherins to intermediate filaments in the cytoskeleton. In cardiac muscle, plakophilin-2 is found in desmosome structures located within intercalated discs. Mutations in PKP2 have been shown to be causal in arrhythmogenic right ventricular cardiomyopathy.
Ankyrin-3 (ANK-3), also known as ankyrin-G, is a protein from ankyrin family that in humans is encoded by the ANK3 gene.
The ryanodine-inositol 1,4,5-triphosphate receptor Ca2+ channel (RIR-CaC) family includes Ryanodine receptors and Inositol trisphosphate receptors. Members of this family are large proteins, some exceeding 5000 amino acyl residues in length. This family belongs to the Voltage-gated ion channel (VIC) superfamily. Ry receptors occur primarily in muscle cell sarcoplasmic reticular (SR) membranes, and IP3 receptors occur primarily in brain cell endoplasmic reticular (ER) membranes where they effect release of Ca2+ into the cytoplasm upon activation (opening) of the channel. They are redox sensors, possibly providing a partial explanation for how they control cytoplasmic Ca2+. Ry receptors have been identified in heart mitochondria where they provide the main pathway for Ca2+ entry. Sun et al. (2011) have demonstrated oxygen-coupled redox regulation of the skeletal muscle ryanodine receptor-Ca2+ release channel (RyR1;TC# 1.A.3.1.2) by NADPH oxidase 4.
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