Plakophilin-2 is a protein that in humans is encoded by the PKP2 gene. [5] [6] 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.
Two splice variants of the PKP2 gene have been identified. The first has a molecular weight of 97.4 kDa (881 amino acids) and the second of molecular weight of 92.7 kDa (837 amino acids). [7] [8] A processed pseudogene with high similarity to this locus has been mapped to chromosome 12p13. [6]
Plakophilin-2 is a member of the armadillo repeat and plakophilin protein family. Plakophilin proteins contain nine central, conserved armadillo repeat domains flanked by N-terminal and C-terminal domains. [9] Alternately spliced transcripts encoding protein isoforms have been identified. [10]
Plakophilin 2 localizes to cell desmosomes and nuclei and binds plakoglobin, desmoplakin, and the desmosomal cadherins via N-terminal head domain. [11] [12]
Plakophilin 2 functions to link cadherins to intermediate filaments in the cytoskeleton. In cardiomyocytes, plakophilin-2 is found at desmosome structures within intercalated discs, which link adjacent sarcolemmal membranes together. [13] The desmosomal protein, desmoplakin, is the core constituent of the plaque which anchors intermediate filaments to the sarcolemma by its C-terminus and indirectly to sarcolemmal cadherins by its N-terminus, facilitated by plakoglobin and plakophilin-2. [14] Plakophilin is necessary for normal localization and content of desmoplakin to desmosomes, which may in part be due to the recruitment of protein kinase C alpha to desmoplakin. [15]
Ablation of PKP2 in mice severely disrupts normal heart morphogenesis. Mutant mice are embryonic lethal and exhibit deficits in the formation of adhering junctions in cardiomyocytes, including the dissociation of desmoplakin and formation of cytoplasmic granular aggregates around embryonic day 10.5-11. Additional malformation included reduced trabeculation, cytoskeletal disarray and cardiac wall rupture. [16] Further studies demonstrated that plakophilin-2 coordinate with E-cadherin is required to properly localize RhoA early in actin cytoskeletal rearrangement in order to properly couple the assembly of adherens junctions to the translocation of desmosome precursors in newly formed cell-cell junctions. [17]
Plakophilin-2 over time has shown to be more than components of cell-cell junctions; rather the plakophilins are emerging as versatile scaffolds for various signaling pathways that more globally modulate diverse cellular activities. [9] Plakophilin-2 has shown to localize to nuclei, in addition to desmosomal plaques in the cytoplasm. Studies have shown that plakkophillin-2 is found in the nucleoplasm, complexed in the RNA polymerase III holoenzyme with the largest subunit of RNA polymerase III, termed RPC155. [11]
There are data to support molecular crosstalk between plakophilin-2 and proteins involved in mechanical junctions in cardiomyocytes, including connexin 43, the major component of cardiac gap junctions; the voltage-gated sodium channel Na(V)1.5 and its interacting subunit, ankyrin G; and the K(ATP). Decreased expression of plakophilin-2 via siRNA leads to a decrease in and redistribution of connexin 43 protein, as well as a decrease in coupling of adjacent cardiomyocytes. Studies also showed that GJA1 and plakophilin-2 are components in the same biomolecular complex. [18] Plakophilin-2 also associates with Na(V)1.5, and knockdown of plakophilin-2 in cardiomyocytes alters sodium current properties as well as velocity of action potential propagation. [19] It has also been demonstrated that plakophilin-2 associates with an important component of the Na(V)1.5 complex, ankyrin G, and loss of ankyrin G via siRNA downregulation mislocalized plakophilin-2 and connexin 43 in cardiac cells, which was coordinate with decreased electrical coupling of cells and decreased adhesion strength. [20] These studies were further supported by an investigation in a mouse model harboring a PKP2-heterozygous null mutation, which showed decreased Na(V)1.5 amplitude, as well as a shift in gating and kinetics; pharmacological challenge also induced ventricular arrhythmias. These findings further support the notion that desmosomes crosstalk with sodium channels in the heart, and suggest that the risk of arrhythmias in patients with PKP2 mutations may be unveiled with pharmacological challenge. [21] Evidence has also shown that plakophilin-2 binds to the K(ATP) channel subunit, Kir6.2, and that in cardiomyocytes from haploinsufficient PKP2 mice, K(ATP) channel current density was ~40% smaller and regional heterogeneity of K(ATP) channels was altered, suggesting that plakophilin-2 interacts with K(ATP) and mediates crosstalk between intercellular junctions and membrane excitability. [22]
Mutations in PKP2 have been associated with, have been shown to cause, and are considered common in arrhythmogenic right ventricular cardiomyopathy, which is characterized by fibrofatty replacement of cardiomyocytes, ventricular tachycardia and sudden cardiac death. [23] [24] [25] [26] [27] [28] [29] [30] It is estimated that 70% of all mutations associated with arrhythmogenic right ventricular cardiomyopathy are within the PKP2 gene. [31] These mutations in general appear to disrupt the assembly and stability of desmosomes. [32] Mechanistic studies have shown that certain PKP2 mutations result in instability of the plakophilin-2 protein due to enhanced calpain-mediated degradation. [33]
Specific and sensitive markers of PKP2 and plakoglobin mutation carriers in arrhythmogenic right ventricular cardiomyopathy have been identified to include T-wave inversions, right ventricular wall motion abnormalities, and ventricular extrasystoles. [34] Additionally, immunohistochemical analysis of proteins comprising cardiomyocyte desmosomes has shown to be a highly sensitive and specific diagnostic indicator. [35]
Clinical and genetic characterization of arrhythmogenic right ventricular cardiomyopathy is currently under intense investigation to understand the penetrance associated with PKP2 mutations, as well as other genes encoding desmosomal proteins, in disease progression and outcome. [10] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45]
PKP2 mutations were also found to coexist with sodium channelopathies in patients with Brugada syndrome. [46] [47]
Additionally, plakophilin-2 was found in adherens junctions of cardiac myxomata tumors analyzed, and absent in patients with noncardiac myxomata, suggesting that plakophilin-2 may serve as a valuable marker in the clinical diagnosis of cardiac myxomata. [48]
PKP2 has been shown to interact with:
Intermediate filaments (IFs) are cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate Branchiostoma.
A desmosome, also known as a macula adherens, is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adhesions randomly arranged on the lateral sides of plasma membranes. Desmosomes are one of the stronger cell-to-cell adhesion types and are found in tissue that experience intense mechanical stress, such as cardiac muscle tissue, bladder tissue, gastrointestinal mucosa, and epithelia.
Arrhythmogenic cardiomyopathy (ACM), arrhythmogenic right ventricular dysplasia (ARVD), or arrhythmogenic right ventricular cardiomyopathy (ARVC), most commonly is an inherited heart disease.
Desmin is a protein that in humans is encoded by the DES gene. Desmin is a muscle-specific, type III intermediate filament that integrates the sarcolemma, Z disk, and nuclear membrane in sarcomeres and regulates sarcomere architecture.
Desmoglein-1 is a protein that in humans is encoded by the DSG1 gene. Desmoglein-1 is expressed everywhere in the skin epidermis, but mainly it is expressed in the superficial upper layers of the skin epidermis.
Desmoglein-2 is a protein that in humans is encoded by the DSG2 gene. Desmoglein-2 is highly expressed in epithelial cells and cardiomyocytes. Desmoglein-2 is localized to desmosome structures at regions of cell-cell contact and functions to structurally adhere adjacent cells together. In cardiac muscle, these regions are specialized regions known as intercalated discs. Mutations in desmoglein-2 have been associated with arrhythmogenic right ventricular cardiomyopathy and familial dilated cardiomyopathy.
Desmoplakin is a protein in humans that is encoded by the DSP gene. Desmoplakin is a critical component of desmosome structures in cardiac muscle and epidermal cells, which function to maintain the structural integrity at adjacent cell contacts. In cardiac muscle, desmoplakin is localized to intercalated discs which mechanically couple cardiac cells to function in a coordinated syncytial structure. Mutations in desmoplakin have been shown to play a role in dilated cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy, where it may present with acute myocardial injury; striate palmoplantar keratoderma, Carvajal syndrome and paraneoplastic pemphigus.
Plakoglobin, also known as junction plakoglobin or gamma-catenin, is a protein that in humans is encoded by the JUP gene. Plakoglobin is a member of the catenin protein family and homologous to β-catenin. Plakoglobin is a cytoplasmic component of desmosomes and adherens junctions structures located within intercalated discs of cardiac muscle that function to anchor sarcomeres and join adjacent cells in cardiac muscle. Mutations in plakoglobin are associated with arrhythmogenic right ventricular dysplasia.
Cardiac muscle troponin T (cTnT) is a protein that in humans is encoded by the TNNT2 gene. Cardiac TnT is the tropomyosin-binding subunit of the troponin complex, which is located on the thin filament of striated muscles and regulates muscle contraction in response to alterations in intracellular calcium ion concentration.
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.
The myosin-binding protein C, cardiac-type is a protein that in humans is encoded by the MYBPC3 gene. This isoform is expressed exclusively in heart muscle during human and mouse development, and is distinct from those expressed in slow skeletal muscle (MYBPC1) and fast skeletal muscle (MYBPC2).
Desmocollin-2 is a protein that in humans is encoded by the DSC2 gene. Desmocollin-2 is a cadherin-type protein that functions to link adjacent cells together in specialized regions known as desmosomes. Desmocollin-2 is widely expressed, and is the only desmocollin isoform expressed in cardiac muscle, where it localizes to intercalated discs. Mutations in DSC2 have been causally linked to arrhythmogenic right ventricular cardiomyopathy.
Plakophilin-1 is a protein that in humans is encoded by the PKP1 gene.
Desmocollin-3 is a protein that in humans is encoded by the DSC3 gene.
Plakophilin-3 is a protein that in humans is encoded by the PKP3 gene.
LIM domain binding 3 (LDB3), also known as Z-band alternatively spliced PDZ-motif (ZASP), is a protein which in humans is encoded by the LDB3 gene. ZASP belongs to the Enigma subfamily of proteins and stabilizes the sarcomere during contraction, through interactions with actin in cardiac and skeletal muscles. Mutations in the ZASP gene has been associated with several muscular diseases.
Transmembrane protein 43 is a protein that in humans is encoded by the TMEM43 gene. TMEM43 may have an important role in maintaining nuclear envelope structure by organizing protein complexes at the inner nuclear membrane. Required for retaining emerin at the inner nuclear membrane. However, the localization of TMEM43 in myocardial tissue is controversial discussed. Franke et al. demonstrated that TMEM43 is localized at the intercalated disc but not at the nuclear envelope. In contrast Christensen et al. have shown that TMEM43 is mainly localized at the sarcolemma. Mutations in TMEM43 are associated with ARVD and EDMD7.
Ankyrin-2, also known as Ankyrin-B, and Brain ankyrin, is a protein which in humans is encoded by the ANK2 gene. 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 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.
Desmocollins are a subfamily of desmosomal cadherins, the transmembrane constituents of desmosomes. They are co-expressed with desmogleins to link adjacent cells by extracellular adhesion. There are seven desmosomal cadherins in humans, three desmocollins and four desmogleins. Desmosomal cadherins allow desmosomes to contribute to the integrity of tissue structure in multicellular living organisms.
Frank I. Marcus was an American cardiologist and Emeritus Professor of Medicine at the University of Arizona Health Sciences Center, the author of more than 290 publications in peer-reviewed medical journals and of 90 book chapters. He was considered a world expert on arrhythmogenic right ventricular cardiomyopathy (ARVC) and was a member of the Editorial/Scientific Board of 14 Cardiovascular Journals as well as a reviewer for 26 other medical publications.
