Filamins are a class of proteins that hold two actin filaments at large angles. [1] Filamin protein in mammals is made up of an actin-binding domain at its N-terminus that is followed by 24 immunoglobulin-like repeat modules of roughly 95 amino acids. There are two hinge regions; between repeats 15-16 and 23-24. Filamin gets cleaved at these hinge regions to generate smaller fragments of the protein. Filamin has two actin-binding sites with a V-linkage between them, so that it cross-links actin filaments into a network with the filaments orientated almost at right angles to one another. [2]
Filamin proteins include:
Over-expression of FLNA stops the regeneration of bladder carcinoma (BC) cells, by inhibiting the cell cycle and inducing apoptosis of BC cells. [3] FLNA has also been shown to reduce the mobility and invasion abilities of BC cells. [3]
The FLNa protein is also known for having an important structural function in the cardiovascular system. In particular, it has been studied by the American Society of Hematology for its role in platelet function in the blood. Platelets are known for their role in wound repair as they are able to aggregate and stop bleeding. Megakaryocytes allow for the production of platelets. However, mutations in the FLNa gene have been found to disrupt the process of healing as they limit the production of giant platelets that are needed for healing. [4] This condition is known as Macrothrombocytopenia.
The FLNC protein is important to the functioning of cardiac and skeletal tissue. [5] In particular, mutations to the FLNC can have detrimental effects on cardiac tissue. The FLNC proteins have an important role in the structure of cardiac muscle. They contribute to the z-disk proteins which are heavily found in both cardiac and skeletal muscle tissue. [6] Many studies have been conducted to evaluate the effects of mutations on the FLNC gene to patients that are otherwise healthy in terms of their heart. Studies conducted by the American College of Cardiology Foundation show that missense mutations in the FLNC gene could be precursors for diverse cardiomyopathies. [7] Specifically, hypertrophic cardiomyopathy (HCM) and restrictive cardiomyopathy (RCM) were noted. A further investigation of the link between this protein and cardiac tissue could allow professionals to develop treatments to prevent and treat patients with FLNC mutation induced cardiomyopathies.
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) 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-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.
Nebulette is a cardiac-specific isoform belonging to the nebulin family of proteins. It is encoded by the NEBL gene. This family is composed of 5 members: nebulette, nebulin, N-RAP, LASP-1 and LASP-2. Nebulette localizes to Z-discs of cardiac muscle and appears to regulate the length of actin thin filaments.
An overlap syndrome is a medical condition which shares features of at least two more widely recognised disorders. Examples of overlap syndromes can be found in many medical specialties such as overlapping connective tissue disorders in rheumatology, and overlapping genetic disorders in cardiology.
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.
Filamin A, alpha (FLNA) is a protein that in humans is encoded by the FLNA gene.
ACTC1 encodes cardiac muscle alpha actin. This isoform differs from the alpha actin that is expressed in skeletal muscle, ACTA1. Alpha cardiac actin is the major protein of the thin filament in cardiac sarcomeres, which are responsible for muscle contraction and generation of force to support the pump function of the heart.
Alpha-actinin-2 is a protein which in humans is encoded by the ACTN2 gene. This gene encodes an alpha-actinin isoform that is expressed in both skeletal and cardiac muscles and functions to anchor myofibrillar actin thin filaments and titin to Z-discs.
Filamin-C (FLN-C) also known as actin-binding-like protein (ABPL) or filamin-2 (FLN2) is a protein that in humans is encoded by the FLNC gene. Filamin-C is mainly expressed in cardiac and skeletal muscles, and functions at Z-discs and in subsarcolemmal regions.
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.
Filamin B, beta (FLNB), also known as Filamin B, beta , is a cytoplasmic protein which in humans is encoded by the FLNB gene.
Delta-sarcoglycan is a protein that in humans is encoded by the SGCD gene.
Myosin heavy chain, α isoform (MHC-α) is a protein that in humans is encoded by the MYH6 gene. This isoform is distinct from the ventricular/slow myosin heavy chain isoform, MYH7, referred to as MHC-β. MHC-α isoform is expressed predominantly in human cardiac atria, exhibiting only minor expression in human cardiac ventricles. It is the major protein comprising the cardiac muscle thick filament, and functions in cardiac muscle contraction. Mutations in MYH6 have been associated with late-onset hypertrophic cardiomyopathy, atrial septal defects and sick sinus syndrome.
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.
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.
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