| PRG4 | |||||||||||||||||||||||||||||||||||||||||||||||||||
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| Aliases | PRG4 , CACP, HAPO, JCAP, MSF, SZP, bG174L6.2, proteoglycan 4 | ||||||||||||||||||||||||||||||||||||||||||||||||||
| External IDs | OMIM: 604283 HomoloGene: 130465 GeneCards: PRG4 | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Proteoglycan 4 or lubricin is a proteoglycan that in humans is encoded by the PRG4 gene. [3] [4] [5] It acts as a joint/boundary lubricant. [5]
Lubricin is present in synovial fluid and on the surface (superficial layer) of articular cartilage and therefore plays an important role in joint lubrication and synovial homeostasis. When first isolated, cartilage lubricin was called "superficial zone protein" (SZP). [6] [7] Due to the discovery that the 32-kDa amino terminal fragment of lubricin could stimulate in-vitro megakaryocyte growth, the gene responsible for the expression of lubricin was initially called "megakaryocyte-stimulating factor" (MSF). [8] However, Lubricin, MSF, and SZP are now collectively known as Proteoglycan 4 (hence PRG4 for the gene nomenclature). The evidence that lubricin is actually a proteoglycan is not solid. [9] The expression of lubricin has also been detected and the protein localized in tendon, [10] meniscus, [11] lung, liver, heart, bone, [12] ligament, muscle, and skin. [13] It is present in human plasma, where it binds to neutrophils via L-selectin. [14]
Lubricin shares many properties with other members of the mucin family and similarly plays important roles in protecting cartilage surface from protein deposition and cell adhesion, in inhibiting synovial cell overgrowth, and in preventing cartilage-cartilage adhesion. [15] [16]
Early work on lubricin showed that it was able to lubricate non cartilaginous surfaces as effectively as whole synovial fluid, confirming its important biological lubrication role. [17] Understanding lubricin is key to understanding joint mechanics and friction-based diseases. [18]
The protein encoded by this gene is a approximately 345 kDa [19] specifically synthesized by chondrocytes located at the surface of articular cartilage, and also by synovial lining cells. The cDNA encodes a protein of 1,404 amino acids (human A isoform) with a somatomedin B homology domain, heparin-binding domains, multiple mucin-like repeats, a hemopexin domain, and an aggregation domain. There are 3 consensus sequences for N-glycosylation [5] and more than 168 sites for O-linked glycosylation. [20]
Lubricin is a large glycoprotein that consists of approximately equal proportions of protein and oligosaccharides. The oligosaccharides are O-linked both with and without sialic acid. [14] [20] Electron microscope measurements show that the lubricin molecule is a partially extended flexible rod and, in solution, occupies a smaller spatial domain than would be expected from structural predictions. [21] The large glycosylated region (i. e mucin domain) of lubricin makes it a water-soluble synovial fluid protein. In synovial fluid it interacts with Galectin-3 that improves its lubricating property. [22] [23] Lubricin's unglycosylated regions can interact with cartilage proteins. [24] [25] This characteristic may aid in the molecule's boundary lubricating ability.
Lubricin is a close analog to vitronectin, as both of these proteins contain a somatomedin B-like (SMB) domain and a hemopexin-like chain. These domains play a unique role in cell-cell and cell-extracellular matrix interactions. [26] However, unlike vitronectin, lubricin carries a central mucin-like domain with a large number of repeating KEPAPTT motifs. [27]
In total, lubricin is approximately 200 nm +/- 50 nm in length and has a diameter of a few nanometers. The glycoprotein consists of >5% serine and >20% threonine residues, which give rise to a large number of O-glycosylations. These are thought to contain short polar (Galβ1-3GalNAcα1-Ser/Thr) and negatively charged (NeuAcα2-3Galβ1-3GalNAcα1α1-Ser/Thr) sugar groups. About two thirds of these sugar groups are capped with sialic acid, and the end domains of the glycoprotein are thought to be globular, due to the nature of their protein-like domains. The N-terminus of lubricin is associated with its SMB-like domains, [28] whereas the C-terminus is associated with the hemopexin-like domain. [29] Due to the protein's overall slight negative charge and the fact that the center of the protein carries negatively charged sugar groups, the two end domains are thought to carry much of the protein's positive charge. [16] [21] [30] [20]
Lubricin's complex protein structure is termed "bottle brush," which refers to the large number of densely packed glycosylations on lubricin's backbone. Overall, lubricin's structure is similar to other mucin proteins and bottle brush polymers. This structure is key to its lubricating ability, which is ascribed to interchain repulsion. This leads to trapping of large quantities of solvent and the stabilization of a fluid-like cushioning layer, which enables bottle brush polymers to lower the friction between joints when external pressure is applied. [32] [33]
Furthermore, lubricin's N-terminus is thought to create disulfide bonds between two lubricin monomers. The glycoprotein thus exists as both a monomer and a dimer. [28] The adsorption of lubricin to cartilage surfaces occurs through interactions on its N- and C- terminus, where its bottle brush structure plays a role in both coating and repelling similarly coated cartilage surfaces due to steric repulsion. [34] [35] [24] Lubricin's high degree of hydration is also thought to be involved in repulsion forces generated by lubricin between opposing cartilage surfaces. [36]
Shear studies of lubricin adsorbed between various hydrophilic and hydrophobic surfaces have confirmed the importance of the glycoprotein in boundary lubrication and wear protection in articular joints. [16] Lubricin's bottle brush structure is common among a number of human lubricating glycoproteins, and a number of studies have been conducted to mimic this. [37] Researchers have successfully designed low-friction polymers imitating lubricin's bottle-brush-like structure, further supporting the notion that it is lubricin's architecture which plays an important role in reducing friction. [38] Similarly, another study on zwitterionic polymer brushes, which intended to mimic the structure of bottle-brush polymers present in cartilage, found that the brushes produced super low fouling surfaces and super low friction surfaces. [39]
Lubricin, as MSF, was detected in the urine of patients undergoing bone marrow transplantation during a period of acute thrombocytopenia. [40] Depletion of lubricin function has also been associated with camptodactyly-arthropathy-coxa vara-pericarditis syndrome (CACP), an arthritis-like autosomal recessive disorder. [3]
The locus for autosomal recessive camptodactyly-arthropathy-coxa vara-pericarditis syndrome maps to chromosome 1q25-q31 where the PRG4 gene is located. Cell overgrowth may be primary to the pathogenesis of this protein. [5]
Lubricin’s role in improving tendon gliding has also been studied. While adding lubricin alone fails to affect the tendon gliding resistance, the addition of cd-gelatin plus lubricin significantly lowered the gliding resistance of the tendons. This research can aid in improving the gliding ability of tendon grafts done clinically. [41] Extracorporeal shockwave therapy application has been shown to induce an increased lubricin expression in tendons and septa of rat hindlimbs, which might suggest a beneficial lubricating effect for joints and tissues prone to wear and tear degradation. [42]
Furthermore, the synovial fluid of patients with rheumatoid arthritis and osteoarthritis has been shown to exhibit reduced levels of lubricin when compared to healthy patients. [43] Researchers are currently exploring potential applications of lubricin for treating these and other related diseases. [44] Thus far, adding supplement lubricin has been shown to restore the lubricating ability of synovial fluid from patients with established osteoarthritis. [45] Lubricin has been shown to also play a role in anti-inflammation for osteoarthritis patients. Additionally, reduced lubricin levels have also been observed in the synovial fluid of patients with ACL injuries, and decreased lubricating ability has been found in patients with traumatic synovitis. [46] [47]
Lubricin, which is naturally present in human cornea-eyelid interface, has also been shown to play a key role in reducing friction between the cornea and conjunctiva of the eye. [48] Clinical trials of the use of recombinant lubricin eye drops for treatment of dry eye disease have thus far been relatively successful. [49]
Cartilage is a resilient and smooth type of connective tissue. It is a semi-transparent and non-porous type of tissue. It is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck and the bronchial tubes, and the intervertebral discs. In other taxa, such as chondrichthyans, but also in cyclostomes, it may constitute a much greater proportion of the skeleton. It is not as hard and rigid as bone, but it is much stiffer and much less flexible than muscle. The matrix of cartilage is made up of glycosaminoglycans, proteoglycans, collagen fibers and, sometimes, elastin. It usually grows quicker than bone.
In humans and other primates, the knee joins the thigh with the leg and consists of two joints: one between the femur and tibia, and one between the femur and patella. It is the largest joint in the human body. The knee is a modified hinge joint, which permits flexion and extension as well as slight internal and external rotation. The knee is vulnerable to injury and to the development of osteoarthritis.
The synovial membrane is a specialized connective tissue that lines the inner surface of capsules of synovial joints and tendon sheath. It makes direct contact with the fibrous membrane on the outside surface and with the synovial fluid lubricant on the inside surface. In contact with the synovial fluid at the tissue surface are many rounded macrophage-like synovial cells and also type B cells, which are also known as fibroblast-like synoviocytes (FLS). Type A cells maintain the synovial fluid by removing wear-and-tear debris. As for the FLS, they produce hyaluronan, as well as other extracellular components in the synovial fluid.
A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulating surfaces. This joint unites long bones and permits free bone movement and greater mobility. The synovial cavity/joint is filled with synovial fluid. The joint capsule is made up of an outer layer of fibrous membrane, which keeps the bones together structurally, and an inner layer, the synovial membrane, which seals in the synovial fluid.
Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of two polysaccharides, chitosan and chitin. Glucosamine is one of the most abundant monosaccharides. Produced commercially by the hydrolysis of shellfish exoskeletons or, less commonly, by fermentation of a grain such as corn or wheat, glucosamine has many names depending on country.
Synovial fluid, also called synovia,[help 1] is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its egg white–like consistency, the principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement. Synovial fluid is a small component of the transcellular fluid component of extracellular fluid.
Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and has a considerable amount of collagen. It contains no nerves or blood vessels, and its structure is relatively simple.
Hyaluronic acid, also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans as it is non-sulfated, forms in the plasma membrane instead of the Golgi apparatus, and can be very large: human synovial HA averages about 7 million Da per molecule, or about 20,000 disaccharide monomers, while other sources mention 3–4 million Da.
In cellular biology, mechanotransduction is any of various mechanisms by which cells convert mechanical stimulus into electrochemical activity. This form of sensory transduction is responsible for a number of senses and physiological processes in the body, including proprioception, touch, balance, and hearing. The basic mechanism of mechanotransduction involves converting mechanical signals into electrical or chemical signals.
Aggrecan (ACAN), also known as cartilage-specific proteoglycan core protein (CSPCP) or chondroitin sulfate proteoglycan 1, is a protein that in humans is encoded by the ACAN gene. This gene is a member of the lectican (chondroitin sulfate proteoglycan) family. The encoded protein is an integral part of the extracellular matrix in cartilagenous tissue and it withstands compression in cartilage.
Chitinase-3-like protein 1 (CHI3L1), also known as YKL-40, is a secreted glycoprotein that is approximately 40kDa in size that in humans is encoded by the CHI3L1 gene. The name YKL-40 is derived from the three N-terminal amino acids present on the secreted form and its molecular mass. YKL-40 is expressed and secreted by various cell-types including macrophages, chondrocytes, fibroblast-like synovial cells, vascular smooth muscle cells, and hepatic stellate cells. The biological function of YKL-40 is unclear. It is not known to have a specific receptor. Its pattern of expression is associated with pathogenic processes related to inflammation, extracellular tissue remodeling, fibrosis and solid carcinomas and asthma.
Fibromodulin is a protein that in humans is encoded by the FMOD gene.
Sodium hyaluronate is the sodium salt of hyaluronic acid, a glycosaminoglycan found in various connective tissue of humans.
O-linked glycosylation is the attachment of a sugar molecule to the oxygen atom of serine (Ser) or threonine (Thr) residues in a protein. O-glycosylation is a post-translational modification that occurs after the protein has been synthesised. In eukaryotes, it occurs in the endoplasmic reticulum, Golgi apparatus and occasionally in the cytoplasm; in prokaryotes, it occurs in the cytoplasm. Several different sugars can be added to the serine or threonine, and they affect the protein in different ways by changing protein stability and regulating protein activity. O-glycans, which are the sugars added to the serine or threonine, have numerous functions throughout the body, including trafficking of cells in the immune system, allowing recognition of foreign material, controlling cell metabolism and providing cartilage and tendon flexibility. Because of the many functions they have, changes in O-glycosylation are important in many diseases including cancer, diabetes and Alzheimer's. O-glycosylation occurs in all domains of life, including eukaryotes, archaea and a number of pathogenic bacteria including Burkholderia cenocepacia, Neisseria gonorrhoeae and Acinetobacter baumannii.
Camptodactyly-arthropathy-coxa vara-pericarditis syndrome is a rare autosomal recessive genetic medical condition due to a mutation in the gene proteoglycan 4 (PRG4) – a mucin-type glycoprotein that acts as a lubricant for the cartilage surfaces. This gene is also known as lubricin.
Fibroblast-like synoviocytes (FLS) represent a specialised cell type located inside joints in the synovium. These cells play a crucial role in the pathogenesis of chronic inflammatory diseases, such as rheumatoid arthritis.
Gene therapy for osteoarthritis is the application of gene therapy to treat osteoarthritis (OA). Unlike pharmacological treatments which are administered locally or systemically as a series of interventions, gene therapy aims to establish sustained therapeutic effect after a single, local injection.
The treatment of equine lameness is a complex subject. Lameness in horses has a variety of causes, and treatment must be tailored to the type and degree of injury, as well as the financial capabilities of the owner. Treatment may be applied locally, systemically, or intralesionally, and the strategy for treatment may change as healing progresses. The end goal is to reduce the pain and inflammation associated with injury, to encourage the injured tissue to heal with normal structure and function, and to ultimately return the horse to the highest level of performance possible following recovery.
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Artificial cartilage is a synthetic material made of hydrogels or polymers that aims to mimic the functional properties of natural cartilage in the human body. Tissue engineering principles are used in order to create a non-degradable and biocompatible material that can replace cartilage. While creating a useful synthetic cartilage material, certain challenges need to be overcome. First, cartilage is an avascular structure in the body and therefore does not repair itself. This creates issues in regeneration of the tissue. Synthetic cartilage also needs to be stably attached to its underlying surface i.e. the bone. Lastly, in the case of creating synthetic cartilage to be used in joint spaces, high mechanical strength under compression needs to be an intrinsic property of the material.
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