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).[8][9] Due to the discovery that the 32-kDa amino terminal fragment of lubricin could stimulate in-vitromegakaryocyte growth, the gene responsible for the expression of lubricin was initially called "megakaryocyte-stimulating factor" (MSF).[10] 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.[11] The expression of lubricin has also been detected and the protein localized in tendon,[12]meniscus,[13]lung, liver, heart, bone,[14]ligament, muscle, and skin.[15] It is present in human plasma, where it binds to neutrophils via L-selectin.[16]
The adhesion of Lubricin's N- (blue) and C- (red) termini to two opposing cartilage surfaces undergoing shear stress (𝜏) and normal forces (𝐹_𝑁). Steric repulsion between mucin domains and hydration forces of the trapped solvent layer are thought to give lubricin its characteristic lubrication ability. Two glycoprotein monomers are linked by a disulfide bond in yellow to form a dimer.
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.[17][18]
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.[19] Understanding lubricin is key to understanding joint mechanics and friction-based diseases.[20]
Structure
The protein encoded by this gene is a approximately 345 kDa[21] 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[7] and more than 168 sites for O-linked glycosylation.[22]
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.[16][22]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.[23] 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.[24][25] Lubricin's unglycosylated regions can interact with cartilage proteins.[26][27] 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.[28] However, unlike vitronectin, lubricin carries a central mucin-like domain with a large number of repeating KEPAPTT motifs.[29]
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,[30] whereas the C-terminus is associated with the hemopexin-like domain.[31] 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.[18][23][32][22]
The basic "bottle brush" structure of lubricin, including its mucin, hemopexin-like and somatomedin B (SMB)-like domains. Figure created with BioRender.com[33]
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.[34][35]
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.[30] 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.[36][37][26] Lubricin's high degree of hydration is also thought to be involved in repulsion forces generated by lubricin between opposing cartilage surfaces.[38]
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.[18] 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.[39] 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.[40] 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.[41]
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.[7]
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.[43]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.[44]
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.[45] Researchers are currently exploring potential applications of lubricin for treating these and other related diseases.[46] Thus far, adding supplement lubricin has been shown to restore the lubricating ability of synovial fluid from patients with established osteoarthritis.[47] 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.[48][49]
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.[50] Clinical trials of the use of recombinant lubricin eye drops for treatment of dry eye disease have thus far been relatively successful.[51]
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^ abMarcelino J, Carpten JD, Suwairi WM, Gutierrez OM, Schwartz S, Robbins C, et al. (November 1999). "CACP, encoding a secreted proteoglycan, is mutated in camptodactyly-arthropathy-coxa vara-pericarditis syndrome". Nature Genetics. 23 (3): 319–22. doi:10.1038/15496. PMID10545950. S2CID32556762.
^Flannery CR, Hughes CE, Schumacher BL, Tudor D, Aydelotte MB, Kuettner KE, Caterson B (January 1999). "Articular cartilage superficial zone protein (SZP) is homologous to megakaryocyte stimulating factor precursor and Is a multifunctional proteoglycan with potential growth-promoting, cytoprotective, and lubricating properties in cartilage metabolism". Biochemical and Biophysical Research Communications. 254 (3): 535–41. doi:10.1006/bbrc.1998.0104. PMID9920774.
^Schumacher BL, Block JA, Schmid TM, Aydelotte MB, Kuettner KE (May 1994). "A novel proteoglycan synthesized and secreted by chondrocytes of the superficial zone of articular cartilage". Archives of Biochemistry and Biophysics. 311 (1): 144–52. doi:10.1006/abbi.1994.1219. PMID8185311.
^Jay GD, Britt DE, Cha CJ (March 2000). "Lubricin is a product of megakaryocyte stimulating factor gene expression by human synovial fibroblasts". The Journal of Rheumatology. 27 (3): 594–600. PMID10743795.
^Preissner KT (1993). Biology of vitronectins and their receptors : proceedings of the First International Vitronectin Workshop, Rauischholzhausen Castle, Marburg, Germany, 25-28 August, 1993. Elsevier. pp. 45–52. ISBN0-444-81680-1. OCLC246493326.
^Lord MS, Estrella RP, Chuang CY, Youssef P, Karlsson NG, Flannery CR, Whitelock JM (2012). "Not all lubricin isoforms are substituted with a glycosaminoglycan chain". Connective Tissue Research. 53 (2): 132–41. doi:10.3109/03008207.2011.614364. PMID21966936. S2CID7344867.
^Rees SG, Davies JR, Tudor D, Flannery CR, Hughes CE, Dent CM, Caterson B (November 2002). "Immunolocalisation and expression of proteoglycan 4 (cartilage superficial zone proteoglycan) in tendon". Matrix Biology. 21 (7): 593–602. doi:10.1016/S0945-053X(02)00056-2. PMID12475643.
^Schumacher BL, Schmidt TA, Voegtline MS, Chen AC, Sah RL (May 2005). "Proteoglycan 4 (PRG4) synthesis and immunolocalization in bovine meniscus". Journal of Orthopaedic Research. 23 (3): 562–8. doi:10.1016/j.orthres.2004.11.011. PMID15885476. S2CID1011398..
^Ikegawa S, Sano M, Koshizuka Y, Nakamura Y (2000). "Isolation, characterization and mapping of the mouse and human PRG4 (proteoglycan 4) genes". Cytogenetics and Cell Genetics. 90 (3–4): 291–7. doi:10.1159/000056791. PMID11124536. S2CID39376563.
^Sun Y, Berger EJ, Zhao C, An KN, Amadio PC, Jay G (2006). "Mapping lubricin in canine musculoskeletal tissues". Connective Tissue Research. 47 (4): 215–21. doi:10.1080/03008200600846754. PMID16987753. S2CID46582604..
^Jay GD, Lane BP, Sokoloff L (January 1992). "Characterization of a bovine synovial fluid lubricating factor. III. The interaction with hyaluronic acid". Connective Tissue Research. 28 (4): 245–55. doi:10.3109/03008209209016818. PMID1304440.
^Su JL, Schumacher BL, Lindley KM, Soloveychik V, Burkhart W, Triantafillou JA, et al. (June 2001). "Detection of superficial zone protein in human and animal body fluids by cross-species monoclonal antibodies specific to superficial zone protein". Hybridoma. 20 (3): 149–57. doi:10.1089/027245701750293475. PMID11461663.
^Jay GD (October 2004). "Lubricin and surfacing of articular joints". Current Opinion in Orthopaedics. 15 (5): 355–359. doi:10.1097/01.bco.0000136127.00043.a8.
^Jay GD, Harris DA, Cha CJ (October 2001). "Boundary lubrication by lubricin is mediated by O-linked beta(1-3)Gal-GalNAc oligosaccharides". Glycoconjugate Journal. 18 (10): 807–15. doi:10.1023/a:1021159619373. PMID12441670. S2CID23192748.
^ abSchmidt TA, Plaas AH, Sandy JD (May 2009). "Disulfide-bonded multimers of proteoglycan 4 PRG4 are present in normal synovial fluids". Biochimica et Biophysica Acta (BBA) - General Subjects. 1790 (5): 375–84. doi:10.1016/j.bbagen.2009.03.016. PMID19332105.
^Zappone B, Greene GW, Oroudjev E, Jay GD, Israelachvili JN (February 2008). "Molecular aspects of boundary lubrication by human lubricin: effect of disulfide bonds and enzymatic digestion". Langmuir. 24 (4): 1495–508. doi:10.1021/la702383n. PMID18067335.
^Merberg DM et al. (1993) Comparison of vitronectin and megakaryocyte stimulating factor. In Biology of Vitronectins and their Receptors. (Preissner et al., eds) pp45-52 (Elsevier Science, Amsterdam).
^Taguchi M, Sun YL, Zhao C, Zobitz ME, Cha CJ, Jay GD, et al. (January 2008). "Lubricin surface modification improves extrasynovial tendon gliding in a canine model in vitro". The Journal of Bone and Joint Surgery. American Volume. 90 (1): 129–35. doi:10.2106/JBJS.G.00045. PMID18171967.
^Zhang D, Kearney CJ, Cheriyan T, Schmid TM, Spector M (November 2011). "Extracorporeal shockwave-induced expression of lubricin in tendons and septa". Cell and Tissue Research. 346 (2): 255–62. doi:10.1007/s00441-011-1258-7. PMID22009294. S2CID12089961.
^Lambiase A, Sullivan BD, Schmidt TA, Sullivan DA, Jay GD, Truitt ER, et al. (January 2017). "A Two-Week, Randomized, Double-masked Study to Evaluate Safety and Efficacy of Lubricin (150 μg/mL) Eye Drops Versus Sodium Hyaluronate (HA) 0.18% Eye Drops (Vismed®) in Patients with Moderate Dry Eye Disease". The Ocular Surface. 15 (1): 77–87. doi:10.1016/j.jtos.2016.08.004. PMID27614318.
Morawietz L, Gehrke T, Frommelt L, Gratze P, Bosio A, Möller J, et al. (July 2003). "Differential gene expression in the periprosthetic membrane: lubricin as a new possible pathogenetic factor in prosthesis loosening". Virchows Archiv. 443 (1): 57–66. doi:10.1007/s00428-003-0818-y. PMID12783322. S2CID22232515.
Liu YJ, Lu SH, Xu B, Yang RC, Ren Q, Liu B, et al. (June 2004). "Hemangiopoietin, a novel human growth factor for the primitive cells of both hematopoietic and endothelial cell lineages". Blood. 103 (12): 4449–56. doi:10.1182/blood-2003-06-1825. PMID14976050. S2CID2103324.
