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Lubricin or PRG4 is a mucin like proteoglycan secreted by both chondrocytes and synovial cells. It is encoded by the PRG4 gene. It acts as a lubricant and prevents adhesion of cells to the joint surface. Recent studies have shown that there is a correlation between coefficient of friciton and chondrocyte death and lubricin prevents this by decreasing friction.

History

(Jay et al. 2001) Lubricin was first identified by Swann et al.(1981a, 1985) is the same molecule which accumulates at the articular surface.It was identified early on as a macromolecular that contributed to the lubricating quality of synovial fluid. Swann et al.(1972) with bothe protein and sugar components. Later (Schumacher et al. 1994), identified the same proteoglycan selectively secreted by chondrocytes in the superficial zone of cartilage and has an apparent molecular weight of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and has an apparent molecular weight of 345kDa. they named it superficial zone protein. Later (Flannery et al.1999 and Jay et al. 2000) identified the same protein as lubricin and found that it is a product of megakaryocyte stimulating factor precursor gene expression by human synovial fibroblasts. Its expression is proposed to be important for preventing cell adhesion to the joint surface in addition to maintaining lubrication properties at the cartilage-synovium interface. (Schumacher et la.1999) found that synoviocytes also produce lubricin.

Structure

(Flannery et al. 1999) the core protein is composed of large(76-78 repeats) and small(6-8 repeats) mucin like O-linked oligosaccharide- rich repeat domains that are homologous to somatomedin B and haemopexin domains of vitronectin. (Schumacher et la.1999) A low level substitution with chondroitin sulfate or keratan sulfate GlycosaminoGlycans.

Clinical relevance

(Rhee et al. 2005) Loss of function mutations in PRG4gene/put link cause the human autosomal recessive disorder CACP.

References
 * 1) Sw≥ann, D.A., Radin, E.L., 1972. The molecular basis of articular lubrication. I. Purification and properties of a lubricating fraction from bovine synovial fluid. J. Biol. Chem. 247, 8069–8073.
 * 2) Schumacher, B.L., Block, J.A., Schmid, T.M., Aydelotte, M.B., Kuettner, K.E., 1994. A novel proteoglycan synthesized and secreted by chondrocytes of the superficial zone of articular cartilage. Arch. Biochem. Biophys. 311, 144–152.
 * 3) Schumacher, B.L., Hughes, C.E., Kuettner, K.E., Caterson, B., Aydelotte, M.B., 1999. Immunodetection and partial cDNA sequence of the proteoglycan, superficial zone protein synthesized by cells lining synovial joints. J. Orthop. Res. 17, 110–120.
 * 4) Flannery, C.R., Hughes, C.E., Schumacher, B.L., et al., 1999. Articular cartilage superficial zone proteoglycan (SZP) is homologous to megakaryocyte stimulating factor precursor and is a multifunctional proteoglycan with potential growth-promoting, cytoprotective and lubricating properties in cartilage metabolism. Biochem. Biophys. Res. Commun. 254, 535–541.
 * 5) Jay, G.D., Britt, D.E., Cha, C.-J., 2000. Lubricin is a product of megakaryocyte stimulating factor gene expression by human synovial fibroblasts. J. Rheumatol. 27, 594–600.
 * 6) Jay, G.D., Tantravahi, U., Britt, D.E., Barrach, H.J., Cha, C.-J., 2001. Homology of lubricin and superficial zone protein (SZP): products of megakaryocyte stimulating factor (MSF) gene expression by human synovial fibroblasts and articular chondrocytes localized to chromosome 1q25. J. Orthop. Res. 19, 677–687.
 * 7)  David K. Rhee, Jose Marcelino, MacArthur Baker, Yaoqin Gong, Patrick Smits, Véronique Lefebvre, Gregory D. Jay, Matthew Stewart, Hongwei Wang, Matthew L. Warman, and John D. Carpten. The secreted glycoprotein lubricin protects  cartilage surfaces and inhibits synovial  cell overgrowth. The Journal of Clinical Investigation.115, 622-631.
 * 8)  Waller KA, Zhang LX, Fleming BC, Jay GD (2012) Preventing friction-induced chondrocyte apoptosis: Comparison of human synovial fluid and hylan G-F 20. J Rheumatol 39(7):1473–1480.