title: Both hyaluronan and collagen type II keep proteoglycan 4 (lubricin) at the cartilage surface in a condition that provides low friction during boundary lubrication.
abstract: Wear resistant and ultralow friction in synovial joints is the outcome of a sophisticated synergy between the major macromolecules of the synovial fluid, e.g., hyaluronan (HA) and proteoglycan 4 (PRG4), with collagen type II fibrils and other non-collagenous macromolecules of the cartilage superficial zone (SZ). This study aimed at better understanding the mechanism of PRG4 localization at the cartilage surface. We show direct interactions between surface bound HA and freely floating PRG4 using the quartz crystal microbalance with dissipation (QCM-D). Freely floating PRG4 was also shown to bind with surface bound collagen type II fibrils. Albumin, the most abundant protein of the synovial fluid, effectively blocked the adsorption of PRG4 with HA, through interaction with C and N terminals on PRG4, but not that of PRG4 with collagen type II fibrils. The above results indicate that collagen type II fibrils strongly contribute in keeping PRG4 in the SZ during cartilage articulation in situ. Furthermore, PRG4 molecules adsorbed very well on mimicked SZ of absorbed HA molecules with entangled collagen type II fibrils and albumin was not able to block this interaction. In this last condition PRG4 adsorption resulted in a coefficient of friction (COF) of the same order of magnitude as the COF of natural cartilage, measured with an atomic force microscope in lateral mode.
authors.affiliation: Department of Biomedical Engineering, University of Groningen, University Medical Centre Groningen , Antonius Deusinglaan 1, 9713AV Groningen, The Netherlands.
authors.forename: Sara Ehsani
authors.initials: SE
authors.lastname: Majd
authors.forename: Roel
authors.initials: R
authors.lastname: Kuijer
authors.forename: Alexander
authors.initials: A
authors.lastname: Köwitsch
authors.forename: Thomas
authors.initials: T
authors.lastname: Groth
authors.forename: Tannin A
authors.initials: TA
authors.lastname: Schmidt
authors.forename: Prashant K
authors.initials: PK
authors.lastname: Sharma
mesh_terms: D002356:Cartilage / Q000737:chemistry; D024043:Collagen Type II / Q000737:chemistry*; D017276:Friction; D006023:Glycoproteins / Q000737:chemistry*; D006820:Hyaluronic Acid / Q000737:chemistry*; D008153:Lubrication; D013499:Surface Properties; D000099158:Proteoglycan 4
chemical_list: D024043:Collagen Type II; D006023:Glycoproteins; D006820:Hyaluronic Acid; D000099158:Proteoglycan 4