Fibronectin mediates enhanced wear protection of lubricin during shear

Abstract

Fibronectin (FN) is a glycoprotein found in the superficial zone of cartilage; however, its role in the lubrication and the wear protection of articular joints is unknown. In this work, we have investigated the molecular interactions between FN and various components of the synovial fluid such as lubricin (LUB), hyaluronan (HA), and serum albumin (SA), which are all believed to contribute to joint lubrication. Using a Surface Forces Apparatus, we have measured the normal (adhesion/repulsion) and lateral (friction) forces across layers of individual synovial fluid components physisorbed onto FN-coated mica substrates. Our chief findings are (i) FN strongly tethers LUB and HA to mica, as indicated by high and reversible long-range repulsive normal interactions between surfaces, and (ii) FN and LUB synergistically enhance wear protection of surfaces during shear, as suggested by the structural robustness of FN+LUB layers under pressures up to about 4 MPa. These findings provide new insights into the role of FN in the lubricating properties of synovial fluid components sheared between ideal substrates and represent a significant step forward in our understanding of cartilage damage involved in diseases such as osteoarthritis.

Synergistic Interactions of a Synthetic Lubricin-Mimetic with Fibronectin for Enhanced Wear Protection

“…Although mimLUB is a polyelectrolyte, the numerous counterions in PBS are expected to screen most of the electrostatic interactions between chains so that mimLUB can be treated as a neutral bottlebrush polymer. We identified both long-ranged (LR) and short-ranged (SR) brush regimes in our mimLUB + FN data, similar to what has been observed for LUB + FN (Andresen Eguiluz et al, 2015). Importantly, by using the AdG model to describe FN (alone), we do not intend to imply that FN layers also possess a well-defined brush structure.…”

“…To assess mimLUB tribological properties at the molecular scale and determine the role of FN in mediating these properties, we sheared mimLUB between either bare or FN-coated mica surfaces. Our data indicate that the presence of FN prevents mimLUB from being squeezed out of the shearing junction, which significantly extends its wearless friction regime (up to physiologically relevant pressures), while maintaining friction coefficients similar to those measured across native LUB under identical experimental conditions (Andresen Eguiluz et al, 2015). Collectively, these findings suggest that, when combined with FN, our proposed lubricin-mimetic (mimLUB) provides enhanced wear protection.…”

“…The disks were mounted in a cross-cylindrical configuration and the absolute separation distance between them, D , was measured in real time by multiple beam interferometry (MBI). Additionally, MBI was used to monitor the onset of wear of the shearing surfaces: both shape and intensity of interference fringes were used as indicators of shape (and size) of the contacting junction and presence of shear-induced wear debris (Gourdon and Israelachvili, 2003; Banquy et al, 2014b; Andresen Eguiluz et al, 2015). Before functionalizing the mica surfaces with either mimLUB or FN + mimLUB, mica–mica contact in air was measured to determine the reference distance, D  = 0.…”

“…This remarkable lubrication is believed to arise from the bottlebrush structure of LUB combined with its ability to self-associate into dimers or multimers that anchor robustly to the cartilage surface (Swann et al, 1985). LUB has been reported to bind to cartilage through its carboxyl-terminus (Jones et al, 2007), building a brush-like layer of dimers forming an arc-like (loop) architecture (Zappone et al, 2007, 2008; Andresen Eguiluz et al, 2015). Importantly, LUB is able to bind to various extracellular matrix components, including collagen (COL) (Chang et al, 2013), hyaluronan (HA) (Das et al, 2013), and fibronectin (FN) (Elsaid et al, 2007), a prominent glycoprotein which, in synovial joints, is only found in the superficial zone of cartilage (Balazs, 2009), i.e., at the interface between cartilage and synovial fluid.…”

“…While the interactions of LUB with COL (Chang et al, 2013) and HA (Chang et al, 2008; Greene et al, 2011; Das et al, 2013) have been widely investigated, the role of FN in enhancing LUB-mediated low friction and wear protection at the superficial zone of cartilage has only recently been proposed (Andresen Eguiluz et al, 2015). In this study, we report the characterization of a LUB-mimetic bottlebrush polymer, named mimLUB, through the combination of atomic force microscopy (AFM) and SFA.…”