Dynamics of Aggregation on Sheared Albumin-Fibronectin Thin Film

Abstract

Articular joints are one of the most robust bearing systems found in the natural world. Healthy joints can withstand over 100 million shearing and compressive cycles without wear. This phenomenal lubrication is due to both the cartilage that forms the bearing surface and the synovial fluid (SF) that lubricates the joint. Common approaches that seek to elucidate such remarkable lubrication usually focus on the roles of high molecular weight SF components such as lubricin and hyaluronic acid, however, recent work in our group reports that the smaller globular protein serum albumin (SA) is crucial in mimicking the lubrication behaviour of SF, in particular the formation of shear-induced aggregates. This finding is relevant for artificial joint lubrication because protein aggregates have been recognised to improve wear protection between metal surfaces in-vitro. In this thesis, we investigated the structural, mechanical, and lubrication properties of shear-induced SA aggregates. More specifically, we used the Surface Forces Apparatus and Confocal Microscopy to investigate in detail the response of SA thin films when confined and sheared between model surfaces coated with fibronectin (FN), a glycoprotein present in the superficial zone of cartilage. Our data indicate that, under shear, SA films exhibit instantaneous thickening (dilation) and systematic formation of numerous aggregates accompanied by low friction and efficient surface protection against damage. Additionally, our detailed investigation of shearing conditions reveals that (i) aggregates are comprised of both SA and FN, most likely denatured/unfolded, (ii) aggregates material properties (stiffness, size, shape) are FN concentration-dependent, and (iii) aggregates formation is irreversible, which makes them long-lived (though mobile) and acting as robust ball bearings at the shearing interface. Collectively, our results provide experimental evidence of the complementary roles of both the lubricant and the bearing surface properties in lubrication. Although our findings are based on experiments involving rigid, nonporous surfaces which can hardly be generalized to compliant and porous cartilage surfaces, they apply to other rigid tribosystems such as artificial joints and will certainly advance our understanding of joint implants’ lubrication in SF mediated by protein aggregation, with implications for the future design of artificial joints and therapeutic interventions