Articular Cartilage Proteoglycans As Boundary Lubricants: Structure and Frictional Interaction of Surface-Attached Hyaluronan and Hyaluronan–Aggrecan Complexes

Abstract

Mammalian synovial joints are extremely efficient lubrication systems reaching friction coefficient μ as low as 0.001 at high pressures (up to 100 atm) and shear rates (up to 10(6) to 10(7) Hz); however, despite much previous work, the exact mechanism responsible for this behavior is still unknown. In this work, we study the molecular mechanism of synovial joint lubrication by emulating the articular cartilage superficial zone structure. Macromolecules extracted and purified from bovine hip joints using well-known biochemical techniques and characterized with atomic force microscope (AFM) have been used to reconstruct a hyaluronan (HA)--aggrecan layer on the surface of molecularly smooth mica. Aggrecan forms, with the help of link protein, supramolecular complexes with the surface-attached HA similar to those at the cartilage/synovial fluid interface. Using a surface force balance (SFB), normal and shear interactions between a HA--aggrecan-coated mica surface and bare mica have been examined, focusing, in particular, on the frictional forces. In each stage, control studies have been performed to ensure careful monitoring of the macromolecular surface layers. We found the aggrecan--HA complex to be a much better boundary lubricant than the HA alone, an effect attributed largely to the fluid hydration sheath bound to the highly charged glycosaminoglycan (GAG) segments on the aggrecan core protein. A semiquantitative model of the osmotic pressure is used to describe the normal force profiles between the surfaces and interpret the boundary lubrication mechanism of such layers.

Cell Surface Access Is Modulated by Tethered Bottlebrush Proteoglycans

“…Furthermore, for the PCM to expand to such large sizes, particularly on the RCJ-P cells, which we have shown has no measurable cross-linking in the PCM (19), the HA strands must be at least 18 mm in length (~7.5 MDa, 1 disaccharide~1 nm~400 Da (37)) or it requires that the PCM is comprised in part by free HA strands entangled with surface bound strands (38). This is unlikely, however, as several studies of biomimetic HA-aggrecan systems suggest that such uncross-linked multilayers do not occur (39,40). Finally, we infer differences between the MSC and RCJ-P cell PCMs by comparing the difference in exogenous aggrecan concentration to reach maximal PCM.…”

TAK1 regulates SOX9 expression in chondrocytes and is essential for postnatal development of the growth plate and articular cartilages

“…Type IX collagen is covalently cross-linked to the surface of the type II collagen fibrils and interconnects the type II collagen matrix with other components of the extracellular matrix, including fibronectin, matrilin 3, and cartilage oligomeric protein (COMP) (Parsons et al, 2011). Aggregating proteoglycans are necessary for boundary lubrication of the articular surface and their negative charge is the basis of high osmotic pressure that provides resistance to compressive loads (Seror et al, 2011).…”

Hydration lubrication

“…Injection of HA as a "visco-supplement" for relief of painful joints is to this day a popular clinical treatment, though its benefits (over and above a placebo effect, say) have been questioned [120], and there is little evidence that it acts to reduce the friction in the conditions of articulating joints. In a series of papers, Seror et al [121,122] reconstructed both HA and, separately, the HA/agg complex onto the mica surfaces in the SFB, and measured the resulting boundary lubrication, as summarized in Fig. 18.…”