Normal and Shear Interactions between Hyaluronan–Aggrecan Complexes Mimicking Possible Boundary Lubricants in Articular Cartilage in Synovial Joints

Abstract

Using a surface force balance, normal and shear interactions have been measured between two atomically smooth surfaces coated with hyaluronan (HA), and with HA/aggrecan (Agg) complexes stabilized by cartilage link protein (LP). Such HA/Agg/LP complexes are the most abundant mobile macromolecular species permeating articular cartilage in synovial joints and have been conjectured to be present as boundary lubricants at its surface. The aim of the present study is to gain insight into the extremely efficient lubrication when two cartilage surfaces slide past each other in healthy joints, and in particular to elucidate the possible role in this of the HA/Agg/LP complexes. Within the range of our parameters, our results reveal that the HA/Agg/LP macromolecular surface complexes are much better boundary lubricants than HA alone, likely because of the higher level of hydration, due to the higher charge density, of the HA/Agg/LP layers with respect to the HA alone. However, the friction coefficients (μ) associated with the mutual interactions and sliding of opposing HA/Agg/LP layers (μ ≈ 0.01 up to pressure P of ca. 12 atm, increasing sharply at higher P) suggest that such complexes by themselves cannot account for the remarkable boundary lubrication observed in mammalian joints (up to P > 50 atm).

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.…”

“…[44] and section 5), where the hydrated species are -SO 3 -groups, as well as the relatively weak boundary lubrication effect by the HA and HA/Agg complexes, where the hydrated species are -COO -and -SO 3 -(Ref. [122] and section 8). It would also readily explain why PEO brushes in water [75,76] provide only very moderate lubrication.…”

Liquid Crystals of Self-Assembled DNA Bottlebrushes

“…[11][12][13] In this case the role of the bottlebrush architecture is to provide for a macromolecular structure with an extreme density of charged groups that is highly swollen and holds on to water even under high load.…”

Surface-Active Lipid Linings under Shear Load—A Combined in-Situ Neutron Reflectivity and ATR-FTIR Study

“…Zooming in on the cartilage level the respective boundary lubricant is claimed to consist of the surface-active phospholipids and/or of macromolecular components of the synovial fluid. 2,11,[18][19][20][21][22][23][24][25][26][27][28] For a very simplified model system comprising solid-supported oligolamellar stacks of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer membranes against water and solutions of HA in water we observed a tremendous swelling (by a factor of ~4) and a stabilization of the lipid linings upon incubation with HA at 39 °C, i.e. well above the main phase transition temperature, , of the lipids.…”