Boundary Lubrication Of Cartilage Research Articles

Dependence of nanoscale friction and adhesion properties of articular cartilage on contact load

Boundary lubrication of articular cartilage by conformal, molecularly thin films reduces friction and adhesion between asperities at the cartilage–cartilage contact interface when the contact conditions are not conducive to fluid film lubrication. In this study, the nanoscale friction and adhesion properties of articular cartilage from typical load-bearing and non-load-bearing joint regions were studied in the boundary lubrication regime under a range of physiological contact pressures using an atomic force microscope (AFM). Adhesion of load-bearing cartilage was found to be much lower than that of non-load-bearing cartilage. In addition, load-bearing cartilage demonstrated steady and low friction coefficient through the entire load range examined, whereas non-load-bearing cartilage showed higher friction coefficient that decreased nonlinearly with increasing normal load. AFM imaging and roughness calculations indicated that the above trends in the nanotribological properties of cartilage are not due to topographical (roughness) differences. However, immunohistochemistry revealed consistently higher surface concentration of boundary lubricant at load-bearing joint regions. The results of this study suggest that under contact conditions leading to joint starvation from fluid lubrication, the higher content of boundary lubricant at load-bearing cartilage sites preserves synovial joint function by minimizing adhesion and wear at asperity microcontacts, which are precursors for tissue degeneration.

Atomic force microscope investigation of the boundary-lubricant layer in articular cartilage

To determine the roles of superficial zone protein (SZP), hyaluronan (HA), and surface-active phospholipids (SAPL) in boundary lubrication of articular cartilage through systematic enzyme digestion using trypsin, hyaluronidase, and phospolipase-C (PLC) surface treatments. The friction coefficient of articular cartilage surfaces was measured with an atomic force microscope (AFM) before and after enzyme digestion. Surface roughness, adhesion, and stiffness of the articular surface were also measured to determine the mechanism of friction in the boundary lubrication regime. Histology and transmission electron microscopy were used to visualize the surface changes of treatment groups that showed significant friction changes after enzyme digestion. A significant increase in the friction coefficient of both load-bearing and non load-bearing regions of the joint was observed after proteolysis by trypsin. Treatment with trypsin, hyaluronidase, or PLC did not affect the surface roughness. However, trypsin treatment decreased the adhesion significantly. Results indicate that the protein component at the articular cartilage surface is the main boundary lubricant, with SZP being a primary candidate. The prevailing nanoscale deformation processes are likely plastic and/or viscoelastic in nature, suggesting that plowing is the dominant friction mechanism. The findings of this study indicate that SZP plays an intrinsic and critical role in boundary lubrication at the articular surface of cartilage, whereas the effects of HA and SAPL on the tribological behavior are marginal.

Differential Regulation of Proteoglycan-4 Expression by IL-1.ALPHA. and TGF-.BETA.1 in Rat Condylar Chondrocytes

Proteoglycan 4 (PRG4) is a multifaceted glycoprotein that mediates boundary lubrication of articular cartilage and its dysregulation is associated with impaired lubrication and cartilage destruction in multiple synovial joints. However, the spatiotemporal expression of PRG4 and the associated regulatory networks remain largely unknown in the mandibular condylar cartilage that is responsible for homeostasis and functions of the temporomandibular joint. We here investigated the possible regulatory effects of the interleukin-1α (IL-1α) or/and transforming growth factor-β1 (TGF-β1) on the expression of PRG4 in primary chondrocytes that were isolated from the superficial layer of the condylar cartilage of the 20-day-old male Sprague-Dawley rats. Both IL-1α and TGF-β1 have been implicated in joint destruction and repair. Treatment of primary chondrocytes for 24 h with recombinant human (rh) IL-1α (10 ng/ml) resulted in pronounced reduction in the expression levels of PRG4 mRNA and protein, whereas stimulation with rhTGF-β1 (10 ng/ml) significantly increased the expression levels, as measured by RT-PCR and ELISA, respectively. Moreover, rhTGF-β1 was capable to antagonize the inhibitory effects on the PRG4 expression caused by rhIL-1α and robustly restored its abundance in the cultured condylar chondrocytes. Taken together, our data indicate that PRG4 is synthesized and secreted by condylar cartilage chondrocytes and its expression is differentially regulated by IL-1α and TGF-β1. The rhIL-1α-mediated PRG4 repression is reversible and potently antagonized by rhTGF-β1 in condylar chondrocytes. The observed up-regulation of PRG4 upon rhTGF-β1 treatment further supports the therapeutic application of rhTGF-β1 in the treatment of temporomandibular joint osteoarthritis.

Prevention of cartilage degeneration in a rat model of osteoarthritis by intraarticular treatment with recombinant lubricin

Lubricin, also referred to as superficial zone protein and PRG4, is a synovial glycoprotein that supplies a friction-resistant, antiadhesive coating to the surfaces of articular cartilage, thereby protecting against arthritis-associated tissue wear and degradation. This study was undertaken to generate and characterize a novel recombinant lubricin protein construct, LUB:1, and to evaluate its therapeutic efficacy following intraarticular delivery in a rat model of osteoarthritis (OA). Binding and localization of LUB:1 to cartilage surfaces was assessed by immunohistochemistry. The cartilage-lubricating properties of LUB:1 were determined using a custom friction testing apparatus. A cell-binding assay was performed to quantify the ability of LUB:1 to prevent cell adhesion. Efficacy studies were conducted in a rat meniscal tear model of OA. One week after the surgical induction of OA, LUB:1 or phosphate buffered saline vehicle was administered by intraarticular injection for 4 weeks, with dosing intervals of either once per week or 3 times per week. OA pathology scores were determined by histologic analysis. LUB:1 was shown to bind effectively to cartilage surfaces, and facilitated both cartilage boundary lubrication and inhibition of synovial cell adhesion. Treatment of rat knee joints with LUB:1 resulted in significant disease-modifying, chondroprotective effects during the progression of OA, by markedly reducing cartilage degeneration and structural damage. Our findings demonstrate the potential use of recombinant lubricin molecules in novel biotherapeutic approaches to the treatment of OA and associated cartilage abnormalities.

Induction of chondrogenesis and expression of superficial zone protein (SZP)/lubricin by mesenchymal progenitors in the infrapatellar fat pad of the knee joint treated with TGF-β1 and BMP-7

Superficial zone protein (SZP) is a key mediator of boundary lubrication of articular cartilage in joints. In this investigation, we made the unexpected discovery that SZP was expressed in infrapatellar fat pad (IFP) from bovine knee. Quantitative analysis of secreted proteins in the medium of the IFP stromal cells demonstrated a significant stimulation by TGF-β1 and BMP-7. Real-time PCR analysis revealed the SZP expression was up-regulated by TGF-β1 and BMP-7. Chondrogenically differentiated IFP progenitor cells were stimulated by TGF-β1 and BMP-7 to synthesize and secrete SZP. SZP mRNA was significantly up-regulated by chondrogenic induction for 21 days. These findings indicate that the stimulation of SZP expression by TGF-β and BMP-7 may lead to functional improvement of damaged intraarticular tissues and that IFP progenitor cells may be a potential useful source for inducing superficial zone of articular cartilage by tissue engineering for regeneration of damaged articular cartilage due to osteoarthritis

Boundary lubrication of articular cartilage: Role of synovial fluid constituents

To determine whether the synovial fluid (SF) constituents hyaluronan (HA), proteoglycan 4 (PRG4), and surface-active phospholipids (SAPL) contribute to boundary lubrication, either independently or additively, at an articular cartilage-cartilage interface. Cartilage boundary lubrication tests were performed with fresh bovine osteochondral samples. Tests were performed using graded concentrations of SF, HA, and PRG4 alone, a physiologic concentration of SAPL, and various combinations of HA, PRG4, and SAPL at physiologic concentrations. Static (mu(static, Neq)) and kinetic (<mu(kinetic, Neq)>) friction coefficients were calculated. Normal SF functioned as an effective boundary lubricant both at a concentration of 100% (<mu(kinetic, Neq)> = 0.025) and at a 3-fold dilution (<mu(kinetic, Neq)> = 0.029). Both HA and PRG4 contributed independently to a low mu in a dose-dependent manner. Values of <mu(kinetic, Neq)> decreased from approximately 0.24 in phosphate buffered saline to 0.12 in 3,300 mug/ml HA and 0.11 in 450 mug/ml PRG4. HA and PRG4 in combination lowered mu further at the high concentrations, attaining a <mu(kinetic, Neq)> value of 0.066. SAPL at 200 mug/ml did not significantly lower mu, either independently or in combination with HA and PRG4. The results described here indicate that SF constituents contribute, individually and in combination, both at physiologic and pathophysiologic concentrations, to the boundary lubrication of apposing articular cartilage surfaces. These results provide insight into the nature of the boundary lubrication of articular cartilage by SF and its constituents. They therefore provide insight regarding both the homeostatic maintenance of healthy joints and pathogenic processes in arthritic disease.

Effect of synovial fluid on boundary lubrication of articular cartilage

The lubrication of articulating cartilage surfaces in joints occurs through several distinct modes. In the boundary mode of lubrication, load is supported by surface-to-surface contact, a feature that makes this mode particularly important for maintenance of the normally pristine articular surface. A boundary mode of lubrication is indicated by a kinetic friction coefficient being invariant with factors that influence formation of a fluid film, including sliding velocity and axial load. The objectives of this study were to (1) implement and extend an in vitro articular cartilage-on-cartilage lubrication test to elucidate the dependence of the friction properties on sliding velocity, axial load, and time, and establish conditions where a boundary mode of lubrication is dominant, and (2) determine the effects of synovial fluid (SF) on boundary lubrication using this test. Fresh bovine osteochondral samples were analyzed in an annulus-on-disk rotational configuration, maintaining apposed articular surfaces in contact, to determine static (mu(static) and mu(static),(N(eq)) and kinetic ([mu(kinetic)] and [mu(kinetic),(N(eq))]) friction coefficients, each normalized to the instantaneous and equilibrium (N(eq)) normal loads, respectively. With increasing pre-sliding durations, mu(static) and mu(static),(N(eq)) were similar, and increased up to 0.43 +/- 0.03 in phosphate buffered saline (PBS) and 0.19 +/- 0.01 in SF, whereas [mu(kinetic)] and [mu(kinetic),(N(eq))] were steady. Over a range of sliding velocities of 0.1-1 mm/s and compression levels of 18% and 24%, [mu(kinetic)] was 0.072 +/- 0.010 in PBS and 0.014 +/- 0.003 in SF, and [mu(kinetic),(N(eq))] was 0.093 +/- 0.005 in PBS and 0.018 +/- 0.002 in SF. A boundary mode of lubrication was achieved in a cartilage-on-cartilage test configuration. SF functioned as an effective friction-lowering boundary lubricant for native articular cartilage surfaces.

Expression of superficial zone protein in mandibular condyle cartilage

Superficial zone protein (SZP) has been shown to function in the boundary lubrication of articular cartilages of the extremities. However, the expression of SZP has not been clarified in mandibular cartilage which is a tissue that includes a thick fibrous layer on the surface. This study was conducted to clarify the distribution of SZP on the mandibular condyle and the regulatory effects of humoral factors on the expression in both explants and fibroblasts derived from mandibular condyle. The distribution of SZP was determined in bovine mandibular condyle cartilage, and the effects of interleukin-1beta (IL-1beta) and transforming growth factor-beta (TGF-beta) on SZP expression were examined in condyle explants and fibroblasts derived from the fibrous zone of condyle cartilage. SZP was highly distributed in the superficial zone of intact condyle cartilage. The SZP expression was up-regulated by TGF-beta in both explants and cultured fibroblasts, whereas the expression was slightly down-regulated by IL-1beta. A significant increase in accumulation of SZP protein was also observed in the culture medium of the fibroblasts treated with TGF-beta. These results suggest that SZP plays an important role in boundary lubrication of mandible condylar cartilage, is synthesized locally within the condyle itself, and exhibits differential regulation by cell mediators relevant to mandibular condyle repairing and pathologies.

Homology of lubricin and superficial zone protein (SZP): products of megakaryocyte stimulating factor (MSF) gene expression by human synovial fibroblasts and articular chondrocytes localized to chromosome 1q25

We have previously identified megakaryocyte stimulating factor (MSF) gene expression by synovial fibroblasts as the origin of lubricin in the synovial cavity. Lubricin is a mucinous glycoprotein responsible for the boundary lubrication of articular cartilage. MSF has a significant homology to vitronectin and is composed of 12 exons. RNA was purified from human synovial fibroblasts and articular chondrocytes grown in vitro from tissue explants obtained from subjects without degenerative joint disease. RT-PCR was used with multiple complimentary primer pairs spanning the central mucin expressing exon 6 of the MSF gene and individual exons on both the N- and C-terminal sides of exon 6. Exons 2, 4 and 5 appear to be variably expressed by synovial fibroblasts and articular chondrocytes. Lubricating mucin, in the form of MSF, is expressed by both chondrocytes and synovial fibroblasts in vitro. Both lubricin and superficial zone protein (SZP), a related proteoglycan, share a similar primary structure but could differ in post-translational modifications with O-linked oligosaccharides which are predominant in lubricin and with limited amounts chondroitin and keratan sulfate found in SZP. Since most of the MSF exons are involved in the expression of lubricating mucin, a strong homology to vitronectin persists. It is therefore appropriate to consider that both SZP and lubricin occupy a new class of biomolecules termed tribonectins. Screening of a human genome bacterial artificial chromsome (BAC) library with a cDNA primer pair complimentary for exon 6 identified two clones. Both clones were complimentary for chromosome 1q25 by in situ hybridization. This same locus was previously implicated in camptodactyl-arthropathy-pericarditis syndrome (CAP) by genetic mapping. It is hypothesized that CAP, a large joint arthropathy, may be associated with ineffective boundary lubrication provided by synovial fluid.

A Proposed Model of Boundary Lubrication by Synovial Fluid: Structuring of Boundary Water

On the basis of data obtained from in-vitro friction tests using both cartilage and widely differing artificial surfaces, a general model for boundary lubrication of joint cartilage by synovial fluid is presented. It postulates that one portion of the synovial lubricating glycoprotein (LGP) is adsorbed to the surface. Reduction in surface shear is accomplished by formation of hydration shells about the polar portions of the adsorbed LGP creating a thin layer of viscous structured water at the surface. Mutual electrostatic repulsion between charged polysaccharide moieties aids in separation of the adsorbed surface layers. The hydration shell also serves as a check valve to control the movement of water out of and into the cartilage matrix during motion.