Objective. Mechanical signals are key determinants in tissue morphogenesis, maintenance, and restoration strategies in regenerative medicine, although molecular mechanisms of mechanotransduction remain to be elucidated. This study was undertaken to investigate the mechanotransduction process of expression of superficial zone protein (SZP), a critical joint lubricant.Methods. Regional expression of SZP was first quantified in cartilage obtained from the femoral condyles of immature bovines, using immunoblotting, and visualized by immunohistochemistry. Contact pressure mapping in whole joints was accomplished using pressure-sensitive film and a load application system for joint testing. Friction measurements on cartilage plugs were acquired under boundary lubrication conditions using a pin-on-disk tribometer modified for reciprocating sliding. Direct mechanical stimulation by shear loading of articular cartilage explants was performed with and without inhibition of transforming growth factor ␤ (TGF␤) signaling, and SZP content in media was quantified by enzyme-linked immunosorbent assay.Results. An unexpected pattern of SZP localization in knee cartilage was initially identified, with anterior regions exhibiting high levels of SZP expression. Regional SZP patterns were regulated by mechanical signals and correlated with tribological behavior. Direct relationships were demonstrated between high levels of SZP expression, maximum contact pressures, and low friction coefficients. Levels of SZP expression and accumulation were increased by applying shear stress, depending on location within the knee, and were decreased to control levels with the use of a specific inhibitor of TGF␤ receptor type I kinase and subsequent phospho-Smad2/3 activity.Conclusion. These findings indicate a new role for TGF␤ signaling in the mechanism of cellular mechanotransduction that is especially significant for joint lubrication.Osteoarthritis (OA) is the most common form of arthritis, affecting 12.1% of US adults (1). Treatment of OA is a critical unmet need in biotechnology and medicine for the regeneration of damaged joints and articular cartilage in the elderly. During locomotion, animal joints allow for normal function by minimizing friction and wear (2,3). Superficial zone protein (SZP), a glycoprotein secreted by chondrocytes in the superficial layer of articular cartilage (4,5), is thought to be a key surface molecule or lubricant involved in boundary lubrication. SZP is also known as lubricin (6), megakaryocyte-stimulating factor precursor (7), and PRG4 (5). In addition to its function as a boundary lubricant, SZP inhibits synovial cell overgrowth (7). Down-regulation of SZP has been associated with the pathogenesis of OA (8). Dr. Schmid has received consulting fees, speaking fees, and/or honoraria (less than $10,000 each) from the NIH and Auxilium. He holds a patent for anti-superficial zone protein monoclonal antibody and receives royalties for anti-collagen X monoclonal antibody.

The purpose of this study was to investigate the role of bone morphogenetic proteins (BMPs), such as BMP-7, growth factors, and cytokines, in the accumulation of superficial zone protein (SZP) in bovine articular cartilage. Calf superficial articular cartilage discs and chondrocytes were obtained for explant and monolayer culture systems, respectively. Dose-and time-dependent actions of BMP-7 on SZP accumulation were investigated in both explant and monolayer culture systems. In addition, actions of various morphogens and growth factors [BMP-2, BMP-4, fibroblast growth factor 2 (FGF-2), insulin-like growth factor 1 (IGF-1), platelet-derived growth factor (PDGF), and transforming growth factor b (TGF-b1)], and cytokines [interleukin (IL)-1a, IL-1b, and tumor necrosis factor (TNF-a)] alone, and in combination with BMP-7, on SZP accumulation were investigated in monolayer culture systems. SZP accumulation was quantified in both the cartilage and the medium using SDS-PAGE and subsequent immunoblotting. In both explant and monolayer cultures, BMP-7 increased SZP accumulation in a dose-and time-dependent fashion (p < 0.05). Furthermore, SZP accumulation was significantly increased in monolayer cultures by FGF-2, IGF-1, PDGF, and TGF-b1 (p < 0.05). Both IL-1a and TNF-a significantly reduced SZP accumulation (p < 0.05). The inhibition of SZP accumulation by TNF-a was partially alleviated by concurrent treatment with BMP-7. The results of this investigation provide novel insights into the role of morphogens, especially BMP-7, growth factors, and cytokines in the accumulation of SZP in articular cartilage. This information has clinical implications because stimulation of SZP may ameliorate the pathology of joint function in arthritis. Furthermore, tissue engineering approaches to articular cartilage may depend on the optimal synthesis and assembly of SZP in the superficial zone to ensure functional tissue architecture. ß

Our data reveal that the PFLP with the "kickstand" screw provides more axial stiffness, less torsional stiffness, and equivalent irreversible deformation to cyclic axial loading when compared with the blade plate.

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