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 in vivo measurement of articular cartilage deformation is essential to understand how mechanical forces distribute throughout the healthy tissue and change over time in the pathologic joint. Displacements or strain may serve as a functional imaging biomarker for healthy, diseased, and repaired tissues, but unfortunately intratissue cartilage deformation in vivo is largely unknown. Here, we directly quantified for the first time deformation patterns through the thickness of tibiofemoral articular cartilage in healthy human volunteers. Magnetic resonance imaging acquisitions were synchronized with physiologically relevant compressive loading and used to visualize and measure regional displacement and strain of tibiofemoral articular cartilage in a sagittal plane. We found that compression (of 1/2 body weight) applied at the foot produced a sliding, rigid-body displacement at the tibiofemoral cartilage interface, that loading generated subject- and gender-specific and regionally complex patterns of intratissue strains, and that dominant cartilage strains (approaching 12%) were in shear. Maximum principle and shear strain measures in the tibia were correlated with body mass index. Our MRI-based approach may accelerate the development of regenerative therapies for diseased or damaged cartilage, which is currently limited by the lack of reliable in vivo methods for noninvasive assessment of functional changes following treatment.

No abstract

Articular cartilage is a load bearing and lubricating tissue in animal joints. Heterogeneous deformations arise in the structured and zonal tissue under the application of mechanical load. The character of these deformations is altered by degenerative joint disease. Here, we document an MRI-based technique for determining deformations throughout the volume of the tissue based on displacement encoding with stimulated echoes (DENSE) and a fast spin echo (FSE) readout. A DENSE-FSE technique was designed to image cartilage at 9.4 Tesla in a deformed state during the application of cyclic mechanical loading. Artifact elimination arising from stimulated echoes and FSE was accomplished by radio frequency pulse phase cycling. The articular cartilage of animal joints functions to support load during locomotion and reduce friction and wear at the articular surface (1). Cartilage has a unique zonal structure that varies dramatically in terms of biochemical content (2) and mechanical context (3) through the depth of the thin (i.e., less than 5 mm thick) tissue. Tissue homeostasis and function is altered during degenerative joint diseases such as osteoarthritis (4), which afflicts greater than 10% of the U.S. population (5).Measurement of cartilage deformation under applied mechanical loading by MRI is critical in several research applications. First, MRI provides a tool for the noninvasive and nondestructive evaluation of cartilage mechanical function in normal, diseased, and regenerated tissue. Second, the computed deformations allow for the characterization and estimation of the mechanical environment (which depends on spatial location; 3,6) experienced by chondrocytes, and thus may be important in terms of documenting physical signals for mechanical signal transduction. Third, deformation data permit verification and development of material models for cartilage mechanical behavior. Ultimately, the development of MR techniques at the tissue explant level may be appropriately modified and extended for use at the whole joint or in vivo level with the goal of evaluating the mechanical function of cartilage regeneration or repair techniques. The unique ability of MRI to differentiate between soft tissues, coupled with the ability to determine functional aspects such as mechanical deformations, allow for noninvasive measurements of cartilage deformation at the tissue explant and potentially whole-joint levels.The study of the deformation throughout the volume of cartilage in response to applied loading requires the use of specialized MR-based measurement techniques. One recent cartilage deformation by tag registration (CDTR) technique characterized deformation in cartilage using a loading apparatus with a specialized tagging-based MRI pulse sequence (7). This technique provides a precise description of three-dimensional (3D) strains in cartilage at physiologically relevant loading rates. However, a primary limitation of the CDTR technique was the use of spline fitting to represent tissue motion, which led to a determination of...