Appropriate mechanical stimulation is crucial for maintenance of articular cartilage. Aberrant mechanical loading can lead to the initiation or progression of osteoarthritis. The mechanisms by which chondrocytes sense or respond to mechanical loading in healthy and diseased cartilage are not fully understood. Articular chondrocytes are encapsulated in a complex pericellular matrix composed mainly of fibrillar collagens, hyaluronan, proteoglycans, fibronectin and laminin. This PCM links the chondrocyte to its extracellular environment through cell surface receptors (integrins and CD44), and protects the chondrocyte from osmotic challenge and deformation during mechanical load. Preliminary data indicate that accumulation of extracellular matrix by chondrocytes in culture is concomitant with decreasing rates of ATP release and an increase in intracellular calcium mobilization in response to fluid shear (or stimulation). It is hypothesized therefore that deposition of extracellular matrix alters the mechanical responsiveness and that individual components of the pericellular matrix differentially regulate mechanotransduction in chondrocytes. This hypothesis will be tested in cultured primary chondrocytes using two novel systems: a microplate mechanotransduction assay developed by the principal investigator, and a fluorescence-coupled three-dimensional atomic force microscope capable of delivering precisely measured forces to magnetic beads. Comparison of cell signaling events initiated by chondrocytes on different substrata will help to identify key elements within the pericellular environment that modulate or control the metabolism of chondrocytes during mechanical loading in normal and diseased cartilage. Such comparison will also identify likely chondrocyte mechanoreceptors (such as integrins, or CD44), based on their relative affinities for different ECM molecules. Learning the role of specific ECM molecules in chondrocyte mechanotransduction will increase our understanding of the pathogenesis of osteoarthritis and may aid in designing novel therapies for slowing progression.
