Signal transduction mechanisms of the chondrocyte

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Articular cartilage is a soft tissue covering the surfaces of bones within synovial joints. Cartilage is compressed and relaxed during joint motion leading to changes in osmotic stress as fluid is extruded and then imbibed by the proteoglycan rich matrix. As the only cells of cartilage, chondrocytes play a critical role in maintaining the extracellular matrix. Chondrocytes are sensitive to mechanical and chemical changes in their locale and reciprocate with a variety of biological responses.The long term objective of my research program is to identify and characterize signal transduction mechanisms through which chondrocytes respond biologically to mechanical and chemical changes in cartilage. This advance in knowledge is applicable for cartilage tissue engineers in the design of bioreactor systems. Harnessing the effects of osmotic stress on chondrocyte biology, for example, is scalable at low cost (adding sucrose or water).Primary cilia are non-motile with an axoneme consisting of a 9+0 doublet arrangement of tubulin tubules. Although immotile, the cilium can alter its length by regulating microtubule assembly, disassembly and bidirectional intraflagellar transport. Chondrocyte cilia are 1-2 µm long and host numerous receptors and signaling molecules along their axoneme including the primary chondrocyte osmosensor, transient receptor potential vanilloid 4. In response to osmotic stress, chondrocyte cilia shorten and reorganization of the actin cytoskeleton occurs, the latter requiring the activation of gelsolin. Furthermore, deciliated chondrocytes do not respond to osmotic stimuli and cells lacking gelsolin show a reduction in ciliogenesis. Together these data suggest a critical role for primary cilia and their shortening in chondrocyte transduction of osmotic stress and interplay with the actin cytoskeleton in this transduction. The short term objective of my research program is to investigate the role of the primary cilium and its shortening in chondrocyte transduction of osmotic stress. We will examine if cilia shortening is necessary for chondrocyte osmotransduction and/or if cilia length regulates chondrocyte sensitivity to osmotic stress. Further we will investigate the role of actin dynamics in changes to cilial length. Finally we will manipulate osmotic stress and cilial length in neo-cartilage constructs and measure the effects on chondrogenesis.As such this proposal will provide significant advances in knowledge to the natural sciences and engineering that are critical to the fields of cell biology and tissue engineering. Almost all mammalian cells possess a single primary cilium and many are exposed to osmotic stress, thus our findings will be applicable to numerous cell systems.