Issues of Mechanosensitivity in Osteoblasts

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Mechanical stimulation is critical for development and maintenance of skeletal bone mass. However, the mechanosensitivity of bone cells is rapidly reduced in response to continued loading, probably through adaptation, or habituation, of the cellular mechanisms involved in the transduction of this signal. The earliest measured response to mechanical stimulation in osteoblasts is a rapid increase in intracellular Ca2+ (Ca2+i) that is dependent on both channel-mediated Ca2+ entry and intracellular Ca2+ release (iCaR). We have previously characterized a mechanosensitive channel (MSCC) in osteoblasts that is essential for this early Ca2+i response. The underlying hypothesis of this application is that the desensitization of osteoblasts to mechanical stimulation is the result of downregulation of the MSCC and that this downregulation occurs due to the mechanically-induced increase in organization of the actin cytoskeleton. Due to recent in vivo data showing increased bone formation when loading cycles are separated by intervals of rest, we plan to examine the activation, habituation and restoration of mechanosensitivity in osteoblasts using pulsatile fluid shear. Using cell biologic and molecular techniques coupled with Ca2+i imaging and patch clamp analysis, we will: (1) determine the changes in Ca2+i, MSCC and L-type voltage-sensitive Ca2+ channel (CASCC) kinetics, iCaR and gene expression in MC3T3-E1 osteoblasts in response to pulsatile shear when the interval between loading cycles is increased; (2) examine the Ca2+i response, channel kinetics and iCaR during habituation of MC3T3-E1 cells to shear and determine the interaction of these channels with polymerization of the actin cytoskeleton in this loss of mechanosensitivity, and (3) establish the role of the MSCC and the actin cytoskeleton in restoration of MC3T3-E1 osteoblasts to a mechanosensitive cell. Completion of these studies should provide significant insight into the role of these cellular mechanisms play in the response of osteoblasts to mechanical loading and the loss of mechanosensitivity after repeated stimulation.
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