Abstract
Osteoporosis is a major health problem characterized by low bone mass and microarchitectural deterioration leading to enhanced bone fragility and increased fracture risk. In contrast, the high bone mass trait is characterized by increased bone mass and strength, with a significantly decreased fracture risk. Mechanical loading of bone has been shown to regulate bone formation in the skeleton. How these mechanical signals are transduced into bone forming signals is far from understood. Of the three main cell types in bone; osteoblasts, osteocytes, and osteoclasts, osteocytes are currently thought to be the mechanosensory cells in bone. The studies described here have further elucidated the mechanism of mechanotransduction in bone.|These studies were guided by two related hypotheses. The first hypothesis was mechanically induced fluid flow through the lacuno-canalicular system stimulates osteocytes by fluid shear stress. When stimulated, these osteocytes then activate adjacent osteoblasts. The magnitude of fluid shear stress to which bone cells are exposed, directly determines the bone forming response within the physiological range. The second hypothesis was of the three nitric oxide synthase isozymes, only the endothelial isozyme is mechanically regulated in osteogenic cells. This increase in expression is due to both increased transcription as well as increased stability by the action of ELAVL1.|Murine osteocytes (MLO-Y4) and osteoblasts (2T3) were subjected to pulsatile fluid flow to determine changes in mRNA levels of Ptgs2 (COX-2) and Nos3 (eNOS). The pattern of Ptgs2 expression in these cell types support the hypothesis that osteocytes are the mechanosensors in bone. The data further supports the mechanism of stimulation of bone cells is fluid shear stress and the osteogenic response of these cell types is biphasic in response to increasing magnitudes of fluid shear stress. In MLO-Y4 cells, expression of all three nitric oxide synthase isozymes were regulated by fluid shear stress, while 2T3 cells only expressed Nos3. It is unclear what the relative contribution each of these isozymes plays in the responsiveness of osteocytes to mechanical stimulation. Both cell types demonstrated an initial NO-dependent decrease in mRNA stability that was|abrogated by gadolinium or methylene blue. Further studies showed that other mechanically induced genes are stabilized by the protein Elavil, but Nos3 is not, indicating that its decreased levels following stimulation is most likely due to activation of an unknown destabilizing protein.|The results of these experiments have helped to clarify the mechanism of mechanotransduction in bone and have shown mRNA stabilization to be important in cell signaling following mechanical stimulation.