Osteocyte Signalling in Response to Mechanical Loading after Physical Cellular Damage and Hyperglycemia

Osteocytes are cells that reside in the mineralized bone matrix. They are the cells that sense mechanical loading on the bone; and respond by directing biochemical signals to osteoclasts (for bone resorption) and osteoblast (for bone formation) to remodel the bone. However, it is unknown how mechanical forces affect signaling of osteocytes that are affected by physical damage and pathological chemical environment. In the bone, mechanical loading causes micro-cracks, which cause physical damage to osteocytes and initiate bone remodeling. However, the role of subsequent mechanical loading on osteocyte is unclear. In this study, we have developed an in vitro cell model to study the impact of mechanical loading on osteocytes with physical damage. This model re-created the spatial distribution of osteocyte and signals expression around micro-cracks as in vivo. The results showed that 1) subcellular physical damage upregulates remodeling signals in osteocytes at 24 hr after damage, 2) mechanical loading substantially upregulates these signals for remodeling in osteocytes with physical damage. Hydraulic pressure and fluid flow shear stress are two concurrent mechanical stimuli on osteocytes, since the fluid shear is driven by the pressure gradient in the interstitial fluid in bone. A new mechanical loading system was developed to apply the concurrent mechanical forces. The loading system was able to safely apply physiological loading magnitudes to cells cultured on glass slides. It was found that concurrent pressure and shear stress had an additive effect on the reduction of osteocyte apoptosis. High glucose concentration in diabetic patients has resulted in retarded bone accumulation, bone loss, and an elevated risk of bone fracture. Osteocyte sensitivity to mechanical loading could be a factor in this pathology. The hypothesis is that hyperglycemia suppresses the beneficial effects of fluid flow shear stress on osteocytes. We found that high glucose level abolished mechanical-loading-induced changes in osteocyte signals to regulate bone remodeling, while osmotic control medium having the same elevated osmolarity did not have significant effects. Elevated glucose levels in diabetic patients could have direct effects on osteocytes, adversely influencing osteocyte response to mechanical loading, consequently the bone remodeling process.Ph.D