Mechanobiological Signals in Trabecular Bone and Marrow

Bone adapts to mechanical loading through the coordination of many cell types including osteocytes, osteoblasts, osteoclasts, and stem cells. These cells sense mechanical loading and carry out biochemical signaling in a process known as mechanotransduction. While it is understood that mechanotransduction drives bone adaptation, the mechanisms responsible for mechanotransduction are not well understood. The goal of this work was to gain new understandings of mechanobiological signaling in bone. In order to achieve this, the effects of low magnitude mechanical stimulation, a loading regime that stimulates bone formation in the absence of significant matrix strain, on trabecular bone and bone marrow were investigated.Computational fluid dynamics models quantified the marrow shear stresses occurring during LMMS and demonstrated that marrow is subject to shear stresses above a mechanostimulatory shear stress.

In order to exploit this stimulatory regime, trabecular bone explant bioreactors were designed and validated. These bioreactors supported active bone remodeling with a heterogeneous cell population that experienced mechanical stimuli that affected bone formation.

Before mechanobiology of bone marrow could be studied, osteocyte signaling during LMMS was determined. Their role in mechanosensing of LMMS was investigated by quantifying the percentage of osteocytes expressing sclerostin, a potent Wnt inhibitor, with and without mechanical stimulation using bioreactor culture. The percentage of sclerostin positive osteocytes did not change in response to culture or LMMS. This result suggests that sclerostin signaling is dependent on matrix strain and osteocytes may not be responsible for mechanotransduction during LMMS. To investigate marrow mechanobiology, the presence of primary cilia, a potent mechanosensor, in the trabecular bone compartment was quantified before and after LMMS. They were present on a small fraction of bone marrow cells and osteocytes, and were relatively short within the trabecular bone and marrow compartment, indicating that they may function as mechanosensors in concert with other mechanosensors or that only a small number of cells are responsible for mechanosensing in the trabecular bone and marrow compartment. Cilia expression in the marrow was higher in static cultured explants, but decreased in response to mechanical stimulation. This indicates that cilia may have a chemosensing and mechanosensing role in the marrow or are affected by mechanosensing pathways. In summary, the mechanical environment in trabecular bone during LMMS was quantified. With the results from the computational models, trabecular bone explant biroeactor culture with LMMS offers the ability to study the effects of an alternative mechanobiological signal on cellular sensing and biological function. There is little known about the contribution of bone marrow cells to mechanotransduction and the subsequent signaling that takes place. This explant bioreactor culture system and LMMS provides a tool to study the role of marrow cells in mechanotransduction in order to make new advances in bone mechanobiology.