Mesenchymal stem cells (MSCs) are responsive to mechanical stimuli that play an essential role in directing their differentiation to the chondrogenic lineage. A better understanding of the mechanisms that allow MSCs to respond to mechanical stimuli is important to improving cartilage tissue engineering and regenerative medicine. Hydrostatic pressure (HP) in particular is known to be a primary mechanical force in joints. However, little is known about the underlying mechanisms that facilitate HP mechanotransduction. Understanding the signaling pathways in MSCs in transducing HP to a beneficial biologic response and their interrelationship were the focus of this thesis. Studies used porcine marrow-derived MSCs seeded in agarose gel. Calcium ion Ca⁺⁺ signaling, focal adhesion kinase (FAK) involvement, and sirtuin1 activity were investigated in conjunction with HP application.

Intracellular Ca⁺⁺ concentration was previously shown to be changed with HP application. In our study a bioreactor was used to apply a single application of HP to the MSC-seeded gel structures and observe Ca⁺⁺ signaling via live imaging of a fluorescent calcium indicator in cells. However, no fluctuations in Ca⁺⁺ concentrations were observed with 10 minutes loading of HP. Additionally a problem with the bioreactor design was discovered. First the gel was floating around in the bioreactor even without loading. After stabilizing the gel and stopping it from floating, there were still about 16 µm of movement and deformation in the system. The movement and deformation was analyzed for the gel structure and different parts of the bioreactor.

Furthermore, we investigated the role of FAK in early and late chondrogenesis and also its involvement in HP mechanotransduction. A FAK inhibitor was used on MSCs from day 1 to 21 and showed a dose-dependent suppression of chondrogenesis. However, when low doses of FAK inhibitor added to the MSC culture from day 21 to 42, chondrogenesis was not inhibited. With 4 hour cyclic HP, FAK phosphorylation increased. The beneficial effect of HP was suppressed with overnight addition of the FAK inhibitor to MSC medium, suggesting FAK involvement in HP mechanotransducation by MSCs.

Moreover, sirtuin1 participation in MSC chondrogenesis and mechanotransduction was also explored. The results indicated that overnight sirtuin1 inhibition increased chondrogenic gene expression (Agc, Col2, and Sox9) in MSCs. Additionally, the activity of sirtuin1 was decreased with both 4 hour cyclic hydrostatic pressure and inhibitor application. These two together demonstrated that sirtuin1 inhibition enhances chondrogenesis.

In this research we have investigated the role of Ca⁺⁺ signaling, FAK involvement, and sirtuin1 activity in the mechanotransduction of HP in MSCs. These understandings about the mechanisms regulating the chondrogenesis with respect to HP could have important implications for cartilage tissue engineering and regenerative studies.