Low Intensity Vibrations Restore Nuclear YAP Levels and Acute YAP Nuclear Shuttling in Mesenchymal Stem Cells Subjected to Simulated Microgravity

Abstract

The bone deterioration that astronauts experience in microgravity environments is known to occur in response to the lack of gravity-based tissue stress. Mechanical forces are crucial to maintain healthy bone mass by regulating the function of bone-making osteoblasts as well as the proliferation and differentiation of their progenitors, mesenchymal stem cells (MSC) which replenish osteoblastic cells. Regulation of proliferative function of MSCs in response to mechanical force is in part controlled by the "mechanotransducer" protein YAP (Yes-associated protein) which is shuttled into the nucleus in response to mechanical challenge to induce gene expression necessary for cell proliferation. Our group had recently reported that altered gravity conditions under simulated microgravity (SMG) decreases proliferation of MSCs and that application of daily low intensity vibrations (LIV) during SMG reverses this effect on proliferation. While these findings suggest that LIV may be a promising countermeasure for altered loading, the specific SMG and LIV effects on YAP mechanosignaling are unknown. Therefore, here we tested the effects of SMG and daily LIV treatment on basal nuclear YAP levels as well as on the acute YAP nuclear entry in response to both mechanical and soluble factors in MSCs. MSCs subjected to 72h of SMG, despite decreased nuclear YAP levels across all groups, responded to both LIV and Lysophosphohaditic acid (LPA) treatments by increasing nuclear YAP levels within 6hrs by 49.52% and 87.34%, respectively. Additionally, daily LIV restored the basal decrease seen in SMG as well as nuclear YAP levels as well as restored in part the YAP nuclear entry response to subsequently applied acute LIV and LPA treatments. These results show that rescue of basal YAP levels by LIV may explain previously found proliferative effects of MSCs under SMG and demonstrates that daily LIV is capable of alleviating the inhibition caused by SMG of YAP nuclear shuttling in response to subsequent mechanical and soluble challenge.