Biophysical and gene expression change in living cells by force-induced mechanotransduction

Within the past decade, there has been abounding scientific evidences supporting the notion that mechanical forces are crucial in regulating the physiologic functions of cells and tissues. The importance of engineering principles in studying the biological behavior of cells is no longer in question. Instead, much research is now focused on how mechanical forces are transduced into biochemical activities and biological responses at the cellular and molecular level - a process known as mechanotransduction. This work uses both engineering and biological principles to investigate the different biophysical and gene expression changes of individual cells in response to exogenous forces. We attempt to unravel the mechanism at which forces are transmitted from the apical surface of the cell in to the nucleus. The work presented here provides the first unequivocal evidence that a local surface force can directly alter nuclear functions without intermediate biochemical cascades. We show that a local dynamic force via integrins results in direct displacements of coilin and SMN proteins in Cajal bodies and direct dissociation of coilin-SMN associated complexes. Fluorescence resonance energy transfer changes of coilin-SMN depend on force magnitude, an intact F-actin, cytoskeletal tension, Lamin A/C, or substrate rigidity. Other protein pairs in Cajal bodies exhibit different magnitudes of fluorescence resonance energy transfer. Dynamic cyclic force induces tiny phase lags between various protein pairs in Cajal bodies, suggesting viscoelastic interactions between them. These findings demonstrate that dynamic force-induced direct structural changes of protein complexes in Cajal bodies may represent a unique mechanism of mechanotransduction that impacts on nuclear functions involved in gene expression. We further extend our study to mouse embryonic stem cells (ESCs). Increasing evidence suggests that mechanical factors play a critical role in fate decisions of stem cells. We demonstrate that forces transmitted through different natural extracellular matrix proteins or cell-cell adhesion molecules such as fibronectin, laminin or E-cadherin, have different effects on cell spreading, cell stiffness, Oct3/4 gene expression, and cell proliferation rate. Surprisingly, it was also observed that mouse ESCs do not stiffen when substrate stiffness increases. These cells do not increase spreading on more-rigid substrates either. However, ESCs do increase their basal tractions as substrate stiffness increases. ESCs therefore exhibit mechanical behaviors distinct from those of mesenchymal stem cells and of terminally differentiated cells, and decouple its apical cell stiffness from its basal tractional stresses during the substrate rigidity response. We further elucidate how mechanical forces influence the differentiation of ESCs into spatially organized endoderm, mesoderm, and ectoderm germ layers. ESCs cultured within 3D soft fibrin gels in the absence of Leukemia Inhibitory Factor (LIF) promotes in vivo tissue morphogenesis during vertebrate gastrulation. The results presented demonstrate that mechanical forces play different roles in different force transduction pathways to shape early embryogenesis