Control of cell fate decisions of embryonic stem cells by mechanical forces

Mounting evidences implicate mechanical properties of the substrates, upon which the cells adhere, to influence critical biological functions including cell fate decisions in mesenchymal stem cells. However, how embryonic stem cells respond to forces or underlying substrates is not clear at this time. The work presented here examines how mouse embryonic stem cells (mESCs) respond to externally applied forces and underlying substrates. We examined if mESCs can be directed to differentiate by external local forces through integrin mediated pathway. Surprisingly, we found that cyclic loading of the same stress amplitude can induce cell spreading in mouse embryonic stem cells but not in ~10 times stiffer differentiated cells. The stress induced spreading response was dictated by cell softness, suggesting that it is the intracellular deformation of the cytoskeleton that dictates cell spreading response. A local stress via focal adhesions alone can induce embryonic stem cells to differentiate, in the absence of soluble differentiation factors. Now that we see that mESCs can be directed to differentiation solely by external mechanical forces, we next examined if mESCs can be kept in their pluripotent state by culturing them on soft substrates. We found that soft substrates that match the intrinsic stiffness of the cell can maintain populations of mESC culture homogeneously in an undifferentiated state. The underlying biophysical mechanism is to match matrix substrate stiffness to that of the mESCs which in turn generates low cell-matrix tractions and low colony stiffness correlating well with compact and round colony morphology, expressed high levels of OCT3/4, NANOG, and the Alkaline Phosphatase activity, even in the absence of Leukemia Inhibitory Factor (LIF). The mESCs on the soft substrates formed more efficient embryoid bodies and teratomas than those on rigid substrates. Collectively, these results strongly suggest that mechanics is indispensable in physiological functions of embryonic stem cells