Integrin Molecular Tensions in Live Cells are Altered by Substrate Rigidity

Abstract

Mounting evidence suggests that mammalian cells are capable of sensing the rigidity of their local environment and responding to it. However, it remains unclear whether cells sense cellular force alteration caused by matrix compliance (stress sensing) or sense cytoskeleton remodeling caused by matrix deformation (strain sensing). Because integrins are the major mechano-sensitive receptors that transmit cellular force at the cell-matrix interface, the tension transmitted by integrin molecules (integrin tension) may serve as fundamental mechanical signal in cell mechanotransduction and rigidity sensing. Therefore, measuring and mapping integrin molecular tension in live cells on elastic substrates will shed light to the mechanism of cell rigidity sensing. In this work, we applied integrative tension sensor (ITS), a sensor converting molecular tension above a threshold to fluorescent signal, to calibrate and map integrin tension directly by fluorescence imaging. In experiments, ITS with 54 pN tension threshold was grafted on PDMS (Polydimethylsiloxane) substrates with Young's modulus ranging from 1kPa to 1.8 MPa. Integrin tension above 54 pN in live cells was mapped with high resolution (0.4 µm) and sensitivity on these substrates. The results show that cell spreading area and integrin tension activity both increased with increasing PDMS gel stiffness for CHO-K1, NIH-3T3 and skin fibroblast cell lines. Also, keratocytes migrated faster on softer PDMS substrates and integrin tension activity was the highest on the stiffest PDMS sample. These experiments for the first time monitored integrin tension in live cells on elastic substrates, and clearly demonstrated that the substrate rigidity affects integrin tension activity, providing evidence in favor of stress sensing hypothesis.