Mechanosensing machinery for cells under low substratum rigidity

Mechanical stimuli are essential during development and tumorigenesis. However, how cells sense their physical environment under low rigidity is still unknown. Here we show that low rigidity of collagen gel downregulates β₁-integrin activation, clustering, and focal adhesion kinase (FAK) Y397 phosphorylation, which is mediated by delayed raft formation. Moreover, overexpression of autoclustered β₁-integrin (V737N), but not constitutively active β₁-integrin (G429N), rescues FAKY397 phosphorylation level suppressed by low substratum rigidity. Using fluorescence resonance energy transfer to assess β₁-integrin clustering, we have found that substratum rigidity between 58 and 386 Pa triggers β₁-integrin clustering in a dose-dependent manner, which is highly dependent on actin filaments but not microtubules. Furthermore, augmentation of β₁-integrin clustering enhances the interaction between β₁-integrin, FAK, and talin. Our results indicate that contact with collagen fibrils is not sufficient for integrin activation. However, substratum rigidity is required for integrin clustering and activation. Together, our findings provide new insight into the mechanosensing machinery and the mode of action for epithelial cells in response to their physical environment under low rigidity.