Correlative cryo-ET identifies actin/tropomyosin filaments that mediate cell-substrate adhesion in cancer cells and mechanosensitivity of cell proliferation

The role of actin/tropomyosin filaments in the assembly of cell-substrate adhesions has been investigated and it is now shown by cryo-electron tomography that they are essential for adhesion assembly and also regulate mechanosensing, matrix remodelling and transformation of cells towards a cancer phenotype. The actin cytoskeleton is the primary driver of cellular adhesion and mechanosensing due to its ability to generate force and sense the stiffness of the environment. At the cell's leading edge, severing of the protruding Arp2/3 actin network generates a specific actin/tropomyosin (Tpm) filament population that controls lamellipodial persistence. The interaction between these filaments and adhesion to the environment is unknown. Using cellular cryo-electron tomography we resolve the ultrastructure of the Tpm/actin copolymers and show that they specifically anchor to nascent adhesions and are essential for focal adhesion assembly. Re-expression of Tpm1.8/1.9 in transformed and cancer cells is sufficient to restore cell-substrate adhesions. We demonstrate that knock-out of Tpm1.8/1.9 disrupts the formation of dorsal actin bundles, hindering the recruitment of alpha-actinin and non-muscle myosin IIa, critical mechanosensors. This loss causes a force-generation and proliferation defect that is notably reversed when cells are grown on soft surfaces. We conclude that Tpm1.8/1.9 suppress the metastatic phenotype, which may explain why transformed cells naturally downregulate this Tpm subset during malignant transformation.

Automated summary

The actin/tropomyosin filaments play a crucial role in cell-substrate adhesion assembly and regulation of mechanosensing, matrix remodelling, and transformation towards a cancer phenotype. Knock-out of Tpm1.8/1.9 disrupts the formation of dorsal actin bundles, hindering recruitment of critical mechanosensors, while re-expression can restore cell adhesion.