Mechanosensing by Tropomyosin-Controlled Myosin Contractions

Abstract

Control of cell growth, death or differentiation involves the integration of microenvironmental signals through cell motile processes to produce the desired cellular responses. In the case of cell-matrix interactions, the cell tests the matrix by asking several if-then questions such as is the matrix clustered, can force be generated and later what is the rigidity of the matrix. Using lipid-linked matrix ligands, we found that cells would not spread on the surface unless the liganded integrins were clustered and cells generated forces on the ligands by pulling to barriers (Yu et al., 2011. PNAS 108:20565). Upon activation of cell spreading, the flattening of the cells removed the folds in the membranes until a rise in membrane tension activated contraction to sense rigidity (Gauthier et al., 2011. PNAS 108:14467). Rigidity sensing involved local contraction units that pulled about 100 nm independent of rigidity (Ghassemi et al., 2012 PNAS 109:5325. Recently, we found that local contraction units resembled dynamic sarcomeres with alpha actinin at ends and myosin in the middle (Meacci et al., submitted). Further, generation of force involved repeated step-wise movements of myosin controlled by tropomyosin-1 (Wolfenson et al., submitted). Angstrom level measurements of fibroblasts pulling on elastomeric PDMS pillars showed that pillar displacement occurred in discrete steps of ∼1 nm. In contractile pairs, simultaneous steps of opposing pillars had a total step size of ∼2.2 nm, independent of rigidity. Changes in the stepping patterns on different rigidities indicated that the level of contractile force was critical for sensing pillar stiffness. Importantly, knockdown of tropomyosin-1 caused larger steps and increased forces that resulted in aberrant rigidity sensing. This indicates that the process of cell-matrix adhesion formation involves multiple, sequential steps with if-then decisions based upon the physical as well as biochemical properties of the matrix.