Large and reversible myosin-dependent forces in rigidity sensing

Abstract

Cells sense the rigidity of their environment through localized pinching, which occurs when myosin molecular motors generate contractions within actin filaments anchoring the cell to its surroundings. We present high-resolution experiments performed on these elementary contractile units in cells. Our experimental results challenge the current understanding of molecular motor force generation. Surprisingly, bipolar myosin filaments generate much larger forces per motor than measured in single molecule experiments. Further, contraction to a fixed distance, followed by relaxation at the same rate, is observed over a wide range of matrix rigidities. Lastly, step-wise displacements of the matrix contacts are apparent during both contraction and relaxation. Building upon a generic two-state model of molecular motor collections, we interpret these unexpected observations as spontaneously emerging features of a collective motor behavior. Our approach explains why, in the cellular context, collections of resilient and slow motors contract in a stepwise fashion while collections of weak and fast motors do not. We thus rationalize the specificity of motor contractions implied in rigidity sensing compared to previous in vitro observations.

Cellular contractile forces are nonmechanosensitive

“…p-myosin was arranged in clusters ( fig. S7C), consistent with the appearance of myosin minifilaments (9,20). We then tested the correlation between the density of the large F-actin structures and p-myosin between the pillars and found high positive correlation regardless of rigidity and cell type (correlation coefficients of 0.6 to 0.7 in all cases; Fig.…”

Dissecting fat-tailed fluctuations in the cytoskeleton with active micropost arrays

“…Our cell results suggest a potentially fruitful way of thinking about the architecture of cytoskeletal networks, as our observations could be naturally explained if the cytoskeleton's constituents self-organize into a mechanically marginal state, akin to jammed (35,37,39) and soft glassy materials (11,12,(37)(38)(39). In the cortex, single myosin minifilaments have been found to selfassemble with other proteins into CUs during the cell spreading process that have complex, nonlinear mechanochemical behavior (27) that can be modeled by a collection of 2-state molecular motors (66). In the fully spread cells we studied, we hypothesize that CUs and actin filaments robustly self-organize into tensile networks which are plastic and whose steady states are marginally stable (35), where a small local rearrangement can trigger an avalanche of structural reconfiguration that arrests only when the network reaches a new marginally stable state.…”

Cell Junction Mechanics beyond the Bounds of Adhesion and Tension

“…By contrast, a force-dependent activation of EFGR was recently found to activate vinculin recruitment by cadherin (Sehgal et al, 2018), which was also shown to alter junctional contractility. Using E-cadherin coated substrates, a ligand independent activation of EGFR by E-cadherin ectodomains has also been putatively involved in pulsatile contractile units, resembling those existing at focal adhesion , that could be involved in measuring substrate rigidity (Lohner et al, 2019). Other mechanisms involving CDC42 or Rac1 have also been reported (Collins et al, 2017).…”