Probing the adherens junction [electronic resource] : the effect of mechanical force on the interaction of the cadherin-catenin complex with the actin cytoskeleton
Keywords: Chemical Engineering.

Cadherins are an ancient class of transmembrane proteins that are essential for the formation of multicellular tissues. They link intercellular adhesions to the cellular cytoskeleton, but how they are connected specifically to actin filaments is a hotly debated issue. Genetic and cell culture experiments indicate that E-cadherin, β-catenin, and the actin filament binding protein αE-catenin form a minimal cadherin-catenin complex that binds to the actin cytoskeleton directly in epithelial tissues. However, experiments with purified proteins showed that a stable cadherin-catenin complex can be reconstituted, but it does not bind strongly to actin filaments in solution. Nevertheless, cell culture experiments indicated that the cadherin-catenin complex is under constitutive actomyosin-generated tension, and that this connection is required for mechanotransduction at cadherin-based adhesions. Here, we tested the hypothesis that tension is required to stabilize a linkage between the cadherin-catenin complex and actin filaments, and clarify how the cadherin-catenin complex could interact directly with the actin cytoskeleton in cells. To do this we developed an optical trap-based assay to measure the lifetime of cadherin-catenin complex/actin filament bonds under tension. We observed robust, reproducible binding under force, and the bond lifetime distributions had a biphasic force dependence. The mean lifetimes increased from ~60 ms at low force to ~1.2 s at ~10 pN, after which they decreased. A two-state catch bond model is consistent with the biphasic mean lifetime distribution and the presence of two distinct lifetime subpopulations. In this model, bonds between a cadherin-catenin complex and an actin filament form in a weakly bound state and quickly dissociate, but rapidly transition to a strongly bound state as applied force increases. Long lifetimes are achieved in this state until higher forces accelerate dissociation from the strongly bound state. Additionally, the presence of the D1 domain of vinculin, a known binding partner of αE-catenin, changes the binding interaction between the cadherin-catenin complex and the actin filament such that αE-catenin appears to be preferentially locked in its strongly bound state, leading to a much longer-lived bond to the actin filament at low forces. Our data and kinetic model reconcile previous in vitro and in vivo work by demonstrating that the cadherin-catenin complex binds robustly to actin filaments under force. Thus, it seems that direct cadherin-catenin complex/actin filament binding was not detected in previous solution-based assays because bonds were not strengthened by tension. The two bound states in our model may correspond to different conformational states of αE-catenin, consistent with previous observations that αE-catenin may undergo changes in conformation in response to actomyosin-generated cytoskeletal tension. The vinculin D1 data suggest that the tension-dependent behavior of the cadherin-catenin complex may be modulated by its binding partners. Our model, combined with previous evidence for cooperative binding of αE-catenin to actin filaments, suggests that this linkage is self-reinforcing and that its stability is dynamically regulated by mechanical force during tissue development and maintenance.