Cristina Bertocchi (Department of Physiology, Pontificia Universidad Católica de Chile)
“ Mechano-molecular mechanisms regulating cell adhesion nanomachines “
Cells sense their physical surroundings through molecular nanomachines regulating force transduction and mechanosensing. One of such complexes is the Adherens Junction (AJ), an adhesion complex mediating cell-cell interaction. Within AJ, the protein a-catenin, has been suggested as a force-transducing element. However, it is unlikely that a single dominant tether could bear the load of and regulate such sophisticated and precise system to provide the high morphological plasticity observed in epithelial tissues. Indeed, we have previously unveiled a mechanism by which conformational switch of vinculin (a partner protein of a-catenin)
could selectively engage the actin cytoskeleton in response to regulatory mechanochemical signals, effectively functioning as a molecular clutch, to mediate intercellular interactions. This finding implies that the force transduction and regulation at AJ may be more complex than a single tether functioning through a simple on-off model. Using a super-resolution microscopy approach, we report that vinculin, once activated, could form a direct structural connection with b-catenin, which can bypass a-catenin. Direct vinculin/b-catenin interaction can support mechanical tension and contributes to the stabilization of the cadherin-catenin complexes. We thus propose a multi-step mechanochemical process to mediate mechanical connection and the force-dependent molecular mechanics modulating AJ reinforcement in complex biological functions.
Hiroaki Hirata (平田 宏聡) (Graduate School of Medicine, Nagoya University; 名古屋大学大学院医学系研究科)
“ Molecular and biophysical bases for modulation of the focal adhesion-actin cytoskeleton system in response to extracellular substrate rigidity “
Rigidity of the extracellular substrate largely influences cell adhesion and morphogenesis. On soft substrates, cells cannot stabilize cell-substrate adhesion, resulting in a failure of cell spreading on the substrates. While cell-substrate adhesion is primarily mediated by focal adhesions, their stabilization depends on formation of the stable linkage of the actin cytoskeleton with the focal adhesions. Here, we propose a potential mechanism for rigidity-dependent regulation of the actin cytoskeleton-focal adhesion linkage. Talin molecules linking between the retrograding actin cytoskeleton and the adhesion receptor integrin at focal adhesions are mechanically extended in a manner dependent on the extracellular substrate rigidity. Even though the talin-actin bond is short-lived, extension of talin exposes cryptic binding sites for the actin-binding protein vinculin, causing reinforcement of the talin-actin bond by vinculin. Once cell adhesion is stabilized, phosphorylation of another actin-binding protein calponin 3 regulates contractile force generation by actomyosin in response to rigidity of the extracellular substrate. Rigid substrates promote calponin 3 phosphorylation by MEKK1, which increases actomyosin-based generation of contractile force. Thus, modulation at multiple layers of the focal adhesion-actin cytoskeleton system would underlie proper responses of cell adhesion and morphology against rigidity of the extracellular substrate.