Cadherin Specificity in Adhesion and Mechanotransduction

Cadherins mediate Ca2+ dependent cell-cell junction in all animal tissues. Tight regulation of cadherin adhesion plays critical roles in diverse biological processes. Differential binding between cadherin subtypes is widely believed to mediate cell sorting during embryogenesis. However, a fundamental unanswered question is whether cell sorting is dictated by the biophysical properties of cadherin bonds. Chapter 2 describes the atomic force microscope measurements of the strengths and dissociation rates of homophilic and heterophilic cadherin bonds. Measurements conducted with chicken N-cadherin, canine E-cadherin, and Xenopus C-cadherin demonstrated that all three cadherins cross-react and form multiple, intermolecular bonds. The mechanical and kinetic properties of the heterophilic bonds are similar to the homophilic interactions. The quantified bond parameters, together with previously reported adhesion energies were further compared with in vitro cell aggregation and sorting assays. Trends in quantified biophysical properties of the different cadherin bonds do not correlate with sorting outcomes. These results suggest that cell sorting in vivo and in vitro is not governed solely by biophysical differences between cadherin subtypes. Although the knowledge of molecular mechanism of cadherin adhesion is fundamental to understanding various processes in morphogenesis and the regulation of cell junctions, it remains largely unclear. In Chapter 3, single bond force measurement was used to directly address the functional roles of Trp2 and adhesive domains in C-cadherin interaction. The bond rupture forces between the cadherin ectodomains, domain deletion fragments, and Trp2 point mutant were measured and compared. The results, together with surface force measurements (Maruthamuthu, Ph.D thesis 2009) demonstrated that Trp2 residue both mediates the N-terminal interaction of cadherins and exerts an inter-domain allosteric effect on multiple domains far away from EC1. The findings of the allosteric coupling between classic cadherin ectodomains reconcile several previous models of cadherin adhesion and may have important implications for the regulation of cadherin-based cell adhesion. The importance of mechanical force to development, differentiation, and normal physiology is increasingly acknowledged. Although the classic cadherin complex is a good candidate for tension sensing in tissues, direct evidence for such a role is lacking. In Chapter 4, I examined the cell responses to both external applied forces and soft substrates via classic type I cadherins. Cells respond to applied force through cadherin bonds by reinforcing the bead-cell junction, showing cadherin complexes are force-sensors. Besides, depending on cell context, the cadherin mechanotransduction is subtype-specific. To study how cells respond to passive substrate rigidity, MCF-7 cell spreading on soft gels coated with E-cadherins was examined. The spreading area of cells increases with substrate rigidity. This behavior indicates that the cells are able to sense the substrate stiffness through cadherin receptors and modify their spreading accordingly. These results provide direct evidences that cadherins complexes are mechanosensors.