Cadherin mechanotransduction in morphogenesis: the importance of α-catenin and vinculin

The actomyosin contractile tension that is transmitted and regulated at cadherin-based cell-cell adhesions, greatly contributes to cell shape, cell migration, and coordinated tissue formation and organization. In a process termed mechanotransduction, the protein complexes in cell adhesions are able to sense changes in tension and induce biochemical feedback pathways to respond to these changes. The best understood mechanism is the force-induced recruitment of vinculin to α-catenin in cadherin junctions that leads to strengthening of the adhesion. However, the importance of this mechanism for the development of a living organism has not been directly studied yet. Here we investigated whether α-catenin/vinculin-dependent cadherin mechanotransduction is important for zebrafish development, and how changes in tension at E-cadherin cell-cell contacts might contribute to the total intercellular stress in a remodeling epithelium.

We first generated α-catenin-deficient zebrafish using TALEN gene editing technology. We then specifically disrupted αE-catenin-dependent mechanotransduction while maintaining junction-forming structural capacity, by using an αE-catenin construct lacking the vinculin-binding domain (α-catenin-ΔVBS). αE-catenin mutant embryos show loss of epithelial integrity, which is restored upon expression of either wild-type α-catenin or α-catenin-ΔVBS. However, expression of α-catenin-ΔVBS also perturbed convergence and extension cell movements during gastrulation. Further investigation revealed that the vinculin binding domain of a-catenin is essential for cadherin-dependent morphogenetic cell movements in the developing zebrafish embryo.

Next, we investigated the role of vinculin in zebrafish development directly by generating loss-of-function alleles using TALEN and CRISPR-Cas gene editing technologies. Surprisingly, we found that zygotic loss of the two functional vinculin genes present in zebrafish, did not cause observable defects in early development. However, vinculin A-vinculin B double mutants failed to survive until adulthood, suggesting that vinculin is not needed for the early development and morphogenesis of zebrafish tissues but becomes essential after embryonal stages.

To study the relationship between tension on E-Cadherin and total intercellular tension during tissue remodeling in epithelial monolayers, we characterized a FRET biosensor which measures tension on E-Cadherin molecules in single junctions. Simultaneously, we measured the total intercellular tension in the same junctions using Monolayer Stress Microscopy, which calculates the total monolayer stress from the traction forces exerted by cell-ECM adhesions. We determined that the magnitude of E-Cadherin tension is not predictive of the total level of junctional tension. Instead, we find that E-Cadherin tension rapidly and proportionally changes with total junctional tension during tissue remodeling. This places E-cadherin at an ideal position for mechanically induced feedback signaling into junctional actin organization.

In this thesis we provide the first indication that cadherin mechanotransduction via the mechanosensitive vinculin-binding domain in α-catenin, is essential for morphogenesis in a developing embryo. While the exact role of vinculin in this process remains unresolved, vinculin-deficient zebrafish show lethality during the later development into adulthood. Finally, we have characterized the E-Cadherin-TSMod tension biosensor. Our results, the tools and the zebrafish mutants that we have developed will serve as a basis for future investigations of how changes in tension at cadherin-based cell-cell adhesions influence the formation, morphogenesis, and architecture of tissues and organisms.