The exertion of cell-cell adhesions

Abstract

Regulation of cadherin adhesion in tissue organization and malignant transformation.Cadherins are cell-cell adhesion molecules present in all tissues. Cadherins are transmembrane-proteins that form a homotypic, calcium-dependent complex with cadherins molecules onneighboring cells 1. Inside the cell, they connect to the actin cytoskeleton2 and thus mediate astrong physical connection between cells. As such, cadherins are important in the control of a widerange of fundamental biological processes, including embryonic development, tissuemorphogenesis and vascular homeostasis. The main epithelial cadherin, E-cadherin, organizes intocell-cell adhesion structures called Adherens Junctions (AJ)3 and is a tumor suppressor4, whose lossof expression, mainly through gene-silencing, results in tumor metastasis5-7. However, several typesof metastatic cancer, like certain agressive breast carcinomas (reviewed in8-10), do not show anydownregulation of the E-cadherin protein. This implies that alternative mechanisms must exist to(de-) regulate E-cadherin adhesion.At the start of the research described in this thesis, several reports had shown that cytoskeletaltension is involved in the downregulation of AJs. Tumor tissue was found to be stiffer than normaltissue and artificial increase of tissue stiffness in breast cancer cells resulted in the disorganizationof AJs11. It was shown in our lab, that HGF-induced scattering of epithelial MDCK cells depends onactomyosin-driven cytoskeletal tension12: Myosin activity increased after HGF; the structure of thecytoskeleton changed after HGF; Actin-bundles were formed that connected to cell-cell junctionsto pull them apart; inhibitors of myosin activity prevented all of this from happening. Togetherthese observations indicated that E-cadherin’s adhesive function is sensitive to cytoskeletaltension, but the underlying mechanisms remained completely elusive. The dynamic link between E-cadherin and actin. It had been generally accepted, through a large number of experimental studies, that E-cadherininteracts with actin through catenin and the actin-binding protein catenin. A number of otherproteins had been found associated with the E-cadherin complex, including most notably p120-catenin, actinin and vinculin. P120-catenin was known to regulate E-cadherin’s stability13, 14. Thefunction of actinin and vinculin, two proteins that are also present in integrin-based adhesions,was less clear15.In the year before this PhD research started, this established view on the linkage between E-cadherin and the cytoskeleton had been challenged in two recent papers from James Nelson andWilliam Weis16, 17. They showed that a ternary complex of E-cadherin, -catenin and actin cannot form in vitro. Furthermore, they showed that catenin cannot interact with actin and cateninsimultaneously: It interacts with catenin as a monomer that posesses low affinity for F-actin.Dimerization, which is needed for high affinity for F-actin, excludes the interaction with catenin.This discrepancy between old models and the new observations, and the importance of cadherinadhesion for tissue development and cancer, triggered a lot of new research into the linkagebetween E-cadherin and the actin cytoskeleton.The emerging role of mechanical forces in cadherin adhesionThe one crucial factor that the Nelson/Weis studies did not address was the role of mechanicalforces. From integrin-adhesion research it was known that tension on integrin adhesions, causedby contractility in the associated actomyosin cytoskeleton, is needed for their development fromnascent to mature18. Furthermore, our data and that of others had just indicated that myosinactivity is needed for the formation of stable cadherin adhesions12. The research described in thisthesis was aimed at further elucidating the molecular details of the link between cadherins and theactomyosin cytoskeleton in order to understand how mechanical tension influences the dynamicsand function of cadherin adhesion.