Role of classical cadherins in epidermal junction and barrier formation

Classical cadherins mediate Ca2+-dependent intercellular adhesion and are essential for tissue morphogenesis and maintenance. They are key components of adherens junctions (AJs). In vitro studies in simple epithelial cells indicated an essential role for E-cadherin not only in the formation of AJs but also other intercellular contacts, such as desmosomes and tight junctions. In contrast, in vivo tissue specific knockout studies did not reveal a necessity of E-cadherin in the formation of intercellular junctions, raising the question if classical cadherins are necessary or if other classical cadherins can compensate for the loss of E-cadherin. Therefore, the aim of this thesis was to ask how E-cadherin regulates tight junctions and if E-cadherin has a specific function in the formation of tight junctions. In addition, the question was asked if classical cadherin function is necessary for the formation of other intercellular contacts, such as desmosomes. Using primary keratinocytes as a model for de novo junction formation, it was found that loss of E-cadherin prevents the formation of a functional tight junctional barrier. Surprisingly, the basic assembly of tight junctions was not affected, suggesting that E-cadherin regulates a late step in the formation of a functional barrier. One pathway through which E-cadherin may regulate the functional barrier is by controlling expression levels of the barrier promoting claudin 14. However, knockdown of claudin 14 is insufficient to reduce barrier function, suggestion that other mechanisms contribute to E-cadherin controlled barrier function. Loss of E-cadherin in combination with knock down of the only other epidermal classical cadherin, P-cadherin, resulted in an almost complete loss of intercellular contacts showing that classical cadherins are crucial for desmosome formation. Re-expression of either E- or P-cadherin can rescue not only desmosome formation but also tight junction function, showing that levels but not specific classical cadherin expression is crucial for the functional formation of intercellular junctions. E-cadherin is a tumor suppressor and found to be down regulated in many tumors. In this thesis it was found that loss of E-cadherin in primary keratinocytes is insufficient to enhance migration and proliferation, suggesting that in cancer cells E-cadherin interacts with other, E-cadherin independent pathways in the regulation of growth and migration