Regulation of homophilic and heterophilic adhesion among classical cadherins

Cadherins are calcium-dependent transmembrane adhesion molecules that are responsible for cell-cell adhesion in all solid tissues. There are at least 20 different classical cadherin subtypes. During development, cells that express different cadherin subtypes sort out into distinct tissues. The process of sorting out and cell aggregation studies has led many to believe that cadherins are exclusively homophilic binding molecules -- that is, they bind only to identical cadherin subtypes. The binding and adhesive specificity of the extracellular domains is widely believed to be the underlying reason behind cell sorting. However, more recent studies showed that cadherins bind heterotypically and the heterophilic binding is not substantially different from homophilic cadherin interactions.;In this study, I investigated intracellular changes in response to both homophilic and heterophilic binding. Immobilized functional ectodomains of canine epithelial (E)-cadherin, chicken neural (N)-cadherin and Xenopus cleavage-stage (C)-cadherins were used trigger cadherin-mediated signaling in cells adhering to these ectodomain fragments. I used a biochemical assay to determine the activation of Raci, a small GTPase that is a known regulator of cadherin-mediated responses. E-cadherin receptors activate Rac upon binding to E-, N- and C-cadherin ligands. However, the Rac activation triggered by heterotypic interactions was lower compared to activation following homophilic ligation. I then compared the E-cadherin mediated response with that triggered by N-cadherin receptors that were bound to identical cadherin ectodomains. In contrast to cells expressing E-cadherin, I found no significant Rac activation upon either homophilic or heterophilic cadherin ligation. Further measurements of the projected area of E- and N-cadherin expressing cells showed that homophilic adhesion supported the greatest spread cell area.;Finally, fluorescence imaging assessed cadherin clustering and actin organization in E-cadherin expressing cells cultured on E-, N-, and C-cadherin ectodomains. As expected, the formation of filamentous actin correlated with the Rac activation amplitudes in E-cadherin expressing cells. The results also showed that lateral cadherin clustering, which is thought to be an initial response to cadherin ligation, occurs in E-cadherin expressing cells as a result of both heterophilic and homophilic binding. However, I also showed that lateral cadherin clustering is not a cadherin-specific response. These results are important because they suggest that cadherin-dependent differences in intracellular signaling may play a greater role in cell sorting than the biophysical properties of bonds between ectodomain subtypes.