Integral intercellular adhesion is fundamental for the epithelial cells to exert physiological functions and maintain normal structure. Adherens junctions, along with several other types of junctions, develop at the cell-to-cell contact sites between neighboring cells after confluence. Cadherins are a superfamily of Ca
2+-dependent cell-adhesion molecules.
42 51 Calcium binding with the extracellular portion of the polypeptide chain is a prerequisite for cadherin-mediated cell–cell adhesion. Removal of extracellular Ca
2+ ions by EGTA or calcium-free medium can cause obvious dissociation of RPE cells, as observed by changes in cellular morphology and junction-associated actin. However, function-blocking antibodies against E- and N-cadherin have failed to disrupt intercellular adhesions of RPE cells, though expression of E- and N-cadherin was confirmed by immunocytochemical staining. It is assumed that the formation of Ca
2+-dependent adherens junctions of RPE cells involves many other members of the cadherin superfamily besides E- and N-cadherin. Blocking one or two cadherins may not be enough to disrupt the cell–cell adhesions. Blocking antibodies have been reported to induce disruption of intercellular adhesion in other cell types and, subsequently, changes of cellular function. It has been found that anti-vascular endothelial cadherin (VE-Cad) antibody, as well as EGTA, disrupts adherens junctions of aortic endothelial cells and stimulates VEGF production.
41 In airway epithelial cells, anti-E-cadherin antibody, or low Ca
2+, disrupts intercellular adhesions and increases cell permeability and adenoviral infection.
50 These results indicate that functional changes are closely related to the status of intercellular adhesion. Disruption of cell–cell contacts by either extracellular Ca
2+ manipulation or blocking antibodies of cadherins can trigger changes of a series of cellular functions. It is clear that the cadherin homodimers of the neighboring cells are linked to actin cytoskeleton by catenins and other cytoskeletal protein.
52 53 However, the signaling pathway from cadherins to the effective genes in nuclei remains largely unclear. It has yet to be clarified that the gene expression changes in Ca
2+-mediated cell dissociation model do not result from the direct action of cellular Ca
2+ concentration. There is no evidence thus far showing that low extracellular Ca
2+ itself may change intracellular Ca
2+ levels. However, an effective cell dissociation model without calcium involvement, such as physical removal of certain areas of confluent cells, needs to be developed to clarify the role of Ca
2+ ions.