cDNAs of Cell Adhesion Molecules of Different Specificity Induce Changes in Cell Shape and Border Formation in Cultured $180 Cells

The liver cell adhesion molecule (L-CAM) and N-cadherin or adherens junction-specific CAM (A-CAM) are structurally related cell surface glycoproteins that mediate calcium-dependent adhesion in different tissues. We have isolated and characterized a full-length cDNA clone for chicken N-cadherin and used this clone to transfect S180 mouse sarcoma cells that do not normally express N-cadherin. The transfected cells (S180cadN cells) expressed N-cadherin on their surfaces and resembled S180 cells transfected with L-CAM (S180L cells) in that at confluence they formed an epithelioid sheet and displayed a large increase in the number of adherens and gap junctions. In addition, N-cadherin in S180cadN cells, like L-CAM in S180L ceils, accumulated at cellular boundaries where it was colocalized with cortical actin. In S180L ceils and S180cadN cells, L-CAM and N-cadherin were seen at sites of adherens junctions but were not restricted to these areas. Adhesion mediated by either CAM was inhibited by treatment with cytochalasin D that disrupted the actin network of the transfected cells. Despite their known structural similarities, there was no evidence of interaction between L-CAM and N-cadherin. Doubly transfected cells (S180L/cadN) also formed epithelioid sheets. In these cells, both N-cadherin and L-CAM colocalized at areas of cell contact and the presence of antibodies to both CAMs was required to disrupt the sheets of cells. Studies using divalent antibodies to localize each CAM at the cell surface or to perturb their distributions indicated that in the same cell there were no interactions between L-CAM and N-cadherin molecules. These data suggest that the Ca++-dependent CAMs are likely to play a critical role in the maintenance of epithelial structures and support a model for the segregation of epithelia based on differences in specificity of CAM mediated binding. They also provide further support for the so-called precedence hypothesis that proposes that expression and homophilic binding of CAMs are necessary for formation of junctional structures in epithelia. C ELL adhesion molecules (CAMs) ~ take part in fundamental morphogenetic events such as epithelialmesenchymal transformation (EMT) and reorganization of epithelial boundaries. EMT (18, 35) is the reversible conversion of cells linked in epithelial sheets to loosely associated cells known as mesenchyme. Examples of EMT include the separation and migration of neural crest cells from the neural plate and the formation of the sclerotome in somites. Consistent with their role in cell linkage, the expression of CAMs at the cell surface decreases as mesenchyme is produced and increases when mesenchyme condenses (2). CAMs are also modulated at the cell surface when a contiguous homogeneous epithelium is reorganized into two epitheI. ,4bb~viation~ usedin thispaper: CAM, cell adhesion molecule; A-CAM, adherens junction-specific CAM; L-CAM, liver CAM; N-CAM, neural CAM; EMT, epithelial-mesenchymai transformation. lia that eventually segregate. This is seen, for example, during the differentiation of the neural plate (2, 38) and the differentiation of the otocyst (33). In such processes, the cells remain linked in epithelial sheets and do not undergo EMT. The epithelium initially expresses at least two CAMs, but, as the two epithelia segregate into different structures, the CAMs are differentially expressed in time and space. Evidence has accumulated to suggest that Ca÷+-depen dent CAMs such as liver CAM (L-CAM) (11) and N-cadherin (16, 42) are necessary for the formation of epithelia and for the maintenance of cell junctional structures after they have formed (14, 15, 25). L-CAM was initially purified from embryonic chicken liver but is found in most nonneuronal epithelial cells (39). It is expressed at the surface of the cells as a glycoprotein of 124 kD that binds by a homophilic mechanism. L-CAM appears on the earliest embryonic cells to© The Rockefeller University Press, 0021-9525/90/04/1239/14 $2.00 The Journal of Cell Biology, Volume I10, April 199