Study of the function and regulation of liver N-CAM in Xenopus laevis

The liver of Xenopus laevis is a unique exception in terms of the cell adhesion molecules (CAM) which it expresses. In most species, hepatocytes are characterized by the expression of the epithelial Ca(2+)-dependent CAM E-cadherin or of closely related variants of this molecule (e.g., L-CAM); in Xenopus liver, however, the levels of expression of epithelial cadherins is very low while a thyroxine-inducible isoform of N-CAM is expressed in postmetamorphic hepatocytes. Since Xenopus liver N-CAM is localized in regions of contact between hepatocytes, it has been proposed that it might be involved in mediating hepatocyte adhesion in this species. In this study, we demonstrate that N-CAM can indeed act as a functional adhesion molecule in the liver of Xenopus and that its expression is correlated with a number of profound morphological changes of this organ. After thyroxine treatment, hepatocytes are no longer organized in long loose cords but in compact lobules of cells. Furthermore, at the ultrastructural level, plasma membranes are in much closer proximity with the appearance of electron-dense material in areas of closer contact. We have established two novel culture systems for premetamorphic Xenopus hepatocytes as adherent and non-adherent cells, and we describe the induction of expression of N-CAM in these cells. Given the difference in the profile of adhesion molecules present in the liver of Xenopus and of other species, our results are discussed in view of the importance of the expression of a specific set of cell adhesion molecules in defining the development of homologous organs in different species.