Neural cell adhesion molecules during embryonic induction and development of the kidney

The neural cell adhesion molecules (N-CAM) are a family of related glycoproteins with Mr of 180, 140 and 120 x 10(3) (180K etc.). In the embryo, they are often highly sialylated and migrate as a diffuse band of 170-250K. N-CAM are found in non-neural tissues and we have now studied the expression of N-CAM in the developing mouse kidney. During kidney development, a unique conversion of a mesenchyme to an epithelium occurs and it is thought that this is mediated by an increase in cell adhesivity. By immunofluorescence, we show that N-CAM is present already at onset of kidney development on the cells of the uninduced nephrogenic mesenchyme. After induction, when the cells convert into an epithelium, they lose N-CAM gradually and instead begin to express uvomorulin, another primary CAM. By using an organ culture model, we could rather precisely show that N-CAM and uvomorulin are coexpressed for a short period, but, when epithelial cell polarization is evident, only uvomorulin is present on the epithelium, whereas N-CAM is confined to the surrounding mesenchyme. Immunoblotting for N-CAM revealed that the 'embryonic' form of N-CAM, the broad 170-250K band was not present in the embryonic kidney, which instead expressed the three distinct 180K, 140K and 120K bands typical of adult neurones. The 180K and 140K bands were gradually lost during development and were no longer detectable in adult kidneys. By using an N-CAM cDNA, we detected three different mRNAs of 7.4, 6.7 and 4.3 kb in the developing kidney, but this expression was restricted to the embryonic and early postnatal stages. No transcripts were detectable in adult kidneys. The studies do not support the hypothesis that N-CAM expression in the kidney is turned on by embryonic induction. Rather, we suggest that N-CAM are important adhesives for the predetermined, but not yet induced, nephrogenic mesenchyme.