Cellular mechanisms involving luminal formation of the conducting airway at pseudoglandular to columnar stages of lung morphogenesis is unknown. Mesenchymal-derived epimorphin (EPM) plays an essential role in luminal formation of the epithelial tube acting on mesenchyme and epithelium by a yet undefined mechanism. EPM mRNA and the protein are expressed in the mouse lung mesenchyme throughout development from early pseudoglandular stage to the adult (Ped Res 37:66A 1995). Thus, I hypothesized that the EPM effect on luminal formation is likely regulated by the downstream of the EPM peptide and that it exerts its morphogenetic effects through cell surface membrane receptor(s). As the first step, I generated a fusion protein containing the mid third of EPM peptide. When the culture well was pre-coated with the fusion protein and 10T1/2 cells were plated, cells adhered to the well 15 fold more than the well pre-coated with a control protein. Iodinated 125I-fusion protein binding to the cell was saturable and the Scatchard analysis showed a single class of binding sites, binding capacity of 2.3 × 104 per cell and KD (the affinity) at 5.6 × 10-9 M. When the fusion protein was immobilized to Sepharose and used as affinity column matrix, 2 protein bands, 200 and 100 k dalton respectively, were separated by PAGE in a reduced condition from the cell membrane fraction. Both proteins bind to wheat germ agglutinin and thus are glycoproteins. The sizes of these proteins are different from the subunits of c-met proto-oncogene product, Met, which is related to lumen formation of the kidney tubules and mammary ducts. In addition, Met protein was not detected in the mouse lung by immunohistochemistry. I speculate that EPM exerts its effect through the pathway differently from Met and interacts with specific cell membrane receptors of 200 and/or 100 k dalton giving signals for luminal formation during airway morphogenesis. Characterization of these proteins should give insight into the mechanism of EPM action.
