摘要
The tenascins are a family of extracellular matrix proteins with distinct developmental 'patterns of expression. We initially identified tenascin X(TN-X) because it overlaps the active human 21-hydroxylase gene.Human 21-hydroxylase gene deletions occur frequently,but the TN-X gene is never involved,suggesting that TN-X is an essential protein. To gain insight into its functions,we evaluated TN-X expression during development in the rat.RNase protection and immunoblotting experiments demonstrated that TN-X is widely expressed late in development,with greatest expression in heart and skeletal muscle. By in situ hybridization,the earliest expression of TN-X was at E12 and was confined to migrating epicardial cells.Epicardial TN-X expression continues untill the entire heart is invested with epicardium. Late expression was by a subset of cells scattered through the myocardium,possible fibroblasts. RNase protection assay showed abundant expression by primary cultures of non-myocytes but not myocytes.After E14, there was also intense hybridization to proliferating smooth muscle cells of the ductus arteriosus.For cell binding experiments,we expressed the C-terminus of TN-X in vitro. Neither cardiac fibroblasts or ductal smooth muscle cells adhere to this purified protein,but TN-X does prevent cell banding to fibronectin.This antiadhesive effect is consistent with a role in regulation of cell migration. Because the epicardium is essential for coronary vascular development and possibly for pattening of myocardium, expression of TN-X by migrating epicardium and embryonic heart fibroblasts is consistent with an essential role for this protein during vertebrate development.
The tenascins are a family of extracellular matrix proteins with distinct developmental 'patterns of expression. We initially identified tenascin X(TN-X) because it overlaps the active human 21-hydroxylase gene.Human 21-hydroxylase gene deletions occur frequently,but the TN-X gene is never involved,suggesting that TN-X is an essential protein. To gain insight into its functions,we evaluated TN-X expression during development in the rat.RNase protection and immunoblotting experiments demonstrated that TN-X is widely expressed late in development,with greatest expression in heart and skeletal muscle. By in situ hybridization,the earliest expression of TN-X was at E12 and was confined to migrating epicardial cells.Epicardial TN-X expression continues untill the entire heart is invested with epicardium. Late expression was by a subset of cells scattered through the myocardium,possible fibroblasts. RNase protection assay showed abundant expression by primary cultures of non-myocytes but not myocytes.After E14, there was also intense hybridization to proliferating smooth muscle cells of the ductus arteriosus.For cell binding experiments,we expressed the C-terminus of TN-X in vitro. Neither cardiac fibroblasts or ductal smooth muscle cells adhere to this purified protein,but TN-X does prevent cell banding to fibronectin.This antiadhesive effect is consistent with a role in regulation of cell migration. Because the epicardium is essential for coronary vascular development and possibly for pattening of myocardium, expression of TN-X by migrating epicardium and embryonic heart fibroblasts is consistent with an essential role for this protein during vertebrate development.
