Expression and characterization of endothelial cell receptors in hematopietic cells

Receptor tyrosine kinases(RTK), TIE, TEK, FLT and FLK are reported to be expressed in vascular endothelial cells. Vascular endothelial cells and hematopoietic cells are derived from the common progenitors, hemangioblasts. Thus, it is interesting to see the expression and function of endothelial cell RTK on hematopoietic cells. We have established monoclonal antibodies against murlne and human RTK and examined their expression in hematopoietic cells. TIE and TEK belong to unclassified RTK and they showed high homology. Both genes were expressed in a similar fashion in adult tissues and primitive hematopoietic cells. Flow cytometry shows that TIE and TEK were mainly expressed in hematopoietic stem cell fractions; Lin-Kit⁺ fraction of bone marrow cells and fetal liver cells. In human bone marrow cells, TIE was expressed on hematopoietic progenitors including primitive stem cells, CD34^highCD38- cells, while TEK was expressed in CD34^kw cells. FLT and FLK are receptors for VEGF and belong to class V.FLT was expressed on TER1l9-positive erythroblasts in fetal liver cells and FLK-1 was expressed on AGM region. We have establshed the cell culture system of AGM cells using stromal cells.Cultured AGM cells contained a significant number of Lin-Kit⁺Sca-t⁺ cells, which were able to reconstitute hematopoiesis in lethally irradiated mice. We would like to discuss about the expression of RTK on these cells.