Cell junctions in dyserythropoiesis

Characterization of membrane homing receptors in two cloned murine hemopoietic progenitor cell lines

To characterize homing receptors that are responsible for the recognition and specific binding of hemopoietic progenitor cells to the stroma, we synthesized and 125I-labeled a number of neoglycoproteins. We used these neoglycoproteins as ligand to detect receptors on the membrane of two cloned murine hemopoietic progenitor cell lines, B6SUT and FDCP-1. Both cell lines demonstrated membrane receptors with galactosyl and mannosyl, but not fucosyl, specificities. B6SUT galactosyl receptors showed a single receptor population with a Kd of about 2.3 X 10(-7) M and 10(6) receptors per cell. Mannosyl receptors demonstrated two components with high and low affinities respectively with Kd of about 2.5 X 10(-8) M and 1.0 X 10(-7) M, and respectively about 7.4 X 10(5) and 3.7 X 10(5) receptors per cell. Comparable data were also obtained for FDCP-1. Displacement experiments indicated that radioactive ligands bound to receptors could be increasingly displaced by homologous cold ligand giving typical sigmoid-shaped curves. Cold mannosyl probe could also displace radioactive galactosyl probe in a similar manner, but cold galactosyl probe displaced radioactive mannosyl ligand with a curve demonstrating two phases, further suggesting two receptor components for the mannosyl ligand. Mature murine neutrophils and red cells as well as human neutrophils, monocytes, and red cells showed no receptors. The functional significance of these receptors in binding to stromal cells was demonstrated by quantitation of the binding of 51Cr-labeled progenitor cells to the cloned stromal cell line, D2X, before and after enzymatic removal of various carbohydrate residues of membrane glycoconjugates. Enzymatic removal of galactosyl and mannosyl, but not fucosyl, residues almost totally eliminated the binding. The findings strongly suggest that these homing receptors are present on the surface of early hemopoietic progenitor cells. With maturation the cells lose their receptors, so that mature cells can be released into the blood stream.