Phosphatidylserine-positive erythrocytes bind to immobilized and soluble thrombospondin-1 via its heparin-binding domain
Bioethics and Medical Ethics
Medicine and Health Sciences
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AbstractPhosphatidylserine (PS)-dependent erythrocyte adhesion to endothelium and sub-endothelial matrix components is mediated in part via thrombospondin (TSP). While TSP exhibits multiple cell-binding domains, the PS-binding site on TSP is unknown. Since a cell-binding domain for anionic heparin is located at the amino-terminus, we hypothesized that PS-positive red cells (PS+ve-RBCs) bind to this domain. We demonstrate that both heparin and its low-molecular-weight derivative enoxaparin (0.5-50u/ml) inhibited PS+ve-RBC adhesion to immobilized TSP in a concentration-dependent manner (21-77% inhibition, P<0.05). Pre-incubation of immobilized TSP with an antibody against the heparin-binding domain blocked PS+ve-RBC adhesion to TSP. Antibodies that recognize the collagen- and the carboxy-terminal CD47-binding domain on TSP had no effect on this process. While pre-incubation of PS+ve-RBCs with TSP-peptides from the heparin-binding domain containing the specific heparin-binding motif KKTRG inhibited PS+ve-erythrocyte adhesion to matrix TSP (P<0.001), these peptides in the immobilized form supported PS-mediated erythrocyte adhesion. A TSP-peptide lacking the binding-motif neither inhibited nor supported PS+ve-RBC adhesion. Additional experiments show that soluble-TSP also interacted with PS+ve-RBCs via its heparin-binding domain. Our results demonstrate that PS-positive erythrocytes bind to both immobilized and soluble TSP via its heparin-binding domain and that both heparin and enoxaparin, at clinically relevant concentrations, block this interaction. Other studies have shown that heparin inhibited P-selectin- and soluble-TSP-mediated sickle erythrocyte adhesion to endothelial cells. Our results taken together with the previously documented findings provide a rational basis for clinical use of heparin or its low-molecular-weight derivatives as therapeutic agents in treating vaso-occlusive pain in patients with sickle cell disease.