A novel role for PECAM-1 in megakaryocytokinesis and recovery of platelet counts in thrombocytopenic mice

Abstract

During thrombopoiesis, maturing megakaryocytes (MKs) migrate within the complex bone marrow stromal microenvironment from the proliferative osteoblastic niche to the capillary-rich vascular niche where proplatelet formation and platelet release occurs. This physiologic process involves proliferation, differentiation, migration, and maturation of MKs before platelet production occurs. In this study, we report a role for the glycoprotein PECAM-1 in thrombopoiesis. We show that following induced thrombocytopenia, recovery of the peripheral platelet count is impaired in PECAM-1-deficient mice. Whereas MK maturation, proplatelet formation, and platelet production under in vitro conditions were unaffected, we identified a migration defect in PECAM-1-deficient MKs in response to a gradient of stromal cell-derived factor 1 (SDF1), a major chemokine regulating MK migration within the bone marrow. This defect could be explained by defective PECAM-1 ؊/؊ MK polarization of the SDF1 receptor CXCR4 and an increase in adhesion to immobilized bone marrow matrix proteins that can be ex- IntroductionMegakaryocytopoiesis involves proliferation and differentiation of megakaryocyte (MK) progenitors to a large, terminally differentiated cell with a multi-lobulated, polyploid nucleus. Nuclear maturation, a process known as endoreplication, proceeds in concert with cytoplasmic maturation and expression of platelet surface markers including the glycoprotein receptors IIb/IIIa, GPIb, GPIX, and GPVI. As the MK matures and differentiates, it migrates to sinusoidal bone marrow endothelial cells where it forms transendothelial projections called proplatelets that release 1000 to 5000 platelets per MK into the intravascular space. [1][2][3][4][5] Thrombopoietin (TPO) participates in the humoral regulation of thrombopoiesis. TPO is produced constitutively in the liver and by bone marrow stromal cells and its levels are regulated by binding to the receptor c-Mpl expressed on platelets. 6 This has the net effect of reducing the concentration of TPO in the circulation and thereby inhibiting differentiation of progenitor cells along the MK lineage. Thus, megakaryocytopoiesis and platelet count are directly regulated by circulating TPO. 2 A key step in thrombopoiesis is migration of maturing MKs from the proliferative osteoblastic niche within the bone marrow microenvironment, where hematopoietic stem cells reside, to the capillary-rich vascular niche, where proplatelets are formed. 6 This process is regulated by a variety of chemokines and cytokines, as well as by adhesive interactions with interstitial cells and extracellular matrix proteins. For example, the chemokine SDF1 directs movement of MK progenitors through its receptor CXCR4 from the proliferative "osteoblastic niche" to the "vascular niche," where platelets are formed. 6 SDF1 therefore acts in combination with TPO to promote differentiation of MK progenitor cells to mature MKs. 7,8 In addition, SDF1 promotes the interaction and transmigration of mature MKs through bone marrow endothel...

The pericyte and stromal cell marker CD248 (endosialin) is required for efficient lymph node expansion

“…Dunn chambers (Weber Scientific International) were used as described previously [22]. Briefly, 100 ng/mL PDGF (Sigma) was used in the outer chamber for transfected MG63 cell lines.…”

Platelet power: sticky problems for sticky parasites?

“…Furthermore, although not primary receptors involved in binding, the recruitment of other receptors after initial tethering could nonetheless be important for stabilization of the platelet-infected erythrocyte complex or for triggering functions from them, as is known for other immunological synapses. PECAM-1 is particularly interesting in this respect because it is known to be a ligand for the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family of variant surface antigens [9], binds glycosaminoglycans [10] and has been shown to inhibit platelet responses [11,12], suggesting that PECAM-1 triggering might be advantageous to the parasite.…”

“…Furthermore, although not primary receptors involved in binding, the recruitment of other receptors after initial tethering could nonetheless be important for stabilization of the platelet-infected erythrocyte complex or for triggering functions from them, as is known for other immunological synapses. PECAM-1 is particularly interesting in this respect because it is known to be a ligand for the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family of variant surface antigens [9], binds glycosaminoglycans [10] and has been shown to inhibit platelet responses [11,12], suggesting that PECAM-1 triggering might be advantageous to the parasite.These issues certainly need to be explored, and the availability of increasing numbers of mice deficient in various platelet-adhesion receptors and ligands might provide novel insights into the role of platelets in protection from malaria, especially under hydrodynamic shear flow in the bloodstream [13]. In addition, many of these receptors (including CD36 and gC1qR/HABP1/p32) are expressed by other important immune cells, including dendritic cells, neutrophils and B cells.…”

An alternatively spliced isoform of PECAM-1 is expressed at high levels in human and murine tissues, and suggests a novel role for the C-terminus of PECAM-1 in cytoprotective signaling

“…It is possible that WT mouse strains express a different array of isoforms of PECAM-1 in leukocytes and/or in endothelial cells, and this in part accounts for the differences in leukocyte transmigration seen between strains. PECAM-1 has also been implicated in modulating platelet signaling (Falati et al, 2006;Jones et al, 2001;Patil et al, 2001), as well as murine megakaryocytopoiesis (Dhanjal et al, 2007;Wu et al, 2007). Once again, it is not known whether certain alternatively spliced isoforms of PECAM-1 are more efficient than others at eliciting these responses.…”