Hemostasis keeps the stem cell niche in order

In this issue of Blood, Dhahri and colleagues demonstrate that the fibrinolytic system not only drives hematopoietic regeneration but also contributes to stress adaption of stem cell niches by expanding mesenchymal stromal cells (MSCs) through endothelial cKit signaling.¹ 

The bone marrow (BM) preserves long-term repopulating hematopoietic stem cells (LT-HSCs) for life-long blood cell production and immunity. Hematopoietic stem cells (HSCs) reside predominantly in perivascular niches formed by sinusoidal endothelial and mesenchymal cells.²  The endosteum of highly vascularized trabecular bone is enriched in mesenchymal progenitors and osteoblasts that also support LT-HSCs in particular after irradiation-induced BM stress. In adult hematopoiesis, the majority of LT-HSCs remain in a quiescent, nonmotile state supported by several factors provided by BM mesenchymal cells. These adhesive interactions of matrix- or stromal cell–expressed ligands for HSC integrins are enforced by paracrine signals, including mesenchymal cell-expressed stem cell factor (Kit ligand, KitL) and stromal cell derived factor 1 (CXC-type chemokine receptor 12, CXCL12) interacting with cKit and CXCR4 on HSC, respectively. Emerging evidence further suggests that the localization of HSCs to vascular niches is not uniform, but rather dynamically changes in response to stress and hematopoietic demand to assure HSC maintenance, trafficking, and proliferation.

Activation of the fibrinolytic system and plasminogen facilitates regeneration after injury, hemostatic clot formation, or thrombosis. Plasmin is the key protease that removes fibrin, but it also activates matrix metalloproteinase-9 (MMP-9), supporting matrix remodeling. In the context of myelosuppression, activation of plasminogen by tissue-type plasminogen activator (tPA) is crucial for hematopoietic progenitor and stem cells (HPSCs) to restore multilineage hematopoiesis and for animals to survive myeloablative BM stress. MMP-9 promotes the release of KitL and thereby HPSC progenitor cell proliferation and differentiation.³  In granulocyte colony-stimulating factor–induced HPSC mobilization, the plasminogen–MMP-9 axis furthermore decreases BM CXCL12 levels and conversely increases HPSC CXCR4 expression required for HPSC mobility.⁴ 

Dhahri and colleagues now demonstrate an additional facet by which the fibrinolytic system shapes the BM niche for HSC maintenance. They observed that murine BM CD45⁻/TER119⁻/Sca-1⁺/LeptinR⁺/platelet-derived growth factor receptor α⁺ (PDGFRα⁺) MSCs were expanded in number in mice deficient in the major inhibitor of tPA, ie, plasminogen activator inhibitor 1. Administration of the clinically used tPA, but not of urokinase-type plasminogen activator, to wild-type mice was shown to expand MSCs through activation of plasminogen and MMP-9. This plasmin pathway was triggered by the known MMP-9–mediated release of KitL from MSCs that then engages the endothelium, rather than HPSCs, to indirectly promote MSC proliferation. Using a species-mismatched coculture system, the authors provide evidence that activated cKit⁺ endothelial cells secrete the MSC growth factors, platelet-derived growth factor-BB (PDGF-BB), and fibroblast growth factor 2 (FGF2) (see figure). These 2 growth factors, but not the epidermal growth factor that was induced by tPA stimulation of BM mononuclear fractions, synergize to upregulate PDGFRα expression in MSCs. However, blockade of FGF2 did not prevent the expansion of MSCs in vivo, indicating additional complexity. In addition, tPA treatment also expanded BM endothelial cells, albeit not as pronounced as seen with MSCs.

This study provides new insight into stress adaptation of the BM microenvironment through a crosstalk of 2 major stromal cell types, MSCs and endothelial cells. Successful maintenance of HPSCs in the vascular niche involves additional players. In particular, megakaryocytes have emerged as important niche cells that maintain HSC quiescence during homeostasis. Megakaryocyte-derived transforming growth factor β1 (TGFβ1) acts on HSC, as does the CXC motif ligand 4 (CXCL4, platelet factor 4), regulating HSC cell cycle activity.^5,6  In chemotherapy-induced BM stress, megakaryocytes play a critical role for HPSC regeneration, demonstrating that HSC progeny are central modulators and niche components during the adaptations of the hematopoiesis to stress and injury conditions.⁵  Megakaryocytes have also been shown to synthesize coagulation factors and thus yield thrombin that cleaves osteopontin, a specific ligand for HSC-expressed integrin α4β1.⁷  This modification of the extracellular matrix environment by thrombin renders HSC quiescent, indicating that coagulation as well as fibrinolysis participates in remodeling of the BM microenvironment for HSC maintenance under stress conditions.

The coagulation system has even broader roles in the regulation of HSC quiescence and mobilization. In coagulation factor VIII–deficient mice, the trabecular bone structure is altered, and reduced thrombin production in hemophilic mice can influence HPSC mobilization.⁸  The balance between coagulation activation and its control by the anticoagulant pathway appears to be particularly important to maintain HSC quiescence or induce their mobilization. HSCs express the anticoagulant receptor thrombomodulin that in vessel wall cell typically sequesters thrombin and thereby enables thrombin-thrombomodulin–mediated activation of the anticoagulant protease protein C (PC). Therapeutic application of anticoagulant signals by infusion of soluble thrombomodulin or activated PC (aPC) markedly improves the survival of mice from radiation injury.⁹  HSCs also express the endothelial protein C receptor (EPCR, Procr), a stem cell marker found also in other stem cells and a coreceptor for aPC-biased agonist signaling through PAR1.¹⁰  EPCR-aPC-PAR1 signaling regulates HSC cdc42 activity and downstream α4β1-dependent adhesion and thus preserves HSC during myeloablative stress. This maintenance of LT-HSC quiescence by anticoagulant proteases is counteracted by thrombin that induces metalloproteinase-dependent shedding of EPCR, cdc42 activation, and HSC motility.

These studies document the diverse effects by which the hemostatic system regulates the BM niche and hematopoiesis. Activation of these pathways during injury, stress, and infection allows for a rapid response of hematopoiesis to fulfill increased demand. Reconstitution of the stressed BM with megakaryocytes, HSC signaling by anticoagulant proteases, and the demonstrated expansion of stromal cells by the fibrinolytic system likely serve in a coordinated effort to rebalance hematopoiesis and assure maintenance of LT-HSC when stress factors are resolved. Strengthening these pathways may benefit recovery from stem cell transplantation, radiation therapy, and chemotherapy.