Shiri Gur-Cohen

Distinct bone marrow blood vessels differentially regulate haematopoiesis.

Bone marrow endothelial cells (BMECs) form a network of blood vessels that regulate both leukocyte trafficking and haematopoietic stem and progenitor cell (HSPC) maintenance. However, it is not clear how BMECs balance these dual roles, and whether these events occur at the same vascular site. We found that mammalian bone marrow stem cell maintenance and leukocyte trafficking are regulated by distinct blood vessel types with different permeability properties. Less permeable arterial blood vessels maintain haematopoietic stem cells in a low reactive oxygen species (ROS) state, whereas the more permeable sinusoids promote HSPC activation and are the exclusive site for immature and mature leukocyte trafficking to and from the bone marrow. A functional consequence of high permeability of blood vessels is that exposure to blood plasma increases bone marrow HSPC ROS levels, augmenting their migration and differentiation, while compromising their long-term repopulation and survival. These findings may have relevance for clinical haematopoietic stem cell transplantation and mobilization protocols.

Monocytes-macrophages that express α-smooth muscle actin preserve primitive hematopoietic cells in the bone marrow

Hematopoietic stem and progenitor cells (HSPCs) are regulated by various bone marrow stromal cell types. Here we identified rare activated bone marrow monocytes and macrophages with high expression of α-smooth muscle actin (α-SMA) and the cyclooxygenase COX-2 that were adjacent to primitive HSPCs. These myeloid cells resisted radiation-induced cell death and further upregulated COX-2 expression under stress conditions. COX-2-derived prostaglandin E2 (PGE2) prevented HSPC exhaustion by limiting the production of reactive oxygen species (ROS) via inhibition of the kinase Akt and higher stromal-cell expression of the chemokine CXCL12, which is essential for stem-cell quiescence. Our study identifies a previously unknown subset of α-SMA+ activated monocytes and macrophages that maintain HSPCs and protect them from exhaustion during alarm situations.

Reactive Oxygen Species Regulate Hematopoietic Stem Cell Self-Renewal, Migration and Development, As Well As Their Bone Marrow Microenvironment

SIGNIFICANCE Blood forming, hematopoietic stem cells (HSCs) mostly reside in the bone marrow in a quiescent, nonmotile state via adhesion interactions with stromal cells and macrophages. Quiescent, proliferating, and differentiating stem cells have different metabolism, and accordingly different amounts of intracellular reactive oxygen species (ROS). Importantly, ROS is not just a byproduct of metabolism, but also plays a role in stem cell state and function. RECENT ADVANCES ROS levels are dynamic and reversibly dictate enhanced cycling and myeloid bias in ROS(high) short-term repopulating stem cells, and ROS(low) quiescent long-term repopulating stem cells. Low levels of ROS, regulated by intrinsic factors such as cell respiration or nicotinamide adenine dinucleotide phosphate-oxidase (NADPH oxidase) activity, or extrinsic factors such as stem cell factor or prostaglandin E2 are required for maintaining stem cell self-renewal. High ROS levels, due to stress and inflammation, induce stem cell differentiation and enhanced motility. CRITICAL ISSUES Stem cells need to be protected from high ROS levels to avoid stem cell exhaustion, insufficient host immunity, and leukemic transformation that may occur during chronic inflammation. However, continuous low ROS production will lead to lack of stem cell function and opportunistic infections. Ultimately, balanced ROS levels are crucial for maintaining the small stem cell pool and host immunity, both in homeostasis and during stress situations. FUTURE DIRECTIONS Deciphering the signaling pathway of ROS in HSC will provide a better understanding of ROS roles in switching HSC from quiescence to activation and vice versa, and will also shed light on the possible roles of ROS in leukemia initiation and development.

FGF-2 expands murine hematopoietic stem and progenitor cells via proliferation of stromal cells, c-Kit activation, and CXCL12 down-regulation

Cytokine-induced expansion of hematopoietic stem and progenitor cells (HSPCs) is not fully understood. In the present study, we show that whereas steady-state hematopoiesis is normal in basic fibroblast growth factor (FGF-2)-knockout mice, parathyroid hormone stimulation and myeloablative treatments failed to induce normal HSPC proliferation and recovery. In vivo FGF-2 treatment expanded stromal cells, including perivascular Nestin(+) supportive stromal cells, which may facilitate HSPC expansion by increasing SCF and reducing CXCL12 via mir-31 up-regulation. FGF-2 predominantly expanded a heterogeneous population of undifferentiated HSPCs, preserving and increasing durable short- and long-term repopulation potential. Mechanistically, these effects were mediated by c-Kit receptor activation, STAT5 phosphorylation, and reduction of reactive oxygen species levels. Mice harboring defective c-Kit signaling exhibited abrogated HSPC expansion in response to FGF-2 treatment, which was accompanied by elevated reactive oxygen species levels. The results of the present study reveal a novel mechanism underlying FGF-2-mediated in vivo expansion of both HSPCs and their supportive stromal cells, which may be used to improve stem cell engraftment after clinical transplantation.

PAR1 signaling regulates the retention and recruitment of EPCR-expressing bone marrow hematopoietic stem cells

Retention of long-term repopulating hematopoietic stem cells (LT-HSCs) in the bone marrow is essential for hematopoiesis and for protection from myelotoxic injury. We report that signaling cascades that are traditionally viewed as coagulation related also control retention of endothelial protein C receptor–positive (EPCR+) LT-HSCs in the bone marrow and their recruitment to the blood via two pathways mediated by protease activated receptor 1 (PAR1). Thrombin-PAR1 signaling induces nitric oxide (NO) production, leading to EPCR shedding mediated by tumor necrosis factor-α–converting enzyme (TACE), enhanced CXCL12-CXCR4–induced motility and rapid stem and progenitor cell mobilization. Conversely, bone marrow blood vessels provide a microenvironment enriched with activated protein C (aPC) that retains EPCR+ LT-HSCs by limiting NO generation, reducing Cdc42 activity and enhancing integrin VLA4 affinity and adhesion. Inhibition of NO production by aPC-EPCR-PAR1 signaling reduces progenitor cell egress from the bone marrow, increases retention of bone marrow NOlow EPCR+ LT-HSCs and protects mice from chemotherapy-induced hematological failure and death. Our study reveals new roles for PAR1 and EPCR in controlling NO production to balance maintenance and recruitment of bone marrow EPCR+ LT-HSCs, with potential clinical relevance for stem cell transplantation.

Physiologic corticosterone oscillations regulate murine hematopoietic stem/progenitor cell proliferation and CXCL12 expression by bone marrow stromal progenitors.

The role of corticosterone (Cort), the immune system’s major stress hormone, in the regulation of hematopoietic stem and progenitor cells (HSPCs) and their dynamic bone marrow (BM) microenvironment is currently unknown. We report that corticotropin-releasing factor receptor 1 (CRFR1) mutant mice with chronically low Cort levels showed aberrant HSPC regulation, having higher HSPC numbers and upregulation of the chemokine CXCL12, phenotypes that were restored by Cort supplementation. Expanded stromal progenitors known to support HSPCs were also observed in these low-Cort-containing mice. A similar phenotype was induced in wild-type (WT) mice by Metyrapone, a Cort synthesis inhibitor. Conversely, high Cort exposure induced HSPC apoptosis, reduced long-term BM repopulation and decreased stromal progenitor cell numbers. We documented circadian oscillations of Cort in WT BM but not in CRFR1 mutant mice, leading to diminished circadian BM CXCL12 fluctuations and increased number of circulating HSPCs in these mice. Finally, low Cort induced expansion of stromal progenitors, CXCL12 expression, HSPC proliferation and BM repopulation capacity, involving Notch1 signaling. This was associated with upregulation of the Notch ligand, Jagged1, in BM myeloid cells. Our results suggest that daily physiologic Cort oscillations are critical for balanced HSPC proliferation and function involving Notch1 signaling and their supportive BM microenvironment.

Fibroblast growth factor signaling promotes physiological bone remodeling and stem cell self-renewal.

Purpose of reviewFibroblast growth factor (FGF) signaling activates many bone marrow cell types, including various stem cells, osteoblasts, and osteoclasts. However, the role of FGF signaling in regulation of normal and leukemic stem cells is poorly understood. This review highlights the physiological roles of FGF signaling in regulating bone marrow mesenchymal and hematopoietic stem and progenitor cells (MSPCs and HSPCs) and their dynamic microenvironment. In addition, this review summarizes the recent studies which provide an overview of FGF-activated mechanisms regulating physiological stem cell maintenance, self-renewal, and motility. Recent findingsCurrent results indicate that partial deficiencies in FGF signaling lead to mild defects in hematopoiesis and bone remodeling. However, FGF signaling was shown to be crucial for stem cell self-renewal and for proper hematopoietic poststress recovery. FGF signaling activation was shown to be important also for rapid AMD3100 or post 5-fluorouracil-induced HSPC mobilization. In vivo, FGF-2 administration successfully expanded both MSPCs and HSPCs. FGF-induced expansion was characterized by enhanced HSPC cycling without further exhaustion of the stem cell pool. In addition, FGF signaling expands and remodels the supportive MSPC niche cells. Finally, FGF signaling is constitutively activated in many leukemias, suggesting that malignant HSPCs exploit this pathway for their constant expansion and for remodeling a malignant-supportive microenvironment. SummaryThe summarized studies, concerning regulation of stem cells and their microenvironment, suggest that FGF signaling manipulation can serve to improve current clinical stem cell mobilization and transplantation protocols. In addition, it may help to develop therapies specifically targeting leukemic stem cells and their supportive microenvironment.

MT1-MMP and RECK: opposite and essential roles in hematopoietic stem and progenitor cell retention and migration

Migratory capacity is a fundamental property of hematopoietic stem and progenitor cells (HSPCs). This feature is employed in clinical mobilization of HSPCs to the circulation and constitutes the basis for modern bone marrow (BM) transplantation procedures which are routinely used to treat hematological malignancies. Therefore, characterization of new players in the complex process of HSPC motility in steady-state conditions as well as during stress situations is a major challenge. We report that while the metalloproteinase membrane type 1-metalloprotease (MT1-MMP) has an essential role in human HSPC trafficking during granulocyte colony-stimulating factor (G-CSF)-induced mobilization, its inhibitor reversion-inducing cysteine-rich protein with Kazal motifs (RECK) and the adhesion molecule CD44 are required for HSPC retention to the BM in steady-state conditions. The nervous system via Wnt signaling along with HGF/c-Met signaling and the complement cascade play a major role in regulating MT1-MMP increased activity, CD44 cleavage, and RECK-reduced expression during G-CSF-induced mobilization. This review will elaborate on the opposite roles of MT1-MMP and RECK in HSPC migration and retention and suggest targeting them in order to facilitate HSPC mobilization and engraftment upon BM transplantation in patients.

Regulation of long-term repopulating hematopoietic stem cells by EPCR/PAR1 signaling.

The common developmental origin of endothelial and hematopoietic cells is manifested by coexpression of several cell surface receptors. Adult murine bone marrow (BM) long‐term repopulating hematopoietic stem cells (LT‐HSCs), endowed with the highest repopulation and self‐renewal potential, express endothelial protein C receptor (EPCR), which is used as a marker to isolate them. EPCR/protease‐activated receptor‐1 (PAR1) signaling in endothelial cells has anticoagulant and anti‐inflammatory roles, while thrombin/PAR1 signaling induces coagulation and inflammation. Recent studies define two new PAR1‐mediated signaling cascades that regulate EPCR+ LT‐HSC BM retention and egress. EPCR/PAR1 signaling facilitates LT‐HSC BM repopulation, retention, survival, and chemotherapy resistance by restricting nitric oxide (NO) production, maintaining NOlow LT‐HSC BM retention with increased VLA4 expression, affinity, and adhesion. Conversely, acute stress and clinical mobilization upregulate thrombin generation and activate different PAR1 signaling that overcomes BM EPCR+ LT‐HSC retention, inducing their recruitment to the bloodstream. Thrombin/PAR1 signaling induces NO generation, TACE‐mediated EPCR shedding, and upregulation of CXCR4 and PAR1, leading to CXCL12‐mediated stem and progenitor cell mobilization. This review discusses new roles for factors traditionally viewed as coagulation related, which independently act in the BM to regulate PAR1 signaling in bone‐ and blood‐forming progenitor cells, navigating their fate by controlling NO production.

MicroRNA-155 promotes G-CSF-induced mobilization of murine hematopoietic stem and progenitor cells via propagation of CXCL12 signaling

MicroRNA-155 promotes G-CSF-induced mobilization of murine hematopoietic stem and progenitor cells via propagation of CXCL12 signaling