Component Of Hematopoietic Stem Cells Niche Research Articles

Bone Marrow Adipocytes: The Enigmatic Components of the Hematopoietic Stem Cell Niche.

Bone marrow adipocytes (BMA) exert pleiotropic roles beyond mere lipid storage and filling of bone marrow (BM) empty spaces, and we are only now beginning to understand their regulatory traits and versatility. BMA arise from the differentiation of BM mesenchymal stromal cells, but they seem to be a heterogeneous population with distinct metabolisms, lipid compositions, secretory properties and functional responses, depending on their location in the BM. BMA also show remarkable differences among species and between genders, they progressively replace the hematopoietic BM throughout aging, and play roles in a range of pathological conditions such as obesity, diabetes and anorexia. They are a crucial component of the BM microenvironment that regulates hematopoiesis, through mechanisms largely unknown. Previously considered as negative regulators of hematopoietic stem cell function, recent data demonstrate their positive support for hematopoietic stem cells depending on the experimental approach. Here, we further discuss current knowledge on the role of BMA in hematological malignancies. Early hints suggest that BMA may provide a suitable metabolic niche for the malignant growth of leukemic stem cells, and protect them from chemotherapy. Future in vivo functional work and improved isolation methods will enable determining the true essence of this elusive BM hematopoietic stem cell niche component, and confirm their roles in a range of diseases. This promising field may open new pathways for efficient therapeutic strategies to restore hematopoiesis, targeting BMA.

Regulation of hematopoiesis in endosteal microenvironments.

After birth, the hematopoietic system develops along with bone formation in mammals. Osteolineage cells are derived from mesenchymal progenitor cells, and differentiate into several types of bone-forming cells. Of the various types of cell constituents in bone marrow, osteolineage cells have been shown to play important roles in hematopoiesis. Early studies have identified osteoblasts as a hematopoietic stem cell niche component. Since that time, the role of endosteal microenvironment as a critical regulator of hematopoietic stem/progenitor cell (HSC/HPC) behavior has been appreciated particularly under stress conditions, such as cytokine-induced HSC/HPC mobilization, homing/engraftment after bone marrow transplantation, and disease models of leukemia/myelodysplasia. Recent studies revealed that the most differentiated osteolineage cells, i.e., osteocytes, play important roles in the regulation of hematopoiesis. In this review, we provide an overview of recent advances in knowledge of regulatory hematopoietic mechanisms in the endosteal area.

Understanding the role of the microenvironment during definitive hemopoietic development

Hemopoietic stem cells (HSCs) are sustained in a specific microenvironment known as the stem cell niche. Recent studies in adult bone marrow have identified osteoblasts and endothelial cells as two important supportive cell types within the niche and demonstrated that interactions between HSCs and cellular and extracellular components within the endosteal and perivascular regions are critical for HSC regulation. However, the understanding of the role of the microenvironment in definitive HSC establishment, expansion, and maintenance during embryonic development is extremely limited. This review focuses on what is known about the components of each HSC microenvironment at various developmental stages and their known functional roles.

Mesenchymal stem cells as an essential hematopoietic stem cell niche component

Hematopoietic stem cells (HSCs) are capable of self-renewal and multi-lineage differentiation, which are characteristics that enable them to maintain the stem cell population and produce large numbers of blood cells. Quiescent mammalian adult HSCs localize within the bone marrow niche, primarily in the endosteal region of the trabecular bone. These cells scarcely participate in the cell cycle, but are instead maintained by assorted niche cells within the bone marrow, including mesenchymal stem cells (MSCs). This review summarizes the crucial role(s) that MSCs play as niche cells for the support of HSCs.

Tissue Inhibitor of Metalloproteinase-3 (TIMP-3) Regulates Hematopoiesis and Bone Formation In Vivo

BackgroundTissue inhibitor of metalloproteinases-3 (TIMP-3) inhibits matrix metalloproteinases and membrane-bound sheddases. TIMP-3 is associated with the extracellular matrix and is expressed in highly remodeling tissues. TIMP-3 function in the hematopoietic system is unknown.Methodology/Principal FindingsWe now report that TIMP-3 is highly expressed in the endosteal region of the bone marrow (BM), particularly by osteoblasts, endothelial and multipotent mesenchymal stromal cells which are all important cellular components of hematopoietic stem cell (HSC) niches, whereas its expression is very low in mature leukocytes and hematopoietic stem and progenitor cells. A possible role of TIMP-3 as an important niche component was further suggested by its down-regulation during granulocyte colony-stimulating factor-induced mobilization. To further investigate TIMP-3 function, mouse HSC were retrovirally transduced with human TIMP-3 and transplanted into lethally irradiated recipients. TIMP-3 overexpression resulted in decreased frequency of B and T lymphocytes and increased frequency of myeloid cells in blood and BM, increased Lineage-negative Sca-1+KIT+ cell proliferation in vivo and in vitro and increased colony-forming cell trafficking to blood and spleen. Finally, over-expression of human TIMP-3 caused a late onset fatal osteosclerosis.Conclusions/SignificanceOur results suggest that TIMP-3 regulates HSC proliferation, differentiation and trafficking in vivo, as well as bone and bone turn-over, and that TIMP-3 is expressed by stromal cells forming HSC niches within the BM. Thus, TIMP-3 may be an important HSC niche component regulating both hematopoiesis and bone remodeling.

Osteopontin - A New Angiogenic Factor Which Correlates with Circulating Endothelial Cells in Acute Myeloid Leukemia.

Abstract 1004Poster Board I-26 Objectives:There is evidence that acute myeloid leukemia (AML) development and progression could be determined by the expression profiling of specific angiogenic proteins. Previously we have found that the number of circulating endothelial cells (CEC) and endothelial progenitors (CEP) in peripheral blood of AML patients is a good noninvasive marker of angiogenesis and correlates with tumor mass and response to treatment. Osteopontin (OPN) is hematopoietic stem cell niche component which controls tumor growth and angiogenesis. It promotes VEGF dependent tumor progression, induces endothelial cell survival and migration and modulates expression of angiopoietins. The role of OPN in AML is unknown. We evaluated the level of circulating OPN, angiopoietin-1 (ANG-1) and ANG-2 in AML patients. In addition we correlated the levels of angiogenic cytokines with CEC number, known prognostic factors and response to treatment. Methods:OPN, ANG-1 and ANG-2 were measured in plasma samples in 69 newly diagnosed AML patients with median age 53 (range 18-78 years) and 16 controls with median age 36 (range 22-73 years) using enzyme-linked immunosorbent assay (ELISA). According to median level of cytokines patients were subdivided into “low-expressers” and “high”-expressers group. CEC were evaluated by the four colour flow cytometry using a panel of previously described monoclonal antibodies. CEC deriving from angiogenic microvessels were characterized by CD36 expression. Results:The median levels of circulating OPN (162,5 pg/ml) and ANG-2 (5184 pg/ml) were significantly higher in AML patients as compared to healthy controls (70 pg/ml; p<0.0001 and 2624,3 pg/ml; p<0.001 respectively). In contrast, the level of ANG-1 in AML pts (1038 pg/ml) was lower than in control group (1710 pg/ml; p<0.002). OPN plasma level was significantly higher in AML patients with more than 50% of blasts in the bone marrow (197 pg/ml) and elevated LDH activity (170 pg/ml) compared to group with BM blasts <50% (152 pg/ml; p<0.05) and normal LDH (137,5 pg/ml; p<0.03). In analogy, we found the significant positive correlation between ANG-2, but not ANG-1, with WBC count (p=0.05), circulating blast count (CBC) (p<0.005) and LDH activity (p=0.05). There was no statistical association between the levels of OPN, ANG-1, and ANG-2, and patients' age, sex, AML subtype according FAB classification and cytogenetic risk group. Interestingly, in AML patients OPN level correlated positively with the number of CEC deriving from angiogenic microvessels (p<0.02). Moreover, we observed tendency to positive correlation between ANG-2 level and CD36+ CEC count (p=0.09). In addition, in AML we have found significant positive correlation between OPN and ANG-2 level (p<0.03). From the 49 patients receiving intensive induction chemotherapy, 28 (56%) achieved complete remission (CR). We observed that the CR rate in “low-expressers” of OPN (67%) was higher than in the “high-expressers” (33%), however a difference did not reached statistical significance. This may be due to not very large number of evaluated patients. Conclusions:The levels of circulating OPN and ANG-2, but not ANG-1, are significantly higher in AML patients than in healthy subjects and correlate with tumor burden as well as the number of “angiogenic” circulating endothelial cells. This data clearly demonstrate an important role of circulating OPN as well as ANG-2 in angiogenesis and AML biology. A better understanding of precise functions of OPN and angiopoietins may create new options for therapeutic interventions in AML. Disclosures:Robak:Celgene: Consultancy; Roche: Honoraria, Research Funding; Genmab: Research Funding; Cambridge Antibody Technology: Research Funding; GlaxoSmithKline: Honoraria.

Systemic Endocrine Instigation of Indolent Tumor Growth Requires Osteopontin

The effects of primary tumors on the host systemic environment and resulting contributions of the host to tumor growth are poorly understood. Here, we find that human breast carcinomas instigate the growth of otherwise-indolent tumor cells, micrometastases, and human tumor surgical specimens located at distant anatomical sites. This systemic instigation is accompanied by incorporation of bone-marrow cells (BMCs) into the stroma of the distant, once-indolent tumors. We find that BMCs of hosts bearing instigating tumors are functionally activated prior to their mobilization; hence, when coinjected with indolent cells, these activated BMCs mimic the systemic effects imparted by instigating tumors. Secretion of osteopontin by instigating tumors is necessary for BMC activation and the subsequent outgrowth of the distant otherwise-indolent tumors. These results reveal that outgrowth of indolent tumors can be governed on a systemic level by endocrine factors released by certain instigating tumors, and hold important experimental and therapeutic implications.

Characterization and localization of side population (SP) cells in zebrafish kidney hematopoietic tissue

AbstractWe previously showed that side population (SP) cells, characterized by specific Hoechst dye efflux pattern in flow cytometric analysis, were present in teleost kidney hematopoietic tissue, and that kidney SP cells were enriched in hematopoietic stem cells (HSCs). ABCG2/Abcg2 is an ATP-binding cassette (ABC) transporter that is known to be associated with Hoechst dye efflux activity of mammalian HSCs. In the present study, we examined the expression and function of Abcg2 in kidney SP cells from zebrafish (Danio rerio). Although the zebrafish genome has 4 paralogous copies of ABCG2 (zAbcg2a, b, c, and d), zAbcg2a and zAbcg2c mRNA was expressed in kidney SP cells. Transfection of COS-7 cells with zAbcg2a and zAbcg2c showed that zAbcg2a was directly associated with the SP phenotype. These results indicate that zAbcg2a mRNA is a useful marker for zebrafish HSCs. In situ hybridization in kidney tissue showed that zAbcg2a-positive cells were sporadically localized on the surface of renal tubules, and tightly adhered to renal tubule epithelial cells. This result suggests that teleost HSCs adhere to the surface of renal tubules, and that renal tubule epithelial cells are a key component of HSC niche in teleosts.

Osteopontin is a hematopoietic stem cell niche component that negatively regulates stem cell pool size

Stem cells reside in a specialized niche that regulates their abundance and fate. Components of the niche have generally been defined in terms of cells and signaling pathways. We define a role for a matrix glycoprotein, osteopontin (OPN), as a constraining factor on hematopoietic stem cells within the bone marrow microenvironment. Osteoblasts that participate in the niche produce varying amounts of OPN in response to stimulation. Using studies that combine OPN-deficient mice and exogenous OPN, we demonstrate that OPN modifies primitive hematopoietic cell number and function in a stem cell–nonautonomous manner. The OPN-null microenvironment was sufficient to increase the number of stem cells associated with increased stromal Jagged1 and Angiopoietin-1 expression and reduced primitive hematopoietic cell apoptosis. The activation of the stem cell microenvironment with parathyroid hormone induced a superphysiologic increase in stem cells in the absence of OPN. Therefore, OPN is a negative regulatory element of the stem cell niche that limits the size of the stem cell pool and may provide a mechanism for restricting excess stem cell expansion under conditions of niche stimulation.

Osteoblasts Support Early B Lymphoiesis as well as Stem Cell Proliferation and Myelopoiesis: Identification of the Mammalian Cellular Analog of the Bursa of Fabricius.

The self-renewal, survival and differentiation of hematopoietic stem cells (HSC) are greatly influenced by the activities of neighboring osteoblasts and non-osteogenic bone marrow (BM) stromal cells such as fibroblasts, endothelial cells and adipocytes. Previously, we showed that osteoblasts from human long bones support the in vitro self-renewal as well as myeloid differentiation of human CD34+ cord blood cells. Recently, Li's and Scadden's groups provided in vivo evidence indicating a primary role of trabecular osteoblasts as a major component of HSC niche and of stromal osteoblastic cells in facilitating the self-renewal of HSCs. We have now asked whether osteoblasts contribute to early lymphopoiesis as well as myelopoiesis, by measuring the cellular outpout of purified HSCs on isolated osteoblasts alone, or with added non-osteoblast stromal cytokines as well. We prepared mature osteoblasts, as monitored and confirmed by homogeneous OPN and CD61 expression, by pretreating osteoblastic cells isolated from neonatal calvaria of C57BL/6 mice (CD45.2) with 1X10−7 M PTH. Purified OB were then co-cultured for 6 days with Lin− BM cells (CD45.1+) isolated from congenic B6 mice(CD45.1) and labeled with CFSE. Osteoblast coculture stimulated the proliferation of Lin− CD45.1+ BM cells 50-fold during culture, with most cells (87%) remaining tightly adherent to the osteoblast monolayer; no live cells were recovered from Lin− BM cell culture without osteoblasts. In addition to mature granulocytes/monocytes, a substantial amount of CD45.1+B220+ B lymphocytes (about 10% of small size cells gated by forward and side scatter), were detected. In contrast, very few CD45.1+Lin-Sca-1+c-Kit+ (LSK) cells or CD45.1+Lin−Sca-1−c-Kit+ (CMP) cells were detected under these conditions. Most B220+ cells attached to osteoblasts were found to be CD43+CD24+ pre-B cells undergoing division. In contrast to the cells recovered attached to the osteoblasts, the pre-B lymphocytes found in suspension were more mature with phenotype of B220+CD43−CD24+. Prevention of direct contact of Lin− BM cells with osteoblasts by Transwell co-culture abrogated the production of pre-B cells in both adherent and suspension compartments, indicating that physical contact is required for the interaction. Interestingly, when 20ng/ml of SCF, 6ng/ml of IL-3, 10ng/ml of IL-6 and 25ng/ml TPO were added to osteoblast/Lin− cell co-culture, B lymphpoiesis was repressed, while the production of CD45.1+LSK HSCs and CMPs was significantly enhanced. These data demonstrate a direct role of osteoblasts in inducing and supporting the early development of B lymphocytes from HSCs or/and common lymphoid progenitors. Additional cytokines, perhaps provided in specific in vivo niches by non-osteogenic stromal cells, cooperate with the stimulatory signals from osteoblasts to promote the survival and expansion of HSCs. Taken together, these results suggest that osteoblasts may be the mammalian analog of the avian Bursa of Fabricius, and that their local degree of proximity to non-osteogenic stromal cells may define specific microniches for stem cell survival, myelopoiesis and/or B lymphopoiesis.