Wnt signaling regulates hemopoiesis through stromal cells

Yolk sac development, function and role in rodent pregnancy

During the early phases of embryonic development, the yolk sac serves as an initial placenta in many animal species. While in some, this role subsides around the end of active organogenesis, it continues to have important functions in rodents, alongside the chorio-allantoic placenta. The yolk sac is the initial site of hematopoiesis in many animal species including primates. Cells of epiblastic origin form blood islands that are the forerunners of hematopoietic cells and of the primitive endothelial cells that form the vitelline circulation. The yolk sac is also a major route of embryonic and fetal nutrition apparently as long as it functions. In mammals and especially rodents, macro and micronutrients are absorbed by active pinocytosis into the visceral yolk sac, degraded and the degradation products (i.e., amino acids) are then transferred to the embryo. Interference with the yolk sac function may directly reflect on embryonic growth and development, inducing congenital malformations or in extreme damage, causing embryonic and fetal death. In rodents, many agents were found to damage the yolk sac (i.e., anti-yolk sac antibodies or toxic substances interfering with yolk sac pinocytosis) subsequently affecting the embryo/fetus. Often, the damage to the yolk sac is transient while embryonic damage persists. In humans, decreased yolk sac diameter was associated with diabetic pregnancies and increased diameter was associated with pregnancy loss. In addition, culture of rat yolk sacs in serum obtained from pregnant diabetic women or from women with autoimmune diseases induced severe damage to the visceral yolk sac epithelium and embryonic malformations. It can be concluded that as a result of the crucial role of the yolk sac in the well-being of the early embryo, any damage to its normal function may severely and irreversibly affect further development of the embryo/fetus.

Wnt regulation of hematopoietic stem cell development and disease

Hematopoietic stem cells (HSCs) are multipotent stem cells that give rise to all cells of the blood and most immune cells. Due to their capacity for unlimited self-renewal, long-term HSCs replenish the blood and immune cells of an organism throughout its life. HSC development, maintenance, and differentiation are all tightly regulated by cell signaling pathways, including the Wnt pathway. Wnt signaling is initiated extracellularly by secreted ligands which bind to cell surface receptors and give rise to several different downstream signaling cascades. These are classically categorized either β-catenin dependent (BCD) or β-catenin independent (BCI) signaling, depending on their reliance on the β-catenin transcriptional activator. HSC development, homeostasis, and differentiation is influenced by both BCD and BCI, with a high degree of sensitivity to the timing and dosage of Wnt signaling. Importantly, dysregulated Wnt signals can result in hematological malignancies such as leukemia, lymphoma, and myeloma. Here, we review how Wnt signaling impacts HSCs during development and in disease.

Only the Co-Transcriptional Activity of β-Catenin Is Required for the Local Regulatory Effects in Hypertrophic Chondrocytes on Developmental Bone Modeling

In hypertrophic chondrocytes, β-catenin has two roles. First, it locally suppresses the differentiation of osteoclasts at the chondro-osseous junction by maintaining the pro-osteoclastic factor receptor activator of NF-κB ligand (RANKL) at low levels. Second, it promotes the differentiation of osteoblast-precursors from chondrocytes. Yet, β-catenin is a dual-function protein, which can either participate in cell-cell adherens junctions or serve as a transcriptional co-activator in canonical Wnt signaling interacting with T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) transcription factors. Hence, whenever studying tissue-specific requirements of β-catenin using a conventional conditional knockout approach, the functional mechanisms underlying the defects in the conditional mutants remain ambiguous. To decipher mechanistically which of the two molecular functions of β-catenin is required in hypertrophic chondrocytes, we used different approaches. We analyzed the long bones of newborn mice carrying either the null-alleles of Lef1 or Tcf7, or mice in which Tcf7l2 was conditionally deleted in the hypertrophic chondrocytes, as well as double mutants for Lef1 and Tcf7l2, and Tcf7 and Tcf7l2. Furthermore, we analyzed Ctnnb1 mutant newborns expressing a signaling-defective allele that retains the cell adhesion function in hypertrophic chondrocytes. None of the analyzed Tcf/Lef single or double mutants recapitulated the previously published phenotype upon loss of β-catenin in hypertrophic chondrocytes. However, using this particular Ctnnb1 allele, maintaining cell adhesion function, we show that it is the co-transcriptional activity of β-catenin, which is required in hypertrophic chondrocytes to suppress osteoclastogenesis and to promote chondrocyte-derived osteoblast differentiation. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

LRRC17 regulates the bone metabolism of human bone marrow mesenchymal stem cells from patients with idiopathic necrosis of femoral head through Wnt signaling pathways: A preliminary report

Idiopathic necrosis of the femoral head (INFH) is a common disease with unknown cause. Its successful treatment relies on the repair of the necrotic bone. The application of autologous mesenchymal stem cells (MSCs) has shown great promise in saving the patients from undergoing total hip arthroplasty. Leucine-rich repeat-containing 17 (LRRC17) is less expressed in patients with femoral head necrosis and LRRC17 can inhibit bone degradation. However, it remains unknown whether LRRC17 plays a role in the pathogenesis of INFH. The present study aimed to investigate the potential role and mechanism of LRRC17 in the pathogenesis and treatment of INFH. It was found that despite the similar cell morphology and MSC surface marker expressions of human bone marrow MSCs (BMSCs) isolated from patients with INFH (INFH-hBMSC) and femoral neck fracture (FNF) (FNF-hBMSC), INFH-hBMSC had higher percentage of apoptosis (P<0.05), as well as lower osteogenic potential and higher adipogenic potential (both P<0.05). However, there was no difference in cell proliferation between FNF-hBMSC and INFH-hBMSC (P>0.05). It was also confirmed that the expression of LRRC17 was lower in the bone tissue and hBMSCs from patients with INFH compared with patients with FNF (P<0.05). Overexpression of LRRC17 promoted osteogenesis and inhibited the adipogenesis in hBMSCs, accompanied with the increase of Wnt3a and β-catenin expressions, and the decrease of Wnt5a and receptor activator of nuclear factor κ-B ligand (Rankl) expressions (all, P<0.05). Furthermore, knockout of LRRC17 in hBMSCs inhibited the expression levels of osteogenic and promoted adipogenic markers, while decreasing Wnt3a and β-catenin expressions, and increasing Wnt5a and Rankl expressions (all, P<0.05). The present preliminary study suggested that imbalanced bone metabolism may be involved in the pathogenesis of INFH. The modulation of the LRRC17 gene may delay or even restore the balance of osteogenic and adipogenic differentiation in autologous BMSCs derived from patients with INFH, providing a new target for the treatment of INFH.

Divergent effects of Wnt5b on IL-3- and GM-CSF-induced myeloid differentiation

The multiple specialized cell types of the hematopoietic system originate from differentiation of hematopoietic stem cells and progenitors (HSPC), which can generate both lymphoid and myeloid lineages. The myeloid lineage is preferentially maintained during ageing, but the mechanisms that contribute to this process are incompletely understood. Here, we studied the roles of Wnt5a and Wnt5b, ligands that have previously been linked to hematopoietic stem cell ageing and that are abundantly expressed by both hematopoietic progenitors and bone-marrow derived niche cells. Whereas Wnt5a had no major effects on primitive cell differentiation, Wnt5b had profound and divergent effects on cytokine-induced myeloid differentiation. Remarkably, while IL-3- mediated myeloid differentiation was largely repressed by Wnt5b, GM-CSF-induced myeloid differentiation was augmented. Furthermore, in the presence of IL-3, Wnt5b enhanced HSPC self-renewal, whereas in the presence ofGM-CSF, Wnt5b accelerated differentiation, leading to progenitor cell exhaustion. Our results highlight discrepancies between IL-3 and GM-CSF, and reveal novel effects of Wnt5b on the hematopoietic system.

Wnt Signalling in Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a group of malignant diseases of the haematopoietic system. AML occurs as the result of mutations in haematopoietic stem/progenitor cells, which upregulate Wnt signalling through a variety of mechanisms. Other mechanisms of Wnt activation in AML have been described such as Wnt antagonist inactivation through promoter methylation. Wnt signalling is necessary for the maintenance of leukaemic stem cells. Several molecules involved in or modulating Wnt signalling have a prognostic value in AML. These include: β-catenin, LEF-1, phosphorylated-GSK3β, PSMD2, PPARD, XPNPEP, sFRP2, RUNX1, AXIN2, PCDH17, CXXC5, LLGL1 and PTK7. Targeting Wnt signalling for tumour eradication is an approach that is being explored in haematological and solid tumours. A number of preclinical studies confirms its feasibility, albeit, so far no reliable clinical trial data are available to prove its utility and efficacy.

Spatially Restricted Stromal Wnt Signaling Restrains Prostate Epithelial Progenitor Growth through Direct and Indirect Mechanisms

Cell-autonomous Wnt signaling has well-characterized functions in controlling stem cell activity, including in the prostate. While niche cells secrete Wnt ligands, the effects of Wnt signaling in niche cells per se are less understood. Here, we show that stromal cells in the proximal prostatic duct near the urethra, a mouse prostate stem cell niche, not only produce multiple Wnt ligands but also exhibit strong Wnt/β-catenin activity. The non-canonical Wnt ligand Wnt5a, secreted by proximal stromal cells, directly inhibits proliefration of prostate epithelial stem or progenitor cells whereas stromal cell-autonomous canonical Wnt/β-catenin signaling indirectly suppresses prostate stem or progenitor activity via the transforming growth factor β (TGFβ) pathway. Collectively, these pathways restrain the proliferative potential of epithelial cells in the proximal prostatic ducts. Human prostate likewise exhibits spatially restricted distribution of stromal Wnt/β-catenin activity, suggesting a conserved mechanism for tissue patterning. Thus, this study shows how distinct stromal signaling mechanisms within the prostate cooperate to regulate tissue homeostasis.

Alterations in WNT Signaling in Leukemias

The WNT/β-catenin signaling pathway plays an important role in the differentiation and proliferation of hematopoietic cells. In recent years, special attention has been paid to the role of impairments in the WNT signaling pathway in pathogenesis of malignant neoplasms of the hematopoietic system. Disorders in the WNT/β-catenin signaling in leukemias identified to date include hypersensitivity to the WNT ligands, epigenetic repression of WNT antagonists, overexpression of WNT ligands, impaired β-catenin degradation in the cytoplasm, and changes in the activity of the TCF/Lef transcription factors. At the molecular level, these impairments involve overexpression of the FZD protein, hypermethylation of the SFRP, DKK, WiF, Sox, and CXXC gene promoters, overexpression of Lef1 and plakoglobin, mutations in GSK3β, and β-catenin phosphorylation by the BCR-ABL kinase. This review is devoted to the systematization of these data.

Mouse Yolk Sac Hematopoiesis

The yolk sac is the first observed site of hematopoiesis during mouse ontogeny. Primitive erythroid cells are the most well-recognized cell lineages produced from this tissue. In addition to primitive erythroid cells, several types of hematopoietic cells are present, including multipotent hematopoietic progenitors. Yolk sac-derived blood cells constitute a transient wave of embryonic and fetal hematopoiesis. However, recent studies have demonstrated that some macrophage and B cell lineages derived from the early yolk sac may persist to adulthood. This review discusses the cellular basis of mouse yolk sac hematopoiesis and its contributions to embryonic and adult hematopoietic systems.

Novel therapeutic strategies to target leukemic cells that hijack compartmentalized continuous hematopoietic stem cell niches

Acute myeloid leukemia and acute lymphoblastic leukemia cells hijack hematopoietic stem cell (HSC) niches in the bone marrow and become leukemic stem cells (LSCs) at the expense of normal HSCs. LSCs are quiescent and resistant to chemotherapy and can cause relapse of the disease. HSCs in niches are needed to generate blood cell precursors that are committed to unilineage differentiation and eventually production of mature blood cells, including red blood cells, megakaryocytes, myeloid cells and lymphocytes. Thus far, three types of HSC niches are recognized: endosteal, reticular and perivascular niches. However, we argue here that there is only one type of HSC niche, which consists of a periarteriolar compartment and a perisinusoidal compartment. In the periarteriolar compartment, hypoxia and low levels of reactive oxygen species preserve the HSC pool. In the perisinusoidal compartment, hypoxia in combination with higher levels of reactive oxygen species enables proliferation of progenitor cells and their mobilization into the circulation. Because HSC niches offer protection to LSCs against chemotherapy, we review novel therapeutic strategies to inhibit homing of LSCs in niches for the prevention of dedifferentiation of leukemic cells into LSCs and to stimulate migration of leukemic cells out of niches. These strategies enhance differentiation and proliferation and thus sensitize leukemic cells to chemotherapy. Finally, we list clinical trials of therapies that tackle LSCs in HSC niches to circumvent their protection against chemotherapy.

Human NK cell development requires CD56-mediated motility and formation of the developmental synapse

While distinct stages of natural killer (NK) cell development have been defined, the molecular interactions that shape human NK cell maturation are poorly understood. Here we define intercellular interactions between developing NK cells and stromal cells which, through contact-dependent mechanisms, promote the generation of mature, functional human NK cells from CD34(+) precursors. We show that developing NK cells undergo unique, developmental stage-specific sustained and transient interactions with developmentally supportive stromal cells, and that the relative motility of NK cells increases as they move through development in vitro and ex vivo. These interactions include the formation of a synapse between developing NK cells and stromal cells, which we term the developmental synapse. Finally, we identify a role for CD56 in developmental synapse structure, NK cell motility and NK cell development. Thus, we define the developmental synapse leading to human NK cell functional maturation.

Noncanonical Wnt signaling in stromal cells regulates B-lymphogenesis through interleukin-7 expression

The regulation of early B cell development and the interaction of hematopoietic precursors with stromal cells in the bone marrow (BM) are controlled by various secreted signaling molecules. Several recent studies showed Wnt signaling involved in B-lymphogenesis through stromal cells. However, the molecules modulated by Wnt signaling in stromal cells regulating B-lymphogenesis have not been identified yet. Interleukin (IL)-7 and CXC chemokine ligand (CXCL) 12 are known to be express in stromal cells, and both molecules are essential for B-lymphogenesis. In the present study, we examined the role of Wnt signaling in regulating IL-7 and CXCL12 expression and in affecting B-lymphogenesis. In mouse stromal ST2 cells, expression of IL-7 and CXCL12 mRNA was augmented by noncanonical Wnt5a. When mouse BM-derived cells were cultured on Wnt5a-overexpressing ST2 cells, an increased number of B220+/IgM- B-lymphoid precursor cells was observed. These results show that Wnt5a regulates IL-7 gene expression in stromal cells and suggest the possibility that noncanonical Wnt regulates B-lymphogenesis via IL-7 expression in stromal cells.

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Noncanonical Wnt signaling enhances IL-7 gene expression in ST2 cells.

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ST2 cells well maintain B cells in the coculture system.

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Noncanonical Wnt5a overexpressed ST2 cells increase the number of B cell progenitors.

Endothelial progenitor cell dysfunction in myelodysplastic syndromes: possible contribution of a defective vascular niche to myelodysplasia

We set a model to replicate the vascular bone marrow niche by using endothelial colony forming cells (ECFCs), and we used it to explore the vascular niche function in patients with low-risk myelodysplastic syndromes (MDS). Overall, we investigated 56 patients and we observed higher levels of ECFCs in MDS than in healthy controls; moreover, MDS ECFCs were found variably hypermethylated for p15INK4b DAPK1, CDH1, or SOCS1. MDS ECFCs exhibited a marked adhesive capacity to normal mononuclear cells. When normal CD34 + cells were co-cultured with MDS ECFCs, they generated significant lower amounts of CD11b + and CD41 + cells than in co-culture with normal ECFCs. At gene expression profile, several genes involved in cell adhesion were upregulated in MDS ECFCs, while several members of the Wingless and int (Wnt) pathways were underexpressed. Furthermore, at miRNA expression profile, MDS ECFCs hypo-expressed various miRNAs involved in Wnt pathway regulation. The addition of Wnt3A reduced the expression of intercellular cell adhesion molecule-1 on MDS ECFCs and restored the defective expression of markers of differentiation. Overall, our data demonstrate that in low-risk MDS, ECFCs exhibit various primary abnormalities, including putative MDS signatures, and suggest the possible contribution of the vascular niche dysfunction to myelodysplasia.

Regulation of bone metabolism by Wnt signals

Wnt ligands play a central role in the development and homeostasis of various organs through β-catenin-dependent and -independent signalling. The crucial roles of Wnt/β-catenin signals in bone mass have been established by a large number of studies since the discovery of a causal link between mutations in the low-density lipoprotein receptor-related protein 5 (Lrp5) gene and alternations in human bone mass. The activation of Wnt/β-catenin signalling induces the expression of osterix, a transcription factor, which promotes osteoblast differentiation. Furthermore, this signalling induces the expression of osteoprotegerin, an osteoclast inhibitory factor in osteoblast-lineage cells to prevent bone resorption. Recent studies have also shown that Wnt5a, a typical non-canonical Wnt ligand, enhanced osteoclast formation. In contrast, Wnt16 inhibited osteoclast formation through β-catenin-independent signalling. In this review, we discussed the current understanding of the Wnt signalling molecules involved in bone formation and resorption.

Wnt Signaling: Role in Regulation of Haematopoiesis

Hematopoietic stem cells (HSCs) are a unique population of bone marrow cells which are responsible for the generation of various blood cell lineages. One of the significant characteristics of these HSCs is to self-renew, while producing differentiating cells for normal hematopoiesis. Deregulation of self-renewal and differentiation leads to the hematological malignancies. Several pathways are known to be involved in the maintenance of HSC fate among which Wnt signaling is a crucial pathway which controls development and cell fate determination. Wnt signaling also plays a major role in differentiation, self-renewal and maintenance of HSCs. Wnt ligands activate three major pathways including planar cell polarity, Wnt/β-catenin and Wnt/Ca(2+). It has been shown that Wnt/β-catenin or canonical pathway regulates cell proliferation, survival and differentiation in HSCs, deregulation of this pathway leads to hematological malignancies. Wnt non-canonical pathway regulates calcium signaling and planar cell polarity. In this review, we discuss various signaling pathways induced by Wnt ligands and their potential role in hematopoiesis.

The regulation of osteoclast differentiation by Wnt signals

Wnt ligands activate β-catenin-dependent canonical and -independent noncanonical signaling pathways. Wnt regulates many physiological events such as the development of organs and bone metabolism. In contrast, failed signaling leads to pathological conditions including cancer and osteoporosis. Analyses of loss-of-function mutations in the low-density lipoprotein receptor-related protein (Lrp) 5 gene revealed that Lrp5 acted as a co-receptor of Wnt/β-catenin signals and positively regulated bone mass in humans and mice. Many players in Wnt signals including sclerostin, an osteocyte-derived Wnt antagonist, also have since been found to influence bone mass. Bone mass is regulated by the activities of bone-forming osteoblasts, -resorbing osteoclasts and matrix-embedded osteocytes. The roles of Wnt/β-catenin signals in osteoblastogenesis and osteoclastogenesis have been established by the findings of a large number of in vitro and in vivo studies. In contrast, the roles of noncanonical Wnt signals in bone metabolism are only now being examined. In this review, we introduced and discussed recent information on the roles of Wnt signals in bone resorption.

Wnt16 regulates osteoclast differentiation in conjunction with Wnt5a

The canonical Wnt/β-catenin signaling pathway in osteoblast-lineage cells inhibits osteoclastogenesis through the expression of osteoprotegerin (Opg), a decoy receptor of receptor activator of Nf-κb (Rank) ligands. Wnt5a, a typical non-canonical Wnt ligand, enhances the expression of Rank in osteoclast precursors, which, in turn, promotes the Rank ligand (Rankl)-induced formation of osteoclasts. In contrast, Wnt16 and Wnt4 have been shown to inhibit the Rankl-induced formation of osteoclasts through non-canonical Wnt signals. However, the relationships among these Wnt ligands in osteoclastogenesis remained to be elucidated. We herein showed that Wnt16, but not Wnt4, inhibited the Rankl-induced osteoclastogenesis in bone marrow-derived macrophage (BMM) cultures. Wnt3a and Wnt4 inhibited the 1α,25-dihydroxy vitamin D3 (1,25D3)-induced osteoclastogenesis in co-cultures prepared from wild-type mice, but not in those from Opg(-/-) nice. Wnt16 inhibited the 1,25D3-induced formation of osteoclasts in both wild-type and Opg(-/-) co-cultures. Wnt16, Wnt4, and Wnt3a failed to inhibit the pit-forming activity of osteoclasts. Wnt16 failed to inhibit the Wnt5a-induced expression of Rank in osteoclast precursors. In contrast, Wnt5a abrogated the inhibitory effects of Wnt16 on Rankl-induced osteoclastogenesis. These results suggested that Wnt16 inhibited osteoclastogenesis, but not the function of osteoclasts and that Wnt16, an inhibitory Wnt ligand for osteoclastogenesis, regulates bone resorption in conjunction with Wnt5a.

Cytokine secretion profiling of human mesenchymal stem cells by antibody array

Mesenchymal stem cells (MSCs) provide not only cell sources for connective tissues but also the control of hematopoiesis and immune response. A multitude of cytokines and growth factors secreted from MSCs are known to confer such multifunctional functionality, but their overall spectrum and the respective expression strength have not been thoroughly illustrated. In this study, we have obtained the comprehensive cytokine secretion profile of human bone marrow (BM)-derived MSCs, with the use of an antibody array recognizing 120 cytokines and chemokines. The array membrane incubated with the secretion media of the cells featured a predominant hybridization signal for IL-6 and moderately elevated signals for IL-8, TIMP-2, MCP-1, VEGF and OPG. This cytokine secretion profile was found to be common to all cell lines from three different donors, and also similar but not identical to that of umbilical cord blood-derived cells, suggesting that the trophic nature of the MSCs might depend slightly on the cell origin but not on individuality of the donors. Our results here may provide the molecular basis for further studies on MSC-assisted biological processes, such as connective tissue homeostasis, hematopoiesis and immune modulation.

Regulation of hematopoiesis by activators and inhibitors of Wnt signaling from the niche

Hematopoietic stem cells (HSCs) are a rare population of somatic stem cells that have the ability to regenerate the entire mature blood system in a hierarchical way for the duration of an adult life. Adult HSCs reside in the bone marrow niche. Different niche cell types and molecules regulate the balance of HSC dormancy and activation as well as HSC behavior in both normal and malignant hematopoiesis. Here, we describe the interplay of HSCs and their niche, in particular the involvement of the Wnt signaling pathway. Although the prevailing notion has been that malignant transformation of HSCs is the main cause of leukemia, evidence is mounting that disruption of niche regulation by transformed hematopoietic cells, which may overexpress Wnt signaling or intrinsic stromal defects in gene expression, is at least a collaborative factor in leukemogenesis. Thus, insights into the normal and altered functions of niche components will help to obtain a better understanding of normal and malignant hematopoiesis and how environmental factors affect these processes.

Earliest hematopoietic progenitors at embryonic day 9 preferentially generate B-1 B cells rather than follicular B or marginal zone B cells

The lymphoid potential of the hematopoietic system is observed as early as embryonic day 9 (E9) before transplantable hematopoietic stem cells (HSCs) appear at E11 in mice. However, it is largely unknown as to which cell fraction is responsible for the initial wave of lymphopoiesis and whether these earliest lymphocytes make any contributions to the adult lymphoid system. We previously isolated the earliest hematolymphoid progenitors at E9 that had CD45(+)c-Kit(+)AA4.1(+) phenotypes. In this study, the differentiation potency into B cell subsets of the E9 hematolymphoid progenitors was examined in detail. In culture, E9 hematolymphoid progenitors produced B220(-/low) B cell progenitors in striking contrast to adult BM c-Kit(+)Sca-1(+)Lin(-) cells. Upon in vivo transplantation, B cell progenitors derived from E9 hematolymphoid progenitors preferentially differentiated into the B-1 B lymphocyte subset, whereas their differentiation into B-2 B lymphocyte subsets [follicular B (FoB), marginal zone B (MZB) cells] was inefficient. Of note, these donor B lymphocytes permanently repopulated in host mice, even if adult mice were used as recipients. These results suggest that B cell progenitors produced from an initial wave of definitive hematopoiesis before authentic HSCs appear could be a permanent source for, at least, the B-1 B lymphocyte subset.

Roles of Wnt signals in bone resorption during physiological and pathological states

Osteoclasts, multinucleated giant cells, are responsible for bone resorption in physiological and pathological conditions such as osteoporosis and rheumatoid arthritis. Osteoclasts develop from the monocyte/macrophage lineage under the strict control of bone-forming osteoblasts. Osteoblast-lineage cells express two cytokines essential for osteoclast differentiation, colony-stimulating factor-1, and receptor activator of nuclear factor κB ligand (RANKL) and also express osteoprotegerin, a soluble decoy receptor for RANKL. The signaling molecule Wnt has been shown to be important for the differentiation of osteoblasts through β-catenin-dependent canonical and β-catenin-independent noncanonical pathways. Recent studies have established that Wnt-mediated signals are also crucial for bone resorption in both physiological and pathological conditions. In this review, we introduce recent advances in roles of Wnt signaling in bone formation and bone resorption.

Genome-wide miRNA profiling of mantle cell lymphoma reveals a distinct subgroup with poor prognosis

miRNA deregulation has been implicated in the pathogenesis of mantle cell lymphoma (MCL). Using a high-throughput quantitative real-time PCR platform, we performed miRNA profiling on cyclin D1-positive MCL (n = 30) and cyclin D1-negative MCL (n = 7) and compared them with small lymphocytic leukemia/lymphoma (n = 12), aggressive B-cell lymphomas (n = 138), normal B-cell subsets, and stromal cells. We identified a 19-miRNA classifier that included 6 up-regulated miRNAs and 13 down regulated miRNA that was able to distinguish MCL from other aggressive lymphomas. Some of the up-regulated miRNAs are highly expressed in naive B cells. This miRNA classifier showed consistent results in formalin-fixed paraffin-embedded tissues and was able to distinguish cyclin D1-negative MCL from other lymphomas. A 26-miRNA classifier could distinguish MCL from small lymphocytic leukemia/lymphoma, dominated by 23 up-regulated miRNAs in MCL. Unsupervised hierarchical clustering of MCL patients demonstrated a cluster characterized by high expression of miRNAs from the polycistronic miR17-92 cluster and its paralogs, miR-106a-363 and miR-106b-25, and associated with high proliferation gene signature. The other clusters showed enrichment of stroma-associated miRNAs, and also had higher expression of stroma-associated genes. Our clinical outcome analysis in the present study suggested that miRNAs can serve as prognosticators.

Natural killer cell differentiation from hematopoietic stem cells: a comparative analysis of heparin- and stromal cell-supported methods

Natural killer (NK) cells differentiated from hematopoietic stem cells (HSCs) may have significant clinical benefits over NK cells from adult donors, including the ability to choose alloreactive donors and potentially more robust in vivo expansion. Stromal-based methods have been used to study the differentiation of NK cells from HSCs. Stroma and cytokines support NK cell differentiation, but may face considerable regulatory hurdles. A recently reported clinical-grade, heparin-based method could serve as an alternative. How the stromal-based and heparin-based approaches compare in terms of NK cell generating efficiency or function is unknown. We show that compared with heparin-based cultures, stroma significantly increases the yield of HSC-derived NK cells by differentiating less-committed progenitors into the NK lineage. NK cells generated by both approaches were similar for most NK-activating and -inhibiting receptors. Although both approaches resulted in a phenotype consistent with CD56(bright) stage IV NK cells, heparin-based cultures favored the development of CD56(+)CD16(+) cells, whereas stroma produced more NK cell immunoglobulin-like receptor-expressing NK cells, both of which are markers of terminal maturation. At day 21, stromal-based cultures demonstrated significantly more IL-22 production, and both methods yielded similar amounts of IFN-γ production and cytotoxicity by day 35. These findings suggest that heparin-based cultures are an effective replacement for stroma and may facilitate clinical trials testing HSC-derived NK cells.

Inflammatory bone loss: pathogenesis and therapeutic intervention

Bone is a tissue undergoing continuous building and degradation. This remodelling is a tightly regulated process that can be disturbed by many factors, particularly hormonal changes. Chronic inflammation can also perturb bone metabolism and promote increased bone loss. Inflammatory diseases can arise all over the body, including in the musculoskeletal system (for example, rheumatoid arthritis), the intestine (for example, inflammatory bowel disease), the oral cavity (for example, periodontitis) and the lung (for example, cystic fibrosis). Wherever inflammatory diseases occur, systemic effects on bone will ensue, as well as increased fracture risk. Here, we discuss the cellular and signalling pathways underlying, and strategies for therapeutically interfering with, the inflammatory loss of bone.

Wnt5a-Ror2 signaling between osteoblast-lineage cells and osteoclast precursors enhances osteoclastogenesis

The signaling molecule Wnt regulates bone homeostasis through β-catenin-dependent canonical and β-catenin-independent noncanonical pathways. Impairment of canonical Wnt signaling causes bone loss in arthritis and osteoporosis; however, it is unclear how noncanonical Wnt signaling regulates bone resorption. Wnt5a activates noncanonical Wnt signaling through receptor tyrosine kinase-like orphan receptor (Ror) proteins. We showed that Wnt5a-Ror2 signaling between osteoblast-lineage cells and osteoclast precursors enhanced osteoclastogenesis. Osteoblast-lineage cells expressed Wnt5a, whereas osteoclast precursors expressed Ror2. Mice deficient in either Wnt5a or Ror2, and those with either osteoclast precursor-specific Ror2 deficiency or osteoblast-lineage cell-specific Wnt5a deficiency showed impaired osteoclastogenesis. Wnt5a-Ror2 signals enhanced receptor activator of nuclear factor-κB (RANK) expression in osteoclast precursors by activating JNK and recruiting c-Jun on the promoter of the gene encoding RANK, thereby enhancing RANK ligand (RANKL)-induced osteoclastogenesis. A soluble form of Ror2 acted as a decoy receptor of Wnt5a and abrogated bone destruction in mouse arthritis models. Our results suggest that the Wnt5a-Ror2 pathway is crucial for osteoclastogenesis in physiological and pathological environments and represents a therapeutic target for bone diseases, including arthritis.

The canonical Wnt pathway shapes niches supportive of hematopoietic stem/progenitor cells

Considerable information has accumulated about components of BM that regulate the survival, self-renewal, and differentiation of hematopoietic cells. In the present study, we investigated Wnt signaling and assessed its influence on human and murine hematopoiesis. Hematopoietic stem/progenitor cells (HSPCs) were placed on Wnt3a-transduced OP9 stromal cells. The proliferation and production of B cells, natural killer cells, and plasmacytoid dendritic cells were blocked. In addition, some HSPC characteristics were maintained or re-acquired along with different lineage generation potentials. These responses did not result from direct effects of Wnt3a on HSPCs, but also required alterations in the OP9 cells. Microarray, PCR, and flow cytometric experiments revealed that OP9 cells acquired osteoblastic characteristics while down-regulating some features associated with mesenchymal stem cells, including the expression of angiopoietin 1, the c-Kit ligand, and VCAM-1. In contrast, the production of decorin, tenascins, and fibromodulin markedly increased. We found that at least 1 of these extracellular matrix components, decorin, is a regulator of hematopoiesis: upon addition of this proteoglycan to OP9 cocultures, decorin caused changes similar to those caused by Wnt3a. Furthermore, hematopoietic stem cell numbers in the BM and spleen were elevated in decorin-knockout mice. These findings define one mechanism through which canonical Wnt signaling could shape niches supportive of hematopoiesis.

Future perspectives: therapeutic targeting of notch signalling may become a strategy in patients receiving stem cell transplantation for hematologic malignancies

The human Notch system consists of 5 ligands and 4 membrane receptors with promiscuous ligand binding, and Notch-initiated signalling interacts with a wide range of other intracellular pathways. The receptor signalling seems important for regulation of normal and malignant hematopoiesis, development of the cellular immune system, and regulation of immune responses. Several Notch-targeting agents are now being developed, including natural receptor ligands, agonistic and antagonistic antibodies, and inhibitors of intracellular Notch-initiated signalling. Some of these agents are in clinical trials, and several therapeutic strategies seem possible in stem cell recipients: (i) agonists may be used for stem cell expansion and possibly to enhance posttransplant lymphoid reconstitution; (ii) receptor-specific agonists or antagonists can be used for immunomodulation; (iii) Notch targeting may have direct anticancer effects. Although the effects of therapeutic targeting are difficult to predict due to promiscuous ligand binding, targeting of this system may represent an opportunity to achieve combined effects with earlier posttransplant reconstitution, immunomodulation, or direct anticancer effects.

Strategies to enhance umbilical cord blood stem cell engraftment in adult patients

Umbilical cord blood (UCB) has been used successfully as a source of hematopoietic stem cells (HSCs) for allogeneic transplantation in children and adults in the treatment of hematologic diseases. However, compared with marrow or mobilized peripheral blood stem cell grafts from adult donors, significant delays in the rates and kinetics of neutrophil and platelet engraftment are noted after UCB transplant. These differences relate in part to the reduced numbers of HSCs in UCB grafts. To improve the rates and kinetics of engraftment of UCB HSC, several strategies have been proposed, including ex vivo expansion of UCB HSCs, addition of third-party mesenchymal cells, intrabone delivery of HSCs, modulation of CD26 expression, and infusion of two UCB grafts. This article will focus on ex vivo expansion of UCB HSCs and strategies to enhance UCB homing as potential solutions to overcome the problem of low stem cell numbers in a UCB graft.

How the niche regulates hematopoietic stem cells

The hematopoietic stem cell (HSC) forms all types of blood cells of the hematopoietic system. In the adult, HSC are mainly quiescent, being mostly in G0/G1 phase of cell cycle during steady-state conditions. However, during hematopoietic stress, the stem cells respond quickly to regenerate the damaged hematopoietic system. To understand how environmental signals affect HSC and its progeny, it is essential to know the lineage relationships and transcriptional mechanisms controlling self-renewal, proliferation and differentiation. Because of the high possible output of blood cells from a single HSC, a tight regulation of these processes is extremely important. An essential component for this control is the marrow microenvironment, in this context also referred to as the HSC niche. The niche is heterogeneous and regulates stem cell metabolism through both surface-bound and soluble factors. Several signaling pathways have been shown to take part in these regulation processes, with Notch and especially Wnt signaling being the best studied ones. Dysregulation of the niche, for instance by environmental exposure, has recently been shown to lead to hematopoietic abnormalities. Thus, to understand the effect of the environment on hematopoiesis, it is of importance to study both HSC, its direct progeny and the cellular components of the niche. Detailed knowledge of the regulatory mechanisms operating between hematopoietic cells and their direct surroundings facilitates the study of how such signaling may be disrupted by environmental exposure.

Secreted frizzled-related protein 1 extrinsically regulates cycling activity and maintenance of hematopoietic stem cells

Secreted frizzled-related protein 1 (Sfrp1) is highly expressed by stromal cells maintaining hematopoietic stem cells (HSCs). Sfrp1 loss in stromal cells increases production of hematopoietic progenitors, and in knockout mice, dysregulates hemostasis and increases Flk2- Cd34- Lin- Sca1+ Kit+ (LSK) cell numbers in bone marrow. Also, LSK and multipotent progenitors (MPPs) resided mainly in the G0/G1 phase of cell cycle, with an accompanying decrease in intracellular beta-catenin levels. Gene-expression studies showed a concomitant decrease Ccnd1 and Dkk1 in Cd34- LSK cells and increased expression of Pparg, Hes1, and Runx1 in MPP. Transplantation experiments showed no intrinsic effect of Sfrp1 loss on the number of HSCs or their ability to engraft irradiated recipients. In contrast, serial transplantations of wild-type HSCs into Sfrp1(-/-) mice show a progressive decrease of wild-type LSK and MPP numbers. Our results demonstrate that Sfrp1 is required to maintain HSC homeostasis through extrinsic regulation of beta-catenin.

Wnt-related molecules and signaling pathway equilibrium in hematopoiesis

There is near consensus that Wnt family molecules establish important gradients within niches where hematopoietic stem cells (HSC) reside. We review recent papers suggesting that a delicate balance is required between competing Wnt ligands and corresponding signaling pathways to maintain HSC integrity. Some steps in the transitions from HSC to lymphoid progenitor seem to be partially reversible and under the influence of Wnts. In addition, it has been recently suggested that HSC can oscillate between dormant versus active or lineage-biased states. We speculate that Wnts control a reflux process that may sustain stem cell self-renewal and differentiation potential.

Canonical Wnt pathway signaling suppresses VCAM-1 expression by marrow stromal and hematopoietic cells

OBJECTIVE

The Wnt family may contribute to hematopoietic stem cell (HSC) maintenance in bone marrow, but many questions remain concerning mechanisms. Vascular cell adhesion molecule-1 (VCAM-1) is expressed in cellular compartments of the bone marrow and might contribute to the HSC niche, but mechanisms concerning its constitutive expression are largely unknown. We now explore the influence of Wnt signaling on cellular adhesion molecule expression by bone marrow stromal and hematopoietic cells.

MATERIALS AND METHODS

Recombinant Wnt ligands, retroviral Wnt transductions and cocultures with Wnt-secreting cells were used to analyze the effect of Wnt on adhesion molecule expression by stromal and hematopoietic cells. In vivo experiments were also done to assess the ability of Wnt3a-induced, VCAM-1 deficient hematopoietic cells to engraft bone marrow.

RESULTS

We now report that the beta-catenin-dependent canonical Wnt signaling pathway negatively regulates VCAM-1 expression on two types of bone marrow cells. Wnt pathway inhibitors, Axin (intracellular) or Dickkopf-1 (extracellular) blocked the regulation of VCAM-1 by diffusible Wnt3a. Interestingly, lipopolysaccharide restored a substantial degree of VCAM-1 expression, suggesting functional cross-talk between Wnt and TLR4 signaling pathways. Decreasing VCAM-1 on HSC-enriched Lin(-) Sca-1(+) c-Kit(Hi) Thy1.1(Lo) cells by exposure to Wnt3a did not prevent their successful transplantation.

CONCLUSIONS

Our results suggest that cells comprising and residing in the HSC niche can respond to Wnt ligands and extinguish VCAM-1. This response may be important for export of hematopoietic cells. Given the known contribution of VCAM-1 to inflammation, this may represent a new avenue for therapeutic intervention.

Wnt3a deficiency irreversibly impairs hematopoietic stem cell self-renewal and leads to defects in progenitor cell differentiation

Canonical Wnt signaling has been implicated in various aspects of hematopoiesis. Its role is controversial due to different outcomes between various inducible Wnt-signaling loss-of-function models and also compared with gain-of-function systems. We therefore studied a mouse deficient for a Wnt gene that seemed to play a nonredundant role in hematopoiesis. Mice lacking Wnt3a die prenatally around embryonic day (E) 12.5, allowing fetal hematopoiesis to be studied using in vitro assays and transplantation into irradiated recipient mice. Here we show that Wnt3a deficiency leads to a reduction in the numbers of hematopoietic stem cells (HSCs) and progenitor cells in the fetal liver (FL) and to severely reduced reconstitution capacity as measured in secondary transplantation assays. This deficiency is irreversible and cannot be restored by transplantation into Wnt3a competent mice. The impaired long-term repopulation capacity of Wnt3a(-/-) HSCs could not be explained by altered cell cycle or survival of primitive progenitors. Moreover, Wnt3a deficiency affected myeloid but not B-lymphoid development at the progenitor level, and affected immature thymocyte differentiation. Our results show that Wnt3a signaling not only provides proliferative stimuli, such as for immature thymocytes, but also regulates cell fate decisions of HSC during hematopoiesis.

Contrasting responses of lymphoid progenitors to canonical and noncanonical Wnt signals

The Wnt family of secreted glycoproteins has been implicated in many aspects of development, but its contribution to blood cell formation is controversial. We overexpressed Wnt3a, Wnt5a, and Dickkopf 1 in stromal cells from osteopetrotic mice and used them in coculture experiments with highly enriched stem and progenitor cells. The objective was to learn whether and how particular stages of B lymphopoiesis are responsive to these Wnt family ligands. We found that canonical Wnt signaling, through Wnt3a, inhibited B and plasmacytoid dendritic cell, but not conventional dendritic cell development. Wnt5a, which can oppose canonical signaling or act through a different pathway, increased B lymphopoiesis. Responsiveness to both Wnt ligands diminished with time in culture and stage of development. That is, only hematopoietic stem cells and very primitive progenitors were affected. Although Wnt3a promoted retention of hematopoietic stem cell markers, cell yields and dye dilution experiments indicated it was not a growth stimulus. Other results suggest that lineage instability results from canonical Wnt signaling. Lymphoid progenitors rapidly down-regulated RAG-1, and some acquired stem cell-staining characteristics as well as myeloid and erythroid potential when exposed to Wnt3a-producing stromal cells. We conclude that at least two Wnt ligands can differentially regulate early events in B lymphopoiesis, affecting entry and progression in distinct differentiation lineages.

Parathyroid hormone effectively induces mobilization of progenitor cells without depletion of bone marrow

OBJECTIVE

Cytokine-mediated mobilization of hematopoietic stem cells has become an established method in the field of autologous and allogenic stem cell transplantation. Furthermore, it presents a new concept in tissue repair and regenerative medicine. In the present study, we explored the potency of parathyroid hormone (PTH) compared to granulocyte colony-stimulating factor (G-CSF) for mobilization of stem cells and its regenerative capacity on bone marrow.

MATERIALS AND METHODS

Healthy mice were either treated with PTH, G-CSF, or saline. Laboratory parameters were analyzed using a hematological cell analyzer. Hematopoietic stem cells characterized by lin(-)/Sca-1(+)/c-kit(+), as well as subpopulations (CD31(+), c-kit(+), Sca-1(+), CXCR4(+)) of CD45(+)/CD34(+) and CD45(+)/CD34(-) cells were measured by flow cytometry. Immunohistology as well as fluorescein-activated cell sorting analyses were utilized to determine the composition and cell-cycle status of bone marrow cells. Serum levels of distinct cytokines (G-CSF, vascular endothelial growth factor [VEGF]) were determined by enzyme-linked immunosorbent assay. Further, circulating cells were measured after PTH treatment in combination with G-CSF or a G-CSF antibody.

RESULTS

Stimulation with PTH showed a significant increase of all characterized subpopulations of bone marrow-derived progenitor cells (BMCs) in peripheral blood (1.5- to 9.8-fold) similar to G-CSF. In contrast to G-CSF, PTH treatment resulted in an enhanced cell proliferation with a constant level of lin(-)/Sca-1(+)/c-kit(+) cells and CD45(+)/CD34(+) subpopulations in bone marrow. Interestingly, PTH application was associated with increased serum levels of G-CSF (2.8-fold), whereas VEGF showed no significant changes. Blocking endogenous G-CSF with an antibody significantly reduced the number of circulating cells after PTH treatment. A combination of PTH and G-CSF showed slight additional effects compared to PTH or G-CSF alone.

CONCLUSION

PTH induces mobilization of progenitor cells effectively, which can be related to an endogenous release of G-CSF. In contrast to G-CSF treatment, PTH does not result in a depletion of bone marrow, which may be mediated by an activation of PTH receptor on osteoblasts. The novel function of PTH on mobilization and regeneration of BMCs may pave the way for new therapeutic options in bone marrow and stem cell transplantation as well as in the field of ischemic disorders.

Wnt-expressing rat embryonic fibroblasts suppress Apo2L/TRAIL-induced apoptosis of human leukemia cells

Wnt signaling enhances cell proliferation and the maintenance of hematopoietic cells. In contrast, cytotoxic ligand Apo2L/TRAIL induces the apoptosis of various transformed cells. We observed that co-culture of human pre-B leukemia cells KM3 and REH with Wnt1- or Wnt3a-producing rat embryonic fibroblasts efficiently suppressed Apo2L/TRAIL-induced apoptosis of the lymphoid cells. This suppression occurs at the early stages of the Apo2L/TRAIL apoptotic cascade and, interestingly, the activation of the Wnt pathway alone in human leukemia cells is not sufficient for their full anti-apoptotic protection. We hypothesize that a stimulus emanating specifically from Wnt1- or Wnt3a-expressing rat fibroblasts is responsible for the observed resistance to Apo2L/TRAIL. This anti-apoptotic signaling was significantly hampered by the inhibition of the MEK1/ERK1/2 or NFkappaB pathways in KM3 and REH cells. Our results imply that paracrine Wnt-related signals could be important for the survival of pre-B cell-derived malignancies.

Activation of Wnt/beta-catenin pathway mediates growth and survival in B-cell progenitor acute lymphoblastic leukaemia

This study investigated the response of acute lymphoblastic leukaemia (ALL) cells to Wnt proteins. Accumulation of beta-catenin was measured by Western blotting and immunofluorescence microscopy. Reverse transcription polymerase chain reaction (RT-PCR) analysis of B-cell progenitor acute lymphoblastic leukaemia (ALL) cells revealed expression of Wnt genes, including WNT2B in 33%, WNT5A in 42%, WNT10B in 58% and WNT16B in 25% of cases. The Wnt receptors, (Frizzled) FZD7 and FZD8 were also expressed in most cases while FZD3, FZD4 and FZD9 were occasionally detected. Stimulation of ALL cells with Wnt-3a activated canonical Wnt signalling with increased expression and nuclear translocation of beta-catenin. This resulted in a 1.7- to 5.3-fold increase in cell proliferation, which was associated with enhanced cell cycle entry. A significant increase in the survival of ALL cells under conditions of serum deprivation was also observed. Microarray analysis and quantitative RT-PCR revealed that activation of the Wnt/beta-catenin pathway led to altered expression of genes involved in cell cycle regulation and apoptosis in normal and leukaemic B-cell progenitors. Our results demonstrate that Wnt-3a provides proliferative and survival cues in ALL cells. This data suggests that targeting the Wnt signalling pathway may be a useful therapeutic strategy in ALL.

Human brain endothelial cells (HUBEC) promote SCID repopulating cell expansion through direct contact

The objective of this study was to re-evaluate the previously published hematopoietic stem cell (HSC) expansion work using human brain endothelial cells (HUBEC). The expansion effect of contact and non-contact conditions was reported to be equivalent by others. However, we report here different results that the expansion can be achieved only with direct contact. We co-cultured human CD34+ cells with and without HUBEC contact for seven days with cytokines and the readouts were CD34+ / CD38 - phenotype and SCID repopulating cell (SRC) frequency. Also tested was the inhibitory effect of Wnt receptor inhibitor Dkk-1 on HUBEC contact ex vivo expansion; whether an increased expression of Wnt3 occurs on the HUBEC surface; and detection of an increased nuclear localization of beta-catenin in CD34+ / CD38- cells in HUBEC contact culture condition. We conclude that the successful expansion by HUBEC contact culture is a candidate explanation based on the Wnt family protein, possibly Wnt3, expression on HUBEC.

Developmental abnormalities in multiple proliferative tissues of Apc(Min/+) mice

Germ-line mutation of the Apc gene has been linked to familial adenomatous polyposis (FAP) that predisposes to colon cancer. Apc(Min/+) mice, heterozygous for the Apc gene mutation, progressively develop small intestinal tumours in a manner that is analogous to that observed in the colon of patients with FAP (Su et al. 1992; Fodde et al. 1994; Moser et al. 1995). We have studied the effects of Apc gene mutation on murine intestinal and extra-intestinal, proliferatively active tissues. We have contrasted the histology to that of the age- and sex-matched wild-type C57BL/6 mice. Histological assessment of the normal appearing intestinal mucosa demonstrates minimal change in size of crypts. In contrast, villi are longer in the ileum of Apc(Min/+) mice relative to C57BL/6 mice at 12 and 15 weeks of age. Vigorous splenic haematopoiesis in Apc(Min/+) mice was seen at 12 and 15 weeks of age, as reflected by marked splenomegaly, increased splenic haematopoietic cells and megakaryocytes. Peripheral blood counts, however, did not differ between C57BL/6 and Apc(Min/+) mice at 15 weeks of age. Lymphoid depletion in Apc(Min/+) mice was characterized by diminished numbers of splenic lymphoid follicles and small intestinal Peyer's patches. The ovaries of 12- and 15-week-old Apc(Min/+) mice exhibited increased numbers of atretic follicles, and estrous cycling by serial vaginal smears showed tendency of elongation in the mutant mice during these age ranges. The testicles of 10-week-old Apc(Min/+) mice showed increased numbers of underdeveloped seminiferous tubules. Collectively, these data suggest that, in addition to its obvious effects upon intestinal adenoma formation, Apc gene mutation causes impairment of developmental and apparent differentiation blockade in proliferative tissues, including those of the haematopoietic system, lymphoid and reproductive tract.

Constitutively Active β-Catenin Promotes Expansion of Multipotent Hematopoietic Progenitors in Culture

This study was designed to investigate one component of the Wnt/beta-catenin signaling pathway that has been implicated in stem cell self-renewal. Retroviral-mediated introduction of stable beta-catenin to primitive murine bone marrow cells allowed the expansion of multipotential c-Kit(low)Sca-1(low/-)CD19(-) CD11b/Mac-1(-)Flk-2(-)CD43(+)AA4.1(+)NK1.1(-)CD3(-)CD11c(-)Gr-1(-)CD45R/B220(+) cells in the presence of stromal cells and cytokines. They generated myeloid, T, and B lineage lymphoid cells in culture, but had no T lymphopoietic potential when transplanted. Stem cell factor and IL-6 were found to be minimal requirements for long-term, stromal-free propagation, and a beta-catenin-transduced cell line was maintained for 5 mo with these defined conditions. Although multipotential and responsive to many normal stimuli in culture, it was unable to engraft several types of irradiated recipients. These findings support previous studies that have implicated the canonical Wnt pathway signaling in regulation of multipotent progenitors. In addition, we demonstrate how it may be experimentally manipulated to generate valuable cell lines.

Wnt expression and canonical Wnt signaling in human bone marrow B lymphopoiesis

BACKGROUND

The early B lymphopoiesis in mammals is regulated through close interactions with stromal cells and components of the intracellular matrix in the bone marrow (BM) microenvironment. Although B lymphopoiesis has been studied for decades, the factors that are implicated in this process, both autocrine and paracrine, are inadequately explored. Wnt signaling is known to be involved in embryonic development and growth regulation of tissues and cancer. Wnt molecules are produced in the BM, and we here ask whether canonical Wnt signaling has a role in regulating human BM B lymphopoiesis.

RESULTS

Examination of the mRNA expression pattern of Wnt ligands, Fzd receptors and Wnt antagonists revealed that BM B progenitor cells and stromal cells express a set of ligands and receptors available for induction of Wnt signaling as well as antagonists for fine tuning of this signaling. Furthermore, different B progenitor maturation stages showed differential expression of Wnt receptors and co-receptors, beta-catenin, plakoglobin, LEF-1 and TCF-4 mRNAs, suggesting canonical Wnt signaling as a regulator of early B lymphopoiesis. Exogenous Wnt3A induced stabilization and nuclear accumulation of beta-catenin in primary lineage restricted B progenitor cells. Also, Wnt3A inhibited B lymphopoiesis of CD133+CD10- hematopoietic progenitor cells and CD10+ B progenitor cells in coculture assays using a supportive layer of stromal cells. This effect was blocked by the Wnt antagonists sFRP1 or Dkk1. Examination of early events in the coculture showed that Wnt3A inhibits cell division of B progenitor cells.

CONCLUSION

These results indicate that canonical Wnt signaling is involved in human BM B lymphopoiesis where it acts as a negative regulator of cell proliferation in a direct or stroma dependent manner.

Expression of beta-catenin by acute myeloid leukemia cells predicts enhanced clonogenic capacities and poor prognosis

Activation of the Wnt/beta-catenin pathway has recently been shown to be crucial to the establishment of leukemic stem cells in chronic myeloid leukemia. We sought to determine whether beta-catenin was correlated to clonogenic capacity also in the acute myeloid leukemia (AML) setting. Eighty-two patients were retrospectively evaluated for beta-catenin expression by Western blot. beta-Catenin was expressed (although at various protein levels) in 61% of patients, and was undetectable in the remaining cases. In our cohort, beta-catenin expression was correlated with the clonogenic proliferation of AML-colony forming cells (AML-CFC or CFU-L) in methylcellulose in the presence of 5637-conditioned medium, and more strikingly with self-renewing of leukemic cells, as assessed in vitro by a re-plating assay. In survival analyses, beta-catenin appeared as a new independent prognostic factor predicting poor event-free survival and shortened overall survival (both with P<0.05). Furthermore, variations in beta-catenin protein levels were dependent on post-transcriptional mechanisms involving the Wnt/beta-catenin pathway only in leukemic cells. Indeed, beta-catenin negative leukemic cells were found to increase beta-catenin in response to Wnt3a agonist in contrast to normal counterparts. Altogether, our data pave the way to the evaluation of Wnt pathway inhibition as a new rationale for eradicating the clonogenic pool of AML cells.

Osteoblastic activation in the hematopoietic stem cell niche

Hematopoietic stem cells (HSC) are rare primitive cells capable of reconstituting all blood cell lineages throughout the life of an individual. The microenvironment in which stem cells reside is essential for their survival, self-renewal, and differentiation. This microenvironment, or HSC niche, has been difficult to define in bone and bone marrow, but recent studies from our laboratory and others have shown that osteoblasts, the bone-forming cells, are an essential regulatory component of this complex cellular network. We established that parathyroid hormone (PTH), through activation of the PTH/PTHrP receptor (PTH1R) in osteoblastic cells, could alter the HSC niche resulting in HSC expansion in vivo and in vitro and improving dramatically the survival of mice receiving bone marrow transplants. These findings are of great clinical appeal, because they suggest that a strategy aimed at modifying supportive cells in a stem cell niche can expand HSC. While a number of molecules have been found to be important for hematopoietic/osteoblastic interactions, we have focused on the Jagged1/Notch signaling pathway, which was necessary for the PTH-dependent HSC expansion. Since the Jagged1/Notch signaling pathway has been implicated in the microenvironmental control of stem cell self-renewal in several organ systems, definition of Jagged1 modulation, which is currently poorly understood, should provide additional molecular targets for stem cell regulation and advance the understanding of stem cell-microenvironmental interactions.

Wnt signalling in osteoblasts regulates expression of the receptor activator of NFkappaB ligand and inhibits osteoclastogenesis in vitro

Reports implicating Wnt signalling in the regulation of bone mass have prompted widespread interest in the use of Wnt mimetics for the treatment of skeletal disorders. To date much of this work has focused on their anabolic effects acting on cells of the osteoblast lineage. In this study we provide evidence that Wnts also regulate osteoclast formation and bone resorption, through a mechanism involving transcriptional repression of the gene encoding the osteoclastogenic cytokine receptor activator of NFkappaB ligand (RANKL or TNFSF11) expressed by osteoblasts. In co-cultures of mouse mononuclear spleen cells and osteoblasts, inhibition of GSK3beta with LiCl or exposure to Wnt3a inhibited the formation of tartrate-resistant acid phosphatase-positive multinucleated cells compared with controls. However, these treatments had no consistent effect on the differentiation, survival or activity of osteoclasts generated in the absence of supporting stromal cells. Activation of Wnt signalling downregulated RANKL mRNA and protein expression, and overexpression of fulllength beta-catenin, but not transcriptionally inactive beta-catenin DeltaC(695-781), inhibited RANKL promoter activity. Since previous studies have demonstrated an absence of resorptive phenotype in mice lacking LRP5, we determined expression of a second Wnt co-receptor LRP6 in human osteoblasts, CD14(+) osteoclast progenitors and mature osteoclasts. LRP5 expression was undetectable in CD14-enriched cells and mature human osteoclasts, although LRP6 was expressed at high levels by these cells. Our evidence of Wnt-dependent regulation of osteoclastogenesis adds to the growing complexity of Wnt signalling mechanisms that are now known to influence skeletal function and highlights the requirement to develop novel therapeutics that differentially target anabolic and catabolic Wnt effects in bone.