Activation of Wnt signaling in hematopoietic regeneration

Granulocyte differentiation of rat bone marrow resident C-kit+ hematopoietic stem cells induced by mesenchymal stem cells could be considered as new option in cell-based therapy

Mesenchymal stem cells (MSCs) are effective in hematopoietic engraftment and tissue repair in stem cell transplantation. In addition, these cells control the process of hematopoiesis by secreting growth factors and cytokines. The aim of the present study is to investigate the effect of rat bone marrow (BM)-derived MSCs on the granulocyte differentiation of rat BM-resident C-kit⁺ hematopoietic stem cells (HSCs). The mononuclear cells were collected from rat BM using density gradient centrifugation and MSCs and C-kit⁺ HSCs were isolated. Then, cells were divided into two groups and differentiated into granulocytes; C-kit⁺ HSCs alone (control group) and co-cultured C-kit⁺ HSCs with MSCs (experimental group). Subsequently, the granulocyte-differentiated cells were collected and subjected to real-time PCR and Western blotting for the assessment of their telomere length (TL) and protein expressions, respectively. Afterwards, culture medium was collected to measure cytokine levels. CD34, CD16, CD11b, and CD18 granulocyte markers expression levels were significantly increased in the experimental group compared to the control group. A significant change was also observed in the protein expression of Wnt and β-catenin. In addition, MSCs caused an increase in the TL of granulocyte-differentiated cells. MSCs could affect the granulocyte differentiation of C-kit⁺ HSCs via increasing TL and Wnt/β-catenin protein expression.

Hematopoietic Stem Cells and Regeneration

Hematopoietic stem cell (HSC) regeneration is the remarkable process by which extremely rare, normally inactive cells of the bone marrow can replace an entire organ if called to do so by injury or harnessed by transplantation. HSC research is arguably the first quantitative single-cell science and the foundation of adult stem cell biology. Bone marrow transplant is the oldest and most refined technique of regenerative medicine. Here we review the intertwined history of the discovery of HSCs and bone marrow transplant, the molecular and cellular mechanisms of HSC self-renewal, and the use of HSCs and their derivatives for cell therapy.

Hematopoietic Stem and Progenitor Cells (HSPCs) and Hematopoietic Microenvironment: Molecular and Bioinformatic Studies of the Zebrafish Models

Hematopoietic stem cells (HSCs) reside in a specialized microenvironment in a peculiar anatomic location which regulates the maintenance of stem cells and controls its functions. Recent scientific progress in experimental technologies have enabled the specific detection of epigenetic factors responsible for the maintenance and quiescence of the hematopoietic niche, which has improved our knowledge of regulatory mechanisms. The aberrant role of RNA-binding proteins and their impact on the disruption of stem cell biology have been reported by a number of recent studies. Despite recent modernization in hematopoietic microenvironment research avenues, our comprehension of the signaling mechanisms and interactive pathways responsible for integration of the hematopoietic niche is still limited. In the past few decades, zebrafish usage with regards to exploratory studies of the hematopoietic niche has expanded our knowledge for deeper understanding of novel cellular interactions. This review provides an update on the functional roles of different genetic and epigenetic factors and molecular signaling events at different sections of the hematopoietic microenvironment. The explorations of different molecular approaches and interventions of latest web-based tools being used are also outlined. This will help us to get more mechanistic insights and develop therapeutic options for the malignancies.

Induction of Wnt signaling antagonists and p21-activated kinase enhances cardiomyocyte proliferation during zebrafish heart regeneration

Heart regeneration occurs by dedifferentiation and proliferation of pre-existing cardiomyocytes (CMs). However, the signaling mechanisms by which injury induces CM renewal remain incompletely understood. Here, we find that cardiac injury in zebrafish induces expression of the secreted Wnt inhibitors, including Dickkopf 1 (Dkk1), Dkk3, secreted Frizzled-related protein 1 (sFrp1), and sFrp2, in cardiac tissue adjacent to injury sites. Experimental blocking of Wnt activity via Dkk1 overexpression enhances CM proliferation and heart regeneration, whereas ectopic activation of Wnt8 signaling blunts injury-induced CM dedifferentiation and proliferation. Although Wnt signaling is dampened upon injury, the cytoplasmic β-catenin is unexpectedly increased at disarrayed CM sarcomeres in myocardial wound edges. Our analyses indicated that p21-activated kinase 2 (Pak2) is induced at regenerating CMs, where it phosphorylates cytoplasmic β-catenin at Ser 675 and increases its stability at disassembled sarcomeres. Myocardial-specific induction of the phospho-mimetic β-catenin (S675E) enhances CM dedifferentiation and sarcomere disassembly in response to injury. Conversely, inactivation of Pak2 kinase activity reduces the Ser 675-phosphorylated β-catenin (pS675-β-catenin) and attenuates CM sarcomere disorganization and dedifferentiation. Taken together, these findings demonstrate that coordination of Wnt signaling inhibition and Pak2/pS675-β-catenin signaling enhances zebrafish heart regeneration by supporting CM dedifferentiation and proliferation.

Contrary Roles of Wnt/β-Catenin Signaling in BMP9-Induced Osteogenic and Adipogenic Differentiation of 3T3-L1 Preadipocytes

Our previous study revealed that 3T3-L1 preadipocytes can differentiate to either osteoblasts or adipocytes in response to bone morphogenic protein 9 (BMP9). In the present study, we try to further investigate whether the Wnt/β-catenin signaling plays a crucial role in this process. It was found that BMP9 effectively activated the Wnt/β-catenin signaling, and induced the expression levels of certain canonical Wnt ligands and their receptors in preadipocytes. Exogenous expression of β-catenin, Wnt1, Wnt3a, and Wnt10b potentiated BMP9-induced alkaline phosphatase (ALP) activity, while β-catenin knockdown or Dickkopf 1 (Dkk1) diminished BMP9-induced ALP activity. Moreover, it was demonstrated that β-catenin overexpression promoted BMP9-induced mineralization, and increased the expression levels of late osteogenic markers osteopontin and osteocalcin. Furthermore, β-catenin inhibited BMP9-induced lipid accumulation and the adipogenic marker adipocyte fatty acid binding protein (aP2). The cell-implantation assay results identified that β-catenin not only augmented BMP9-induced ectopic bone formation, but also blocked adipogenesis in vivo. Mechanistically, it was found that β-catenin and BMP9 synergistically stimulated the osteogenic transcription factors runt-related transcription factor 2 (Runx2) and Osterix (OSX). However, BMP9-induced adipogenic transcription factors, peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT enhancer-binding protein α (C/EBPα), were inhibited by β-catenin. Therefore, these findings suggested that the Wnt/β-catenin signaling, potentially via the modulation of osteogenic and adipogenic transcriptional factors, exerts an opposite effect on BMP9-induced osteogenic and adipogenic differentiation in preadipocytes.

Inhibiting Glycogen Synthase Kinase-3 Mitigates the Hematopoietic Acute Radiation Syndrome in a Sex- and Strain-dependent Manner in Mice

The Radiation and Nuclear Countermeasures Program at the National Institute of Allergy and Infectious Diseases (NIAID) mandated that medical countermeasures for treating Acute Radiation Syndrome (ARS) must have efficacy when administered at least 24 h after radiation exposure. At this time point, many cells within key target tissues, such as the hematopoietic system and the gastrointestinal (GI) tract, will already be dead. Therefore, drugs that promote the regeneration of surviving cells may improve outcomes. The serine/threonine kinase glycogen synthase kinase-3 (GSK-3) regulates stem and progenitor cell self-renewal and regeneration in the hematopoietic and GI compartments. We tested inhibition of GSK-3β by SB216763 24 h after total body irradiation (TBI) and sub-total body irradiation (SBI). Here, we show that subcutaneous administration of SB216763 promotes the regeneration of surviving hematopoietic stem/progenitor cells (HSPCs), including myeloid progenitor cells, and improves survival of C57Bl/6 male mice when administered 24 h after TBI. However, these results were not recapitulated in female C57Bl/6 animals, suggesting a sex difference in GSK-3β signaling in HSPCs. Subcutaneous administration of SB216763 in male mice stimulated activation of Sox2 transcription but failed to induce Sox2 transcription in female C57Bl/6 mice. Using TCF/lef-GFP reporter mice, we examined Wnt signaling in HSPCs of irradiated male and female mice treated with SB216763. GSK-3 inhibition elevated Wnt reporter activity in HSPCs isolated from male but not female mice. SB216763 did not mitigate hematopoietic ARS in males or females of a second strain of wild-type mice, C3H. In addition, administration of SB216763 did not mitigate hematopoietic ARS beyond the currently available standard approved therapy of ciprofloxacin and granulocyte-colony stimulating factor (G-CSF) in male C57Bl/6 mice. Further, SB216763 did not mitigate GI-ARS after SBI in C57Bl/6 male mice. The lack of efficacy in both sexes and multiple strains of mice indicate that SB216763 is not suitable for further drug development as a mitigator of ARS. Our studies demonstrate that activation of Wnt signaling in HSPCs promotes hematopoietic regeneration following radiation exposure, and targeting this pathway downstream of GSK-3β may mitigate ARS in a sex- and strain-independent manner.

MLLT10 in benign and malignant hematopoiesis

Non-random chromosomal translocations involving the putative transcription factor Mixed Lineage Leukemia Translocated to 10 (MLLT10, also known as AF10) are commonly observed in both acute myeloid and lymphoid leukemias and are indicative of a poor prognosis. Despite the well-described actions of oncogenic MLLT10 fusion proteins, the role of wild-type MLLT10 in hematopoiesis is not well characterized. The protein structure and several interacting partners have been described and provide indications as to the potential functions of MLLT10. This review examines these aspects of MLLT10, contextualizing its function in benign and malignant hematopoiesis.

Proteomic Changes in Mouse Spleen after Radiation-Induced Injury and its Modulation by Gamma-Tocotrienol

Gamma-tocotrienol (GT3), a naturally occurring vitamin E isomer, a promising radioprotector, has been shown to protect mice against radiation-induced hematopoietic and gastrointestinal injuries. We analyzed changes in protein expression profiles of spleen tissue after GT3 treatment in mice exposed to gamma radiation to gain insights into the molecular mechanism of radioprotective efficacy. Male CD2F1 mice, 12-to-14 weeks old, were treated with either vehicle or GT3 at 24 h prior to 7 Gy total-body irradiation. Nonirradiated vehicle, nonirradiated GT3 and age-matched naïve animals were used as controls. Blood and tissues were harvested on days 0, 1, 2, 4, 7, 10 and 14 postirradiation. High-resolution mass-spectrometry-based radioproteomics was used to identify differentially expressed proteins in spleen tissue with or without drug treatment. Subsequent bioinformatic analyses helped delineate molecular markers of biological pathways and networks regulating the cellular radiation responses in spleen. Our results show a robust alteration in spleen proteomic profiles including upregulation of the Wnt signaling pathway and actin-cytoskeleton linked proteins in mediating the radiation injury response in spleen. Furthermore, we show that 24 h pretreatment with GT3 attenuates radiation-induced hematopoietic injury in the spleen by modulating various cell signaling proteins. Taken together, our results show that the radioprotective effects of GT3 are mediated, via alleviation of radiation-induced alterations in biochemical pathways, with wide implications on overall hematopoietic injury.

Wnt Signaling in Hematological Malignancies

Leukemia and lymphoma are a wide encompassing term for a diverse set of blood malignancies that affect people of all ages and result in approximately 23,000 deaths in the United States per year (Siegel RL, Miller KD, Jemal A. Cancer statistics, 2016. CA Cancer J Clin. 2016;66(1):7-30.). Hematopoietic stem cells (HSCs) are tissue-specific stem cells at the apex of the hierarchy that gives rise to all of the terminally differentiated blood cells, through progressively restricted progenitor populations, a process that is known to be Wnt-responsive. In particular, the progenitor populations are subject to uncontrolled expansion during oncogenic processes, namely the common myeloid progenitor and common lymphoid progenitor, as well as the myeloblast and lymphoblast. Unregulated growth of these cell-types leads to mainly three types of blood cancers (i.e., leukemia, lymphoma, and myeloma), which frequently exhibit deregulation of the Wnt signaling pathway. Generally, leukemia is caused by the expansion of myeloid progenitors, leading to an overproduction of white blood cells; as such, patients are unable to make sufficient numbers of red blood cells and platelets. Likewise, an overproduction of lymphocytes leads to clogging of the lymph system and impairment of the immune system in lymphomas. Finally, cancer of the plasma cells in the blood is called myeloma, which also leads to immune system failure. Within each of these three types of blood cancers, there are multiple subtypes, usually characterized by their timeline of onset and their cell type of origin. Of these, 85% of leukemias are encompassed by the four most common diseases, that is, acute myeloid leukemia (AML), chronic myeloid leukemia (CML), acute lymphoblastic leukemia (ALL), and chronic lymphocytic leukemia (CLL); AML accounts for the majority of leukemia-related deaths (Siegel RL, Miller KD, Jemal A. Cancer statistics, 2016. CA Cancer J Clin. 2016;66(1):7-30.). Through understanding how HSCs are normally developed and maintained, we can understand how the normal functions of these pathways are disrupted during blood cancer progression; the Wnt pathway is important in regulation of both normal and malignant hematopoiesis. In this chapter, we will discuss the role of Wnt signaling in normal and aberrant hematopoiesis. Our understanding the relationship between Wnt and HSCs will provide novel insights into therapeutic targets.

Different Balance of Wnt Signaling in Adult and Fetal Bone Marrow-Derived Mesenchymal Stromal Cells

Mesenchymal stromal cells (MSCs) are applied as novel therapeutics for their regenerative and immune-suppressive capacities. Clinical applications, however, require extensive expansion of MSCs. Fetal bone marrow-derived MSCs (FBMSCs) proliferate faster than adult bone marrow-derived MSC (ABMSCs). To optimize expansion and function of MSC in general, we explored the differences between ABMSC and FBMSC. Gene expression profiling implicated differential expression of genes encoding proteins in the Wnt signaling pathway, including excreted inhibitors of Wnt signaling, particularly by ABMSC. Both MSC types had a similar basal level of canonical Wnt signaling. Abrogation of autocrine Wnt production by inhibitor of Wnt production-2 (IWP2) reduced canonical Wnt signaling and cell proliferation of FBMSCs, but hardly affected ABMSC. Addition of exogenous Wnt3a, however, induced expression of the target genes lymphocyte enhancer-binding factor (LEF) and T-cell factor (TCF) faster and at lower Wnt3a levels in ABMSC compared to FBMSC. Medium replacement experiments indicated that ABMSC produce an inhibitor of Wnt signaling that is effective on ABMSC itself but not on FBMSC, whereas FBMSC excrete (Wnt) factors that stimulate proliferation of ABMSC. In contrast, FBMSC were not able to support hematopoiesis, whereas ABMSC displayed hematopoietic support sensitive to IWP2, the inhibitor of Wnt factor excretion. In conclusion, ABMSC and FBMSC differ in their Wnt signature. While FBMSC produced factors, including Wnt signals, that enhanced MSC proliferation, ABMSC produced Wnt factors in a setting that enhanced hematopoietic support. Thus, further unraveling the molecular basis of this phenomenon may lead to improvement of clinical expansion protocols of ABMSCs.

Using Zebrafish to Study Pathways that Regulate Hematopoietic Stem Cell Self-Renewal and Migration

This perspective describes the usefulness of zebrafish as a model to study interaction of hematopoietic stem cells with the associated niche in vivo, explains how such interactions influence regeneration, migration, and clonality of HSCs, and defines their fate during differentiation.

Intronless WNT10B-short variant underlies new recurrent allele-specific rearrangement in acute myeloid leukaemia

Defects in the control of Wnt signaling have emerged as a recurrent mechanism involved in cancer pathogenesis and acute myeloid leukaemia (AML), including the hematopoietic regeneration-associated WNT10B in AC133bright leukaemia cells, although the existence of a specific mechanism remains unproven. We have obtained evidences for a recurrent rearrangement, which involved the WNT10B locus (WNT10BR) within intron 1 (IVS1) and flanked at the 5' by non-human sequences whose origin remains to be elucidated; it also expressed a transcript variant (WNT10BIVS1) which was mainly detected in a cohort of patients with intermediate/unfavorable risk AML. We also identified in two separate cases, affected by AML and breast cancer respectively, a genomic transposable short form of human WNT10B (ht-WNT10B). The intronless ht-WNT10B resembles a long non-coding RNA (lncRNA), which suggests its involvement in a non-random microhomology-mediated recombination generating the rearranged WNT10BR. Furthermore, our studies supports an autocrine activation primed by the formation of WNT10B-FZD4/5 complexes in the breast cancer MCF7 cells that express the WNT10BIVS1. Chemical interference of WNT-ligands production by the porcupine inhibitor IWP-2 achieved a dose-dependent suppression of the WNT10B-FZD4/5 interactions. These results present the first evidence for a recurrent rearrangement promoted by a mobile ht-WNT10B oncogene, as a relevant mechanism for Wnt involvement in human cancer.

Immune Modulation of the T Cell Response in Asthma through Wnt10b

Asthma is a chronic inflammatory disease, which is characterized by activation of CD4(+) T helper 2 cells orchestrating an allergic airway response. Whereas the role of Wnt family members in regulating T cell maintenance and maturation is established, their contribution to T cell activation in allergic asthma is not known. We hypothesized that Wnt10b plays a role in the modulation of the allergic airway response and affects T cell activation and polarization. Using an in vivo house dust mite asthma model, Wnt10b-deficient (Wnt10b(-/-)) mice were allergen-sensitized and inflammation, as well as T cell activation, was studied in vivo and in vitro. Wnt10b(-/-) mice exhibited an augmented inflammatory phenotype with an increase in eosinophils in the bronchoalveolar lavage and IL-4 and IL-13 in the lungs when compared with wild-type mice. In vitro studies confirmed an increased T helper type 2 polarization and increased T cell activation of Wnt10b(-/-) cells. Accordingly, the percentage of naive T cells was elevated by the addition of recombinant Wnt10b protein. Finally, Wnt10b(-/-) mice exhibited an increase in the percentage of effector T cells in the lungs after house dust mite sensitization, which indicated a heightened activation state, measured by an increased percentage of CD69(hi)CD11a(hi) cells. These findings suggest that Wnt10b plays an important role in regulating asthmatic airway inflammation through modification of the T cell response and is a prospective target in the disease process.

Repairing quite swimmingly: advances in regenerative medicine using zebrafish

Regenerative medicine has the promise to alleviate morbidity and mortality caused by organ dysfunction, longstanding injury and trauma. Although regenerative approaches for a few diseases have been highly successful, some organs either do not regenerate well or have no current treatment approach to harness their intrinsic regenerative potential. In this Review, we describe the modeling of human disease and tissue repair in zebrafish, through the discovery of disease-causing genes using classical forward-genetic screens and by modulating clinically relevant phenotypes through chemical genetic screening approaches. Furthermore, we present an overview of those organ systems that regenerate well in zebrafish in contrast to mammalian tissue, as well as those organs in which the regenerative potential is conserved from fish to mammals, enabling drug discovery in preclinical disease-relevant models. We provide two examples from our own work in which the clinical translation of zebrafish findings is either imminent or has already proven successful. The promising results in multiple organs suggest that further insight into regenerative mechanisms and novel clinically relevant therapeutic approaches will emerge from zebrafish research in the future.

Concise review: Current concepts in bone marrow microenvironmental regulation of hematopoietic stem and progenitor cells

Hematopoietic stem cell (HSC) behavior is governed in large part by interactions of the blood system with the bone microenvironment. Increasing evidence demonstrates the profound role the local HSC microenvironment or niche plays in normal stem cell function, in therapeutic activation and in the setting of malignancy. A number of cellular and molecular components of the microenvironment have been identified thus far, several of which are likely to provide exciting therapeutic targets in the near future. Clinically effective strategies for niche manipulation, however, require careful study of the interaction of these niche components. Some of the key findings defining these regulatory interactions are explored in this concise review, with special emphasis on potential translational applications.

Inhibiting glycogen synthase kinase-3 mitigates the hematopoietic acute radiation syndrome in mice

Exposure to a nuclear accident or radiological attack can cause death from acute radiation syndrome (ARS), which results from radiation injury to vital organs such as the hematopoietic system. However, the U.S. Food and Drug Administration (FDA) has not approved any medical countermeasures for this specific purpose. With growing concern over nuclear terrorism, there is an urgent need to develop small molecule deliverables that mitigate mortality from ARS. One emerging modulator of hematopoietic stem/progenitor cell (HSPC) activity is glycogen synthase kinase-3 (GSK-3). The inhibition of GSK-3 has been shown to augment hematopoietic repopulation in mouse models of bone marrow transplantation. In this study, we performed an in vitro screen using irradiated bone marrow mononuclear cells (BM-MNCs) to test the effects of four GSK-3 inhibitors: CHIR99021; 6-Bromoindirubin-3'-oxime (BIO); SB415286; and SB216763. This screen showed that SB216763 significantly increased the frequency of c-Kit(+) Lin(-) Sca1(+) (KLS) cells and hematopoietic colony-forming cells in irradiated BM-MNCs. Importantly, administration of a single dose of SB216763 to C57BL/6J mice by subcutaneous injection 24 h after total-body irradiation significantly improved hematopoietic recovery and mitigated hematopoietic ARS. Collectively, our results demonstrate that the GSK-3 inhibitor SB216763 is an effective medical countermeasure against acute radiation injury of the hematopoietic system.

Ovariectomy expands murine short-term hemopoietic stem cell function through T cell expressed CD40L and Wnt10B

Estrogen deficiency expands hemopoietic stem and progenitor cells (HSPCs) and mature blood lineages, but the involved mechanism and the affected HSPC populations are mostly unknown. Here we show that ovariectomy (ovx) expands short-term HSPCs (ST-HSPCs) and improves blood cell engraftment and host survival after bone marrow (BM) transplantation through a dual role of the T-cell costimulatory molecule CD40 ligand (CD40L). This surface receptor is required for ovx to stimulate T-cell production of Wnt10b, a Wnt ligand that activates Wnt signaling in HSPCs and stromal cells (SCs). Moreover, CD40L is required for ovx to increase SC production of the hemopoietic cytokines interleukin (IL)-6, IL-7, and granulocyte macrophage-colony-stimulating factor. Attesting to the relevance of CD40L and Wnt10b, ovx fails to expand ST-HSPCs in CD40L-null mice and in animals lacking global or T-cell expression of Wnt10b. In summary, T cells expressed CD40L, and the resulting increased production of Wnt10b and hemopoietic cytokines by T cells and SCs, respectively, plays a pivotal role in the mechanism by which ovx regulates hemopoiesis. The data suggest that antiestrogens may represent pharmacological targets to improve ST-HSPC function through activation of the microenvironment.

Small-molecule modulation of Wnt signaling via modulating the Axin-LRP5/6 interaction

The Wnt/β-catenin signaling pathway has a crucial role in embryonic development, stem cell maintenance and human disease. By screening a synthetic chemical library of lycorine derivatives, we identified 4-ethyl-5-methyl-5,6-dihydro-[1,3]dioxolo[4,5-j]phenanthridine (HLY78) as an activator of the Wnt/β-catenin signaling pathway, which acts in a Wnt ligand-dependent manner. HLY78 targets the DIX domain of Axin and potentiates the Axin-LRP6 association, thus promoting LRP6 phosphorylation and Wnt signaling transduction. Moreover, we identified the critical residues on Axin for HLY78 binding and showed that HLY78 may weaken the autoinhibition of Axin. In addition, HLY78 acts synergistically with Wnt in the embryonic development of zebrafish and increases the expression of the conserved hematopoietic stem cell (HSC) markers, runx1 and cmyb, in zebrafish embryos. Collectively, our study not only provides new insights into the regulation of the Wnt/β-catenin signaling pathway by a Wnt-specific small molecule but also will facilitate therapeutic applications, such as HSC expansion.

Wnt3a reestablishes osteogenic capacity to bone grafts from aged animals

BACKGROUND

Age-related fatty degeneration of the bone marrow contributes to delayed fracture-healing and osteoporosis-related fractures in the elderly. The mechanisms underlying this fatty change are unknown, but they may relate to the level of Wnt signaling within the aged marrow cavity.

METHODS

Transgenic mice were used in conjunction with a syngeneic bone-graft model to follow the fates of cells involved in the engraftment. Immunohistochemistry along with quantitative assays were used to evaluate Wnt signaling and adipogenic and osteogenic gene expression in bone grafts from young and aged mice. Liposomal Wnt3a protein (L-Wnt3a) was tested for its ability to restore osteogenic potential to aged bone grafts in critical-size defect models created in mice and in rabbits. Radiography, microquantitative computed tomography (micro-CT) reconstruction, histology, and histomorphometric measurements were used to quantify bone-healing resulting from L-Wnt3a or a control substance (liposomal phosphate-buffered saline solution [L-PBS]).

RESULTS

Expression profiling of cells in a bone graft demonstrated a shift away from an osteogenic gene profile and toward an adipogenic one with age. This age-related adipogenic shift was accompanied by a significant reduction (p < 0.05) in Wnt signaling and a loss in osteogenic potential. In both large and small animal models, osteogenic competence was restored to aged bone grafts by a brief incubation with the stem-cell factor Wnt3a. In addition, liposomal Wnt3a significantly reduced cell death in the bone graft, resulting in significantly more osseous regenerate in comparison with controls.

CONCLUSIONS

Liposomal Wnt3a enhances cell survival and reestablishes the osteogenic capacity of bone grafts from aged animals.

CLINICAL RELEVANCE

We developed an effective, clinically applicable, regenerative medicine-based strategy for revitalizing bone grafts from aged patients.

Regeneration-associated WNT signaling is activated in long-term reconstituting AC133bright acute myeloid leukemia cells

Acute myeloid leukemia (AML) is a genetically heterogeneous clonal disorder characterized by two molecularly distinct self-renewing leukemic stem cell (LSC) populations most closely related to normal progenitors and organized as a hierarchy. A requirement for WNT/β-catenin signaling in the pathogenesis of AML has recently been suggested by a mouse model. However, its relationship to a specific molecular function promoting retention of self-renewing leukemia-initiating cells (LICs) in human remains elusive. To identify transcriptional programs involved in the maintenance of a self-renewing state in LICs, we performed the expression profiling in normal (n = 10) and leukemic (n = 33) human long-term reconstituting AC133(+) cells, which represent an expanded cell population in most AML patients. This study reveals the ligand-dependent WNT pathway activation in AC133(bright) AML cells and shows a diffuse expression and release of WNT10B, a hematopoietic stem cell regenerative-associated molecule. The establishment of a primary AC133(+) AML cell culture (A46) demonstrated that leukemia cells synthesize and secrete WNT ligands, increasing the levels of dephosphorylated β-catenin in vivo. We tested the LSC functional activity in AC133(+) cells and found significant levels of engraftment upon transplantation of A46 cells into irradiated Rag2(-/-)γc(-/-) mice. Owing to the link between hematopoietic regeneration and developmental signaling, we transplanted A46 cells into developing zebrafish. This system revealed the formation of ectopic structures by activating dorsal organizer markers that act downstream of the WNT pathway. In conclusion, our findings suggest that AC133(bright) LSCs are promoted by misappropriating homeostatic WNT programs that control hematopoietic regeneration.

Wnt signaling in normal and malignant hematopoiesis

One of the most remarkable characteristics of stem cells is their ability to perpetuate themselves through self-renewal while concomitantly generating differentiated cells. In the hematopoietic system, stem cells balance these mechanisms to maintain steady-state hematopoiesis for the lifetime of the organism, and to effectively regenerate the system following injury. Defects in the proper control of self-renewal and differentiation can be potentially devastating and contribute to the development of malignancies. In this review, we trace the emerging role of Wnt signaling as a critical regulator of distinct aspects of self-renewal and differentiation, its contribution to the maintenance of homeostasis and regeneration, and how the pathway can be hijacked to promote leukemia development. A better understanding of these processes could pave the way to enhancing recovery after injury and to developing better therapeutic approaches for hematologic malignancies.

Wnt5a does not support hematopoiesis in stroma-free, serum-free cultures

Previously we reported that Wnt5a is highly expressed in the murine urogenital ridge-derived UG26-1B6 cells but not embryonic liver-derived EL08-1D2 cells. Mouse long-term repopulating hematopoietic stem cells (LTR-HSC) were maintained in non-contact UG26-1B6 cultures but not EL08-1D2 non-contact cultures, unless Wnt5a was also added to the cultures, suggesting a role for Wnt5a in the in vitro maintenance of LTR-HSC. Here, we investigated if the effect of Wnt5a on adult LTR-HSC activity is HSC-autonomous. To test the effect of Wnt5a on maintenance of LTR-HSC, we performed limiting dilution competitive transplantation assays of murine Lin-Sca1(+) c-kit(+) (LSK) cells cultured for 5 days with TPO and SCF with and without Wnt5a. The effect of Wnt5a on the generation of colony forming units (CFU) and the homing ability of LSK progeny was also tested. No effects were found of Wnt5a on total cell expansion, the number of CFU, or homing ability of day 5 LSK progeny. Furthermore, addition of Wnt5a did not improve, but may have impeded maintenance of LTR-HSC. In conclusion, our data indicate that Wnt5a does not enhance the maintenance and expansion of adult murine LTR-HSCs or committed progenitors cultured in vitro in serum- and stroma-free conditions.

GSK3 inhibition prevents lethal GVHD in mice

Graft-versus-host disease (GVHD) is a major contributor to transplant-related mortality and morbidity after allogeneic stem cell transplantation. Despite advancements in tissue-typing techniques, conditioning regimens, and therapeutic intervention, the incidence rate of GVHD remains high. GVHD is caused by alloreactive donor T cells that infiltrate and destroy host tissues (e.g., skin, liver, and gut). Therefore, GVHD is prevented and treated with therapeutics that suppress proinflammatory cytokines and T-cell function (e.g., cyclosporine, glucocorticoids). Here we report that the small molecule inhibitor of glycogen synthase kinase 3, 6-bromoindirubin 3'-oxime (BIO), prevents lethal GVHD in a humanized xenograft model in mice. BIO treatment did not affect donor T-cell engraftment, but suppressed their activation and attenuated bone marrow and liver destruction mediated by activated donor T cells. Glycogen synthase kinase 3 inhibition modulated the Th1/Th2 cytokine profile in vitro and suppressed activation of signal transducers and activators of transcription 1 and 3 signaling pathways both in vitro and in vivo. Importantly, human T cells derived from BIO-treated mice were able to mediate anti-tumor effects in vitro, and BIO did not affect stem cell engraftment and multilineage reconstitution in a mouse model of transplantation. These data demonstrate that inhibition of glycogen synthase kinase 3 can potentially abrogate GVHD without compromising the efficacy of transplantation.

PTH expands short-term murine hemopoietic stem cells through T cells

Intermittent parathyroid hormone (iPTH) treatment expands hemopoietic stem and progenitor cells (HSPCs), but the involved mechanisms and the affected HSPC populations are mostly unknown. Here we show that T cells are required for iPTH to expand short-term HSPCs (ST-HSPCs) and improve blood cell engraftment and host survival after BM transplantation. Silencing of PTH/PTH-related protein receptor (PPR) in T cells abrogates the effects of iPTH, thus demonstrating a requirement for direct PPR signaling in T cells. Mechanistically, iPTH expands ST-HSPCs by activating Wnt signaling in HSPCs and stromal cells (SCs) through T-cell production of the Wnt ligand Wnt10b. Attesting to the relevance of Wnt10b, iPTH fails to expand ST-HSPCs in mice with Wnt10b(-/-) T cells. Moreover, iPTH fails to promote engraftment and survival after BM transplantation in Wnt10b null mice. In summary, direct PPR signaling in T cells and the resulting production of Wnt10b play a pivotal role in the mechanism by which iPTH expands ST-HSPCs. The data suggest that T cells may provide pharmacologic targets for HSPC expansion.

Attenuated Wnt/β-catenin signalling mediates methotrexate chemotherapy-induced bone loss and marrow adiposity in rats

Cancer chemotherapy often causes significant bone loss, marrow adiposity and haematopoietic defects, yet the underlying mechanisms and recovery potential remain unclear. Wnt/β-catenin signalling is integral to the regulation of osteogenesis, adipogenesis and haematopoiesis; using a rat model, the current study investigated roles of this signalling pathway in changes to bone marrow stromal and haematopoietic cell differentiation after chemotherapy with methotrexate (MTX), a commonly used antimetabolite. MTX treatment in rats (5 daily administrations at 0.75 mg/kg) has previously been found to decrease bone volume and increase marrow fat, which was associated with increased osteoclastogenesis in haematopoietic cells and with an osteogenesis to adipogenesis switch in bone marrow stromal cells of treated rats. In the current study, on day 6 after the first MTX dose we found that accompanying these changes as well as a suppressed haematopoietic cellularity but increased granulocyte/macrophage differentiation potential, there was an increase in mRNA expression of Wnt antagonists sFRP-1 and Dkk-1 in bone, a reduction in nuclear β-catenin protein in bone marrow stromal cells, and decreased mRNA levels of β-catenin target genes lef-1, cyclin D1 and survivin, suggesting reduced activation of Wnt/β-catenin signalling in the bone during MTX-induced damage. Concurrent administration of BIO, a GSK-3β inhibitor that stabilises β-catenin, partially abrogated the MTX-induced transient changes in osteogenic/adipogenic commitment, granulocyte/macrophage lineage differentiation and osteoclast number. These findings demonstrate a potentially important role of Wnt/β-catenin signalling in MTX chemotherapy-induced cellular changes to the bone marrow microenvironment.

Controlling the stem cell compartment and regeneration in vivo: the role of pluripotency pathways

Since the realization that embryonic stem cells are maintained in a pluripotent state through the interplay of a number of key signal transduction pathways, it is becoming increasingly clear that stemness and pluripotency are defined by the complex molecular convergence of these pathways. Perhaps this has most clearly been demonstrated by the capacity to induce pluripotency in differentiated cell types, so termed iPS cells. We are therefore building an understanding of how cells may be maintained in a pluripotent state, and how we may manipulate cells to drive them between committed and pluripotent compartments. However, it is less clear how cells normally pass in and out of the stem cell compartment under normal and diseased physiological states in vivo, and indeed, how important these pathways are in these settings. It is also clear that there is a potential "dark side" to manipulating the stem cell compartment, as deregulation of somatic stem cells is being increasingly implicated in carcinogenesis and the generation of "cancer stem cells." This review explores these relationships, with a particular focus on the role played by key molecular regulators of stemness in tissue repair, and the possibility that a better understanding of this control may open the door to novel repair strategies in vivo. The successful development of such strategies has the potential to replace or augment intervention-based strategies (cell replacement therapies), although it is clear they must be developed with a full understanding of how such approaches might also influence tumorigenesis.

Tcf7 is an important regulator of the switch of self-renewal and differentiation in a multipotential hematopoietic cell line

A critical problem in biology is understanding how cells choose between self-renewal and differentiation. To generate a comprehensive view of the mechanisms controlling early hematopoietic precursor self-renewal and differentiation, we used systems-based approaches and murine EML multipotential hematopoietic precursor cells as a primary model. EML cells give rise to a mixture of self-renewing Lin-SCA+CD34+ cells and partially differentiated non-renewing Lin-SCA-CD34− cells in a cell autonomous fashion. We identified and validated the HMG box protein TCF7 as a regulator in this self-renewal/differentiation switch that operates in the absence of autocrine Wnt signaling. We found that Tcf7 is the most down-regulated transcription factor when CD34+ cells switch into CD34− cells, using RNA–Seq. We subsequently identified the target genes bound by TCF7, using ChIP–Seq. We show that TCF7 and RUNX1 (AML1) bind to each other's promoter regions and that TCF7 is necessary for the production of the short isoforms, but not the long isoforms of RUNX1, suggesting that TCF7 and the short isoforms of RUNX1 function coordinately in regulation. Tcf7 knock-down experiments and Gene Set Enrichment Analyses suggest that TCF7 plays a dual role in promoting the expression of genes characteristic of self-renewing CD34+ cells while repressing genes activated in partially differentiated CD34− state. Finally a network of up-regulated transcription factors of CD34+ cells was constructed. Factors that control hematopoietic stem cell (HSC) establishment and development, cell growth, and multipotency were identified. These studies in EML cells demonstrate fundamental cell-intrinsic properties of the switch between self-renewal and differentiation, and yield valuable insights for manipulating HSCs and other differentiating systems.

The hematopoietic system has provided a leading model for stem cell studies, and there is great interest in elucidating the mechanisms that control the decision of HSC self-renewal and differentiation. This switch is important for understanding hematopoietic diseases and manipulating HSCs for therapeutic purposes. However, because HSCs are currently unable to proliferate extensively in vitro, this severely limits the types of biochemical analyses that can be performed; and, consequently, the mechanisms that control the decision between early-stage HSC self-renewal and differentiation remain unclear. Murine bone marrow derived EML multipotential hematopoietic precursor cells are ideal for studying the switch. EML cells can grow in large culture and give rise to a mixture of self-renewing Lin-SCA+CD34+ cells and partially differentiated non-renewing Lin-SCA-CD34− cells in a cell autonomous fashion. Using RNA–Sequencing and ChIP–Sequencing, we identified and validated the HMG box protein TCF7 as a regulator in this switch and find that it operates in the absence of canonical Wnt signaling. Together with RUNX1, TCF7 regulates a network of transcription factors that characterize the CD34+ cell state. This work serves as a model for studying mechanisms of autonomous and balanced cell fate choice and is ultimately valuable for manipulating HSCs.

Stem cell niche dynamics: from homeostasis to carcinogenesis

The stem cell microenvironment is involved in regulating the fate of the stem cell with respect to self-renewal, quiescence, and differentiation. Mathematical models are helpful in understanding how key pathways regulate the dynamics of stem cell maintenance and homeostasis. This tight regulation and maintenance of stem cell number is thought to break down during carcinogenesis. As a result, the stem cell niche has become a novel target of cancer therapeutics. Developing a quantitative understanding of the regulatory pathways that guide stem cell behavior will be vital to understanding how these systems change under conditions of stress, inflammation, and cancer initiation. Predictions from mathematical modeling can be used as a clinical tool to guide therapy design. We present a survey of mathematical models used to study stem cell population dynamics and stem cell niche regulation, both in the hematopoietic system and other tissues. Highlighting the quantitative aspects of stem cell biology, we describe compelling questions that can be addressed with modeling. Finally, we discuss experimental systems, most notably Drosophila, that can best be used to validate mathematical predictions.

The role of WNT10B in physiology and disease

Wnt10b is a member of the Wnt ligand gene family that encodes for secreted proteins, which activate the ancient and highly conserved Wnt signalling cascade. The Wnt pathway has been shown to be essential for embryonic development, tissue integrity, and stem cell activity, but if deregulated, also causes disease such as cancer. Although the 19 different Wnt ligands found in both human and mouse can activate several branches of the Wnt pathway, WNT10B specifically activates canonical Wnt/β-catenin signalling and thus triggers β-catenin/LEF/TCF-mediated transcriptional programs. In this review, we highlight the unique functions of WNT10B and mechanisms of how WNT10B acts in the immune system, mammary gland, adipose tissue, bone and skin. In these organs, WNT10B has been well established to be involved in signalling networks controlling stemness, pluripotency and cell fate decisions. Deregulation of these processes causes diseases such as breast cancer, obesity and osteoporosis. Compelling evidence suggests that WNT10B is a valuable candidate for the development of therapeutic regimens for human diseases.

Postgenomic technologies targeting the Wnt signaling network

The recent development of high-throughput sequencing technologies and the availability of whole genome sequences of a variety of living organisms, including that of humans, have led to an enormous push in the quest for a comprehensive inquiry for the function of each and every gene discovered in different model organisms. A major conclusion from the sequencing projects was that while forward genetics had been extremely successful in identifying key genes/components of many biological processes, such as signal transduction cascades, the function(s) of the majority of genes in the genome remains a mystery. In this article, we discuss the use of a variety of high-throughput postgenomic tools, including functional genomics, proteomics, and chemical genetics that are being implemented in an exhaustive molecular dissection of a key evolutionarily conserved signal transduction pathway, namely the Wnt/wingless (wg) pathway and its associated signaling network.

Pleiotrophin regulates the expansion and regeneration of hematopoietic stem cells

Hematopoietic stem cell (HSC) self-renewal is regulated by both intrinsic and extrinsic signals. Although some of the pathways that regulate HSC self-renewal have been uncovered, it remains largely unknown whether these pathways can be triggered by deliverable growth factors to induce HSC growth or regeneration. Here we show that pleiotrophin, a neurite outgrowth factor with no known function in hematopoiesis, efficiently promotes HSC expansion in vitro and HSC regeneration in vivo. Treatment of mouse bone marrow HSCs with pleiotrophin caused a marked increase in long-term repopulating HSC numbers in culture, as measured in competitive repopulating assays. Treatment of human cord blood CD34(+)CDCD38(-)Lin(-) cells with pleiotrophin also substantially increased severe combined immunodeficient (SCID)-repopulating cell counts in culture, compared to input and cytokine-treated cultures. Systemic administration of pleiotrophin to irradiated mice caused a pronounced expansion of bone marrow stem and progenitor cells in vivo, indicating that pleiotrophin is a regenerative growth factor for HSCs. Mechanistically, pleiotrophin activated phosphoinositide 3-kinase (PI3K) signaling in HSCs; antagonism of PI3K or Notch signaling inhibited pleiotrophin-mediated expansion of HSCs in culture. We identify the secreted growth factor pleiotrophin as a new regulator of both HSC expansion and regeneration.

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.

Expression analysis of E-cadherin, Slug and GSK3beta in invasive ductal carcinoma of breast

Background

Cancer progression is linked to a partially dedifferentiated epithelial cell phenotype. The signaling pathways Wnt, Hedgehog, TGF-β and Notch have been implicated in experimental and developmental epithelial mesenchymal transition (EMT). Recent findings from our laboratory confirm that active Wnt/β-catenin signaling is critically involved in invasive ductal carcinomas (IDCs) of breast.

Methods

In the current study, we analyzed the expression patterns and relationships between the key Wnt/β-catenin signaling components- E-cadherin, Slug and GSK3β in IDCs of breast.

Results

Of the 98 IDCs analyzed, 53 (54%) showed loss/or reduced membranous staining of E-cadherin in tumor cells. Nuclear accumulation of Slug was observed in 33 (34%) IDCs examined. Loss or reduced level of cytoplasmic GSK3β expression was observed in 52/98 (53%) cases; while 34/98 (35%) tumors showed nuclear accumulation of GSK3β. Statistical analysis revealed associations of nuclear Slug expression with loss of membranous E-cadherin (p = 0.001); nuclear β-catenin (p = 0.001), and cytoplasmic β-catenin (p = 0.005), suggesting Slug mediated E-cadherin suppression via the activation of Wnt/β-catenin signaling pathway in IDCs. Our study also demonstrated significant correlation between GSK3β nuclear localization and tumor grade (p = 0.02), suggesting its association with tumor progression.

Conclusion

The present study for the first time provided the clinical evidence in support of Wnt/β-catenin signaling upregulation in IDCs and key components of this pathway - E-cadherin, Slug and GSK3β with β-catenin in implementing EMT in these cells.

Genetic interaction of PGE2 and Wnt signaling regulates developmental specification of stem cells and regeneration

Interactions between developmental signaling pathways govern the formation and function of stem cells. Prostaglandin (PG) E2 regulates vertebrate hematopoietic stem cells (HSC). Similarly, the Wnt signaling pathway controls HSC self-renewal and bone marrow repopulation. Here, we show that wnt reporter activity in zebrafish HSCs is responsive to PGE2 modulation, demonstrating a direct interaction in vivo. Inhibition of PGE2 synthesis blocked wnt-induced alterations in HSC formation. PGE2 modified the wnt signaling cascade at the level of beta-catenin degradation through cAMP/PKA-mediated stabilizing phosphorylation events. The PGE2/Wnt interaction regulated murine stem and progenitor populations in vitro in hematopoietic ES cell assays and in vivo following transplantation. The relationship between PGE2 and Wnt was also conserved during regeneration of other organ systems. Our work provides in vivo evidence that Wnt activation in stem cells requires PGE2, and suggests the PGE2/Wnt interaction is a master regulator of vertebrate regeneration and recovery.

The stem cell niche

Virtually every tissue of the adult organism maintains a population of putatively slowly-cycling stem cells that maintain homeostasis of the tissue and respond to injury when challenged. These cells are regulated and supported by the surrounding microenvironment, referred to as the stem cell 'niche'. The niche includes all cellular and non-cellular components that interact in order to control the adult stem cell, and these interactions can often be broken down into one of two major mechanistic categories--physical contact and diffusible factors. The niche has been studied directly and indirectly in a number of adult stem cell systems. Herein, we will first focus on the most well-understood niches supporting the germline stem cells in the lower organisms Caenorhabditis elegans and Drosophila melanogaster before concentrating on the more complex, less well-understood mammalian niches supporting the neural, epidermal, haematopoietic and intestinal stem cells.

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.

Intrinsic and extrinsic control of haematopoietic stem-cell self-renewal

When stem cells divide, they can generate progeny with the same developmental potential as the original cell, a process referred to as self-renewal. Self-renewal is driven intrinsically by gene expression in a cell-type-specific manner and is modulated through interactions with extrinsic cues from the environment, such as growth factors. However, despite the prevalence of the term self-renewal in the scientific literature, this process has not been defined at the molecular level. Haematopoietic stem cells are an excellent model for the study of self-renewal because they can be isolated prospectively, manipulated relatively easily and assessed by using well-defined assays. Establishing the principles of self-renewal in haematopoietic stem cells will lead to insights into the mechanisms of self-renewal in other tissues.