Microenvironmental targeting of Wnt/β-catenin signals for hematopoietic stem cell regulation

Non-canonical Wnt signaling participates in Jagged1-induced osteo/odontogenic differentiation in human dental pulp stem cells

Osteoblast differentiation requires the interaction of various cell signaling pathways to modulate cell responses. Notch and Wnt signaling are among the crucial pathways that control numerous biological processes, including osteo/odontogenic differentiation. The aim of the present study was to examine the involvement of Wnt signaling in the Jagged1-induced osteo/odontogenic differentiation in human dental pulp stem cells (hDPSCs). The Wnt-related gene expression was analyzed from publicly available data of Jagged1-treated human dental pulp cells. The mRNA expression of Wnt ligands (WNT2B, WNT5A, WNT5B, and WNT16) and Wnt inhibitors (DKK1, DKK2, and SOST) were confirmed using real-time polymerase chain reaction. Among the Wnt ligands, WNT2B and WNT5A mRNA levels were upregulated after Jagged1 treatment. In contrast, the Wnt inhibitors DKK1, DKK2, and SOST mRNA levels were downregulated. Recombinant WNT5A, but not WNT2B, significantly promoted in vitro mineral deposition by hDPSCs. Wnt signaling inhibition using IWP-2, but not DKK1, inhibited Jagged1-induced alkaline phosphatase (ALP) activity, mineralization, and osteo/odontogenic marker gene expression in hDPSCs. In conclusion, Jagged1 promoted hDPSC osteo/odontogenic differentiation by modulating the non-canonical Wnt pathway.

The Role of Notch and Wnt Signaling in MSC Communication in Normal and Leukemic Bone Marrow Niche

Notch and Wnt signaling are highly conserved intercellular communication pathways involved in developmental processes, such as hematopoiesis. Even though data from literature support a role for these two pathways in both physiological hematopoiesis and leukemia, there are still many controversies concerning the nature of their contribution. Early studies, strengthened by findings from T-cell acute lymphoblastic leukemia (T-ALL), have focused their investigation on the mutations in genes encoding for components of the pathways, with limited results except for B-cell chronic lymphocytic leukemia (CLL); in because in other leukemia the two pathways could be hyper-expressed without genetic abnormalities. As normal and malignant hematopoiesis require close and complex interactions between hematopoietic cells and specialized bone marrow (BM) niche cells, recent studies have focused on the role of Notch and Wnt signaling in the context of normal crosstalk between hematopoietic/leukemia cells and stromal components. Amongst the latter, mesenchymal stromal/stem cells (MSCs) play a pivotal role as multipotent non-hematopoietic cells capable of giving rise to most of the BM niche stromal cells, including fibroblasts, adipocytes, and osteocytes. Indeed, MSCs express and secrete a broad pattern of bioactive molecules, including Notch and Wnt molecules, that support all the phases of the hematopoiesis, including self-renewal, proliferation and differentiation. Herein, we provide an overview on recent advances on the contribution of MSC-derived Notch and Wnt signaling to hematopoiesis and leukemia development.

Ryk modulates the niche activity of mesenchymal stromal cells by fine-tuning canonical Wnt signaling

The importance of modulating the intensity of Wnt signaling has been highlighted in various biological models, but their mechanisms remain unclear. In this study, we found that Ryk—an atypical Wnt receptor with a pseudokinase domain—has a Wnt-modulating effect in bone marrow stromal cells to control hematopoiesis-supporting activities. We first found that Ryk is predominantly expressed in the mesenchymal stromal cells (MSCs) of the bone marrow (BM) compared with hematopoietic cells. Downregulation of Ryk in MSCs decreased their clonogenic activity and ability to support self-renewing expansion of primitive hematopoietic progenitors (HPCs) in response to canonical Wnt ligands. In contrast, under high concentrations of Wnt, Ryk exerted suppressive effects on the transactivation of target genes and HPC-supporting effects in MSCs, thus fine-tuning the signaling intensity of Wnt in BM stromal cells. This ability of Ryk to modulate the HPC-supporting niche activity of MSCs was abrogated by induction of deletion mutants of Ryk lacking the intracellular domain or extracellular domain, indicating that the pseudokinase-containing intracellular domain mediates the Wnt-modulating effects in response to extracellular Wnt ligands. These findings indicate that the ability of the BM microenvironment to respond to extracellular signals and support hematopoiesis may be fine-tuned by Ryk via modulation of Wnt signaling intensity to coordinate hematopoietic activity.

Steady production of immune and blood cells depends on a signaling protein that helps maintain stable stem cell populations within the bone marrow. Hematopoietic stem cells (HSCs), which give rise to blood cells, reside within a supportive “niche” surrounded by mesenchymal stromal cells (MSCs), with extensive communication between the two populations. Researchers led by Il-Hoan Oh at The Catholic University of Korea, Seoul, have now identified a mechanism that MSCs employ to stabilize the niche environment through fine-tuning the signaling intensity of Wnt. Oh and colleagues focused on a signaling pathway that controls the undifferentiated state of HSCs, and showed that these signals are specifically modulated by an MSC protein known as Ryk. Without Ryk, MSCs can no longer promote HSC proliferation. However, when these signals are excessively strong, Ryk helps suppress proliferation to keep HSC numbers under control.

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.

Exosomes as Novel Regulators of Adult Neurogenic Niches

Adult neurogenesis has been convincingly demonstrated in two regions of the mammalian brain: the sub-granular zone (SGZ) of the dentate gyrus (DG) in the hippocampus, and the sub-ventricular zone (SVZ) of the lateral ventricles (LV). SGZ newborn neurons are destined to the granular cell layer (GCL) of the DG, while new neurons from the SVZ neurons migrate rostrally into the olfactory bulb (OB). The process of adult neurogenesis persists throughout life and is supported by a pool of neural stem cells (NSCs), which reside in a unique and specialized microenvironment known as "neurogenic niche". Neurogenic niches are structured by a complex organization of different cell types, including the NSC-neuron lineage, glial cells and vascular cells. Thus, cell-to-cell communication plays a key role in the dynamic modulation of homeostasis and plasticity of the adult neurogenic process. Specific cell-cell contacts and extracellular signals originated locally provide the necessary support and regulate the balance between self-renewal and differentiation of NSCs. Furthermore, extracellular signals originated at distant locations, including other brain regions or systemic organs, may reach the niche through the cerebrospinal fluid (CSF) or the vasculature and influence its nature. The role of several secreted molecules, such as cytokines, growth factors, neurotransmitters, and hormones, in the biology of adult NSCs, has been systematically addressed. Interestingly, in addition to these well-recognized signals, a novel type of intercellular messengers has been identified recently: the extracellular vesicles (EVs). EVs, and particularly exosomes, are implicated in the transfer of mRNAs, microRNAs (miRNAs), proteins and lipids between cells and thus are able to modify the function of recipient cells. Exosomes appear to play a significant role in different stem cell niches such as the mesenchymal stem cell niche, cancer stem cell niche and pre-metastatic niche; however, their roles in adult neurogenic niches remain virtually unexplored. This review focuses on the current knowledge regarding the functional relationship between cellular and extracellular components of the adult SVZ and SGZ neurogenic niches, and the growing evidence that supports the potential role of exosomes in the physiology and pathology of adult neurogenesis.

Lung cancer stem cells: The root of resistance

In the absence of specific treatable mutations, platinum-based chemotherapy remains the gold standard of treatment for lung cancer patients. However, 5-year survival rates remain poor due to the development of resistance and eventual relapse. Resistance to conventional cytotoxic therapies presents a significant clinical challenge in the treatment of this disease. The cancer stem cell (CSC) hypothesis suggests that tumors are arranged in a hierarchical structure, with the presence of a small subset of stem-like cells that are responsible for tumor initiation and growth. This CSC population has a number of key properties such as the ability to asymmetrically divide, differentiate and self-renew, in addition to having increased intrinsic resistance to therapy. While cytotoxic chemotherapy kills the bulk of tumor cells, CSCs are spared and have the ability to recapitulate the heterogenic tumor mass. The identification of lung CSCs and their role in tumor biology and treatment resistance may lead to innovative targeted therapies that may ultimately improve clinical outcomes in lung cancer patients. This review will focus on lung CSC markers, their role in resistance and their relevance as targets for future therapies.

Wnt/β-catenin signaling in bone marrow niche

The bone marrow (BM) niche is a specific physiological environment for hematopoietic and non-hematopoietic stem cells (HSCs). Several signaling pathways (including Wnt/β-catenin) regulate various aspects of stem cell growth, function and death in the BM niche. In addition, the canonical Wnt pathway is crucial for directing self-renewal and differentiation as important mechanisms in many types of stem cells. We review the role of the Wnt/β-catenin pathway in the BM niche and its importance in stem cells. Relevant literature was identified by a PubMed search (1997-2014) of English-language literature by using the following keywords: BM niche, Wnt/β-catenin signaling, osteoblast, osteoclast and bone disease. The Wnt/β-catenin pathway regulates the stability of the β-catenin proto-oncogene. The stabilized β-catenin then translocates to the nucleus, forming a β-catenin-TCF/LEF complex regulating the transcription of specific target genes. Stem cells require β-catenin to mediate their response to Wnt signaling for maintenance and transition from the pluripotent state during embryogenesis. In adult stem cells, Wnt signaling functions at various hierarchical levels to contribute to the specification of the diverse tissues. Aberrant Wnt/β-catenin signaling and its downstream transcriptional regulators are observed in several malignant stem cells and human cancers. Because Wnt signaling can maintain stem cells and cancer cells, the ability to modulate the Wnt pathway either positively or negatively may be of therapeutic relevance. The controlled activation of Wnt signaling might allow us to enhance stem and progenitor cell activity when regeneration is needed.

Regulation of mesenchymal stromal cells through fine tuning of canonical Wnt signaling

Mesenchymal stromal cells (MSCs) have been extensively utilized for various cell therapeutic trials, but the signals regulating their stromal function remain largely unclear. Here, we show that canonical Wnt signals distinctively regulate MSCs in a biphasic manner depending on signal intensity, i.e., MSCs exhibit proliferation and progenitor self-renewal under low Wnt/β-catenin signaling, whereas they exhibit enhanced osteogenic differentiation with priming to osteoblast-like lineages under high Wnt/β-catenin signaling. Moreover, low or high levels of β-catenin in MSCs distinctly regulated the hematopoietic support of MSCs to promote proliferation or the undifferentiated state of hematopoietic progenitors, respectively. A gene expression study demonstrated that different intracellular levels of β-catenin in MSCs induce distinct transcriptomic changes in subsets of genes belonging to different gene function categories. Different β-catenin levels also induced differences in intracellular levels of the β-catenin co-factors, Tcf-1 and Lef-1. Moreover, nano-scale mass spectrometry of proteins that co-precipitated with β-catenin revealed distinctive spectra of proteins selectively interacting with β-catenin at specific expression levels. Together, these results show that Wnt/β-catenin signals can coax distinct transcription milieu to induce different transcription profiles in MSCs depending on the signal intensity and that fine-tuning of the canonical Wnt signaling intensity can regulate the phase-specific functionality of MSCs.

Stemness of T cells and the hematopoietic stem cells: fate, memory, niche, cytokines

Stem cells are able to generate both cells that differentiate and cells that remain undifferentiated but potentially have the same developmental program. The prolonged duration of the protective immune memory for infectious diseases such as polio, small pox, and measles, suggested that memory T cells may have stem cell properties. Understanding the molecular basis for the life-long persistence of memory T cells may be useful to project targeted therapies for immune deficiencies and infectious diseases and to formulate vaccines. In the last decade evidence from different laboratories shows that memory T cells may share self-renewal pathways with bone marrow hematopoietic stem cells. In stem cells the intrinsic self-renewal activity, which depends on gene expression, is known to be modulated by extrinsic signals from the environment that may be tissue specific. These extrinsic signals for stemness of memory T cells include cytokines such as IL-7 and IL-15 and there are other cytokine signals for maintaining the cytokine signature (TH1, TH2, etc.) of memory T cells. Intrinsic and extrinsic pathways that might be common to bone marrow hematopoietic stem cells and memory T lymphocytes are discussed and related to self-renewal functions.

Targeting self-renewal pathways in myeloid malignancies

A fundamental property of hematopoietic stem cells (HSCs) is the ability to self-renew. This is a complex process involving multiple signal transduction cascades which control the fine balance between self-renewal and differentiation through transcriptional networks. Key activators/regulators of self-renewal include chemokines, cytokines and morphogens which are expressed in the bone marrow niche, either in a paracrine or autocrine fashion, and modulate stem cell behaviour. Increasing evidence suggests that the downstream signaling pathways induced by these ligands converge at multiple levels providing a degree of redundancy in steady state hematopoiesis. Here we will focus on how these pathways cross-talk to regulate HSC self-renewal highlighting potential therapeutic windows which could be targeted to prevent leukemic stem cell self-renewal in myeloid malignancies.

Immune and inflammatory pathways are involved in inherent bone marrow ossification

BACKGROUND

Bone marrow plays a key role in bone formation and healing. Although a subset of marrow explants ossifies in vitro without excipient osteoinductive factors, some explants do not undergo ossification. The disparity of outcome suggests a significant heterogeneity in marrow tissue in terms of its capacity to undergo osteogenesis.

QUESTIONS/PURPOSES

We sought to identify: (1) proteins and signaling pathways associated with osteogenesis by contrasting the proteomes of ossified and poorly ossified marrow explants; and (2) temporal changes in proteome and signaling pathways of marrow ossification in the early and late phases of bone formation.

METHODS

Explants of marrow were cultured. Media conditioned by ossified (n = 4) and poorly ossified (n = 4) subsets were collected and proteins unique to each group were identified by proteomic analysis. Proteomic data were processed to assess proteins specific to the early phase (Days 1-14) and late phase (Days 15-28) of the culture period. Pathways involved in bone marrow ossification were identified through bioinformatics.

RESULTS

Twenty-eight proteins were unique to ossified samples and eight were unique to poorly ossified ones. Twelve proteins were expressed during the early phase and 15 proteins were specific to the late phase. Several identified pathways corroborated those reported for bone formation in the literature. Immune and inflammatory pathways were specific to ossified samples.

CONCLUSIONS

The marrow explant model indicates the inflammatory and immune pathways to be an integral part of the osteogenesis process.

Concise review: Multidimensional regulation of the hematopoietic stem cell state

Hematopoietic stem cells (HSCs) are characterized by their unique function to produce all lineages of blood cells throughout life. Such tissue-specific function of HSC is attributed to their ability to execute self-renewal and multilineage differentiation. Accumulating evidence indicates that the undifferentiated state of HSC is characterized by dynamic maintenance of chromatin structures and epigenetic plasticity. Conversely, quiescence, self-renewal, and differentiation of HSCs are dictated by complex regulatory mechanisms involving specific transcription factors and microenvironmental crosstalk between stem cells and multiple compartments of niches in bone marrows. Thus, multidimensional regulatory inputs are integrated into two opposing characters of HSCs-maintenance of undifferentiated state analogous to pluripotent stem cells but execution of tissue-specific hematopoietic functions. Further studies on the interplay of such regulatory forces as "cell fate determinant" will likely shed the light on diverse spectrums of tissue-specific stem cells.

Human adipose-derived stromal cells efficiently support hematopoiesis in vitro and in vivo: a key step for therapeutic studies

Adipose-derived stromal cells (ADSCs) are close relatives of bone marrow mesenchymal stromal cells (BM-MSCs). The ease of access to subcutaneous fat pad and the abundance of stromal precursors make fat tissue an attractive source of stromal cells for clinicians. However, their ability to support hematopoietic stem cells in vitro and in vivo has not been established definitively. Thus, their usefulness in supporting hematopoietic stem cell engraftment is not as clear as with BM-MSCs. In this article, we show that human ADSCs, cultured with a good manufacturing practice medium, maintain in vitro human early and committed hematopoietic progenitors and support their complete differentiation toward myeloid and lymphoid lineages. Compared with BM-MSCs, ADSCs elicit a more precocious early progenitor formation and faster proliferation and differentiation of hematopoietic progenitors. Further, in vivo, when co-injected in NOD.Cg-Prkdc(scid) Il2(rgtm1Wjl)/SzJ (NSG) mice with a low number of human CD34(+) cells, ADSCs enabled the higher production of immature human hematopoietic progenitors and CD45(+) cells when compared with BM-MSCs. As a whole, our results indicate that human ADSCs, isolated and expanded under clinical-grade conditions, support hematopoiesis in vitro and in vivo and thus provide the rationale for their use in supporting hematopoietic reconstitution in clinical settings.

Regulation of CXCL12 expression by canonical Wnt signaling in bone marrow stromal cells

CXCL12 (stromal cell-derived factor-1, SDF-1), produced by stromal and endothelial cells including cells of the bone marrow, binds to its receptor CXCR4 and this axis regulates hematopoietic cell trafficking. Recently, osteoclast precursor cells were found to express CXCR4 and a potential role for the CXCL12-CXCR4 axis during osteoclast precursor cell recruitment/retention and development was proposed as a regulator of bone resorption. We examined the role of canonical Wnt signaling in regulating the expression of CXCL12 in bone marrow stromal cells. In mouse stromal ST2 cells, CXCL12 mRNA was expressed, while its expression was reduced in Wnt3a over-expressing ST2 (Wnt3a-ST2) cells or by treatment with lithium chloride (LiCl). Wnt3a decreased CXCL12 levels in culture supernatants from mouse bone marrow stromal cells. The culture supernatant from Wnt3a-ST2 cells also reduced migratory activity of bone marrow-derived cells in a Transwell migration assay. Silencing of glycogen synthase kinase-3β decreased CXCL12 expression, suggesting that the canonical Wnt signaling pathway regulates CXCL12 expression. In a transfection assay, LiCl down-regulated the activity of a reporter gene, a 1.8kb fragment of the 5'-flanking region of the CXCL12 gene. These results show that canonical Wnt signaling regulates CXCL12 gene expression at the transcriptional level, and this is the first study linking chemokine expression to canonical Wnt signaling.