The signaling protein Wnt4 enhances thymopoiesis and expands multipotent hematopoietic progenitors through beta-catenin-independent signaling

WNT Signaling in Stem Cells: A Look into the Non-Canonical Pathway

Tissue homeostasis is crucial for multicellular organisms, wherein the loss of cells is compensated by generating new cells with the capacity for proliferation and differentiation. At the origin of these populations are the stem cells, which have the potential to give rise to cells with both capabilities, and persevere for a long time through the self-renewal and quiescence. Since the discovery of stem cells, an enormous effort has been focused on learning about their functions and the molecular regulation behind them. Wnt signaling is widely recognized as essential for normal and cancer stem cell. Moreover, β-catenin-dependent Wnt pathway, referred to as canonical, has gained attention, while β-catenin-independent Wnt pathways, known as non-canonical, have remained conspicuously less explored. However, recent evidence about non-canonical Wnt pathways in stem cells begins to lay the foundations of a conceivably vast field, and on which we aim to explain this in the present review. In this regard, we addressed the different aspects in which non-canonical Wnt pathways impact the properties of stem cells, both under normal conditions and also under disease, specifically in cancer.

Wnt signaling regulates ion channel expression to promote smooth muscle and cartilage formation in developing mouse trachea

Ion channels play critical roles in the physiology and function of the nervous system and contractile tissue; however, their role in noncontractile tissue and embryonic development has yet to be understood. Tracheobronchomalacia (TBM) and complete tracheal rings (CTR) are disorders affecting the muscle and cartilage of the trachea and bronchi, whose etiology remains poorly understood. We demonstrated that trachealis muscle organization and polarity are disrupted after epithelial ablation of Wntless (Wls), a cargo receptor critical for the Wnt signaling pathway, in developing trachea. The phenotype resembles the anomalous trachealis muscle observed after deletion of ion channel encoding genes in developing mouse trachea. We sought to investigate whether and how the deletion of Wls affects ion channels during tracheal development. We hypothesize that Wnt signaling influences the expression of ion channels to promote trachealis muscle cell assembly and patterning. Deleting Wls in developing trachea causes differential regulation of genes mediating actin binding, cytoskeleton organization, and potassium ion channel activity. Wnt signaling regulates the expression of Kcnj13, Kcnd3, Kcnj8, and Abcc9 as demonstrated by in vitro studies and in vivo analysis in Wnt5a and β-catenin-deficient tracheas. Pharmacological inhibition of potassium ion channels and Wnt signaling impaired contractility of developing trachealis smooth muscle and formation of cartilaginous mesenchymal condensation. Thus, in mice, epithelial-induced Wnt/β-catenin signaling mediates trachealis muscle and cartilage development via modulation of ion channel expression, promoting trachealis muscle architecture, contractility, and cartilaginous extracellular matrix. In turn, ion channel activity may influence tracheal morphogenesis underlying TBM and CTR.NEW & NOTEWORTHY Ion channels play critical roles in the physiology and function of the nervous system and contractile tissue; however, their role in noncontractile tissue and embryonic development has yet to be understood. In this study, we focused on the role of ion channels in the differentiation and patterning of the large airways of the developing respiratory tract. We identify a mechanism by which Wnt-beta-catenin signaling controls levels of ion channel-encoding genes to promote tracheal differentiation.

The Roles of Secreted Wnt Ligands in Cancer

Wnt ligands are secreted signaling proteins that display a wide range of biological effects. They play key roles in stimulating Wnt signaling pathways to facilitate processes such as tissue homeostasis and regeneration. Dysregulation of Wnt signaling is a hallmark of many cancers and genetic alterations in various Wnt signaling components, which result in ligand-independent or ligand-dependent hyperactivation of the pathway that have been identified. Recently, research is focusing on the impact of Wnt signaling on the interaction between tumor cells and their micro-environment. This Wnt-mediated crosstalk can act either in a tumor promoting or suppressing fashion. In this review, we comprehensively outline the function of Wnt ligands in different tumor entities and their impact on key phenotypes, including cancer stemness, drug resistance, metastasis, and immune evasion. Lastly, we elaborate approaches to target Wnt ligands in cancer therapy.

Single-cell transcriptomic profiling redefines the origin and specification of early adrenogonadal progenitors

Adrenal cortex and gonads represent the two major steroidogenic organs in mammals. Both tissues are considered to share a common developmental origin characterized by the expression of Nr5a1/Sf1. The precise origin of adrenogonadal progenitors and the processes driving differentiation toward the adrenal or gonadal fate remain, however, elusive. Here, we provide a comprehensive single-cell transcriptomic atlas of early mouse adrenogonadal development including 52 cell types belonging to twelve major cell lineages. Trajectory reconstruction reveals that adrenogonadal cells emerge from the lateral plate rather than the intermediate mesoderm. Surprisingly, we find that gonadal and adrenal fates have already diverged prior to Nr5a1 expression. Finally, lineage separation into gonadal and adrenal fates involves canonical versus non-canonical Wnt signaling and differential expression of Hox patterning genes. Thus, our study provides important insights into the molecular programs of adrenal and gonadal fate choice and will be a valuable resource for further research into adrenogonadal ontogenesis.

Roles and action mechanisms of WNT4 in cell differentiation and human diseases: a review

WNT family member 4 (WNT4), which belongs to the conserved WNT protein family, plays an important role in the development and differentiation of many cell types during the embryonic development and adult homeostasis. Increasing evidence has shown that WNT4 is a special ligand that not only activates the β-catenin independent pathway but also acts on β-catenin signaling based on different cellular processes. This article is a summary of the current knowledge about the expression, regulation, and function of WNT4 ligands and their signal pathways in cell differentiation and human disease processes. WNT4 is a promoter in osteogenic differentiation in bone marrow stromal cells (BMSCs) by participating in bone homeostasis regulation in osteoporotic diseases. Non-canonical WNT4 signaling is necessary for metabolic maturation of pancreatic β-cell. WNT4 is also necessary for decidual cell differentiation and decidualization, which plays an important role in preeclampsia. WNT4 promotes neuronal differentiation of neural stem cell and dendritic cell (DC) into conventional type 1 DC (cDC1). Besides, WNT4 mediates myofibroblast differentiation in the skin, kidney, lung, and liver during scarring or fibrosis. On the negative side, WNT4 is highly expressed in cancer tissues, playing a pro-carcinogenic role in many cancer types. This review provides an overview of the progress in elucidating the role of WNT4 signaling pathway components in cell differentiation in adults, which may provide useful clues for the diagnosis, prevention, and therapy of human diseases.

Cell-intrinsic Wnt4 promotes hematopoietic stem and progenitor cell self-renewal

Although intracellular Wnt signaling pathways need to be tightly regulated to promote hematopoietic stem cell self-renewal, the source and identity of important Wnt ligands in the bone marrow is still largely unknown. The noncanonical ligand Wnt4 is expressed in the bone marrow as well as in the stroma, and its overexpression in fetal liver cells facilitates thymic recovery; however, its impact on adult hematopoietic stem cell function remains unclear. Here, we report that the deletion of Wnt4 from hematopoietic cells in mice (Wnt4^Δ/Δ ) resulted in decreased lymphopoiesis at steady state. This was likely at least in part due to the increased proinflammatory environment present in the bone marrow of Wnt4^Δ/Δ mice. Wnt4^Δ/Δ hematopoietic stem cells displayed reduced reconstitution capacity in serial transplants, thus demonstrating defective self-renewal, and they expanded poorly in response to lipopolysaccharide stimulation. This appeared to be the result of the absence of Wnt4 in stem/progenitor cells, as myeloid-restricted Wnt4 deletion had no notable effect. Finally, we observed that Wnt4^Δ/Δ stem/progenitor cells were more quiescent, presenting enhanced levels of stress-associated JNK phosphorylation and p16^INK4a expression, likely contributing to the reduced expansion observed in transplants. In conclusion, our results identify a new, largely autocrine role for Wnt4 in hematopoietic stem cell self-renewal, suggesting that regulation of Wnt signaling in hematopoiesis may not need Wnt secretion and could be independent of morphogen gradients.

Global Transcriptomic Profiling of the Bone Marrow Stromal Microenvironment during Postnatal Development, Aging, and Inflammation

Bone marrow (BM) stromal cells provide the regulatory framework for hematopoiesis and contribute to developmental stage-specific niches, such as those preserving hematopoietic stem cells. Despite advances in our understanding of stromal function, little is known about the transcriptional changes that this compartment undergoes throughout lifespan and during adaptation to stress. Using RNA sequencing, we perform transcriptional analyses of four principal stromal subsets, namely CXCL12-abundant reticular, platelet-derived growth factor receptor (PDGFR)-α⁺Sca1⁺, sinusoidal, and arterial endothelial cells, from early postnatal, adult, and aged mice. Our data reveal (1) molecular fingerprints defining cell-specific anatomical and functional features, (2) a radical reprogramming of pro-hematopoietic, immune, and matrisomic transcriptional programs during the transition from juvenile stages to adulthood, and (3) the aging-driven progressive upregulation of pro-inflammatory gene expression in stroma. We further demonstrate that transcriptomic pathways elicited in vivo by prototypic microbial molecules are largely recapitulated during aging, thereby supporting the inflammatory basis of age-related adaptations of BM hematopoietic function.

Wnt family member 4 (WNT4) and WNT3A activate cell-autonomous Wnt signaling independent of porcupine O-acyltransferase or Wnt secretion

Porcupine O-acyltransferase (PORCN) is considered essential for Wnt secretion and signaling. However, we observed that PORCN inhibition does not phenocopy the effects of WNT4 knockdown in WNT4-dependent breast cancer cells. This suggests a unique relationship between PORCN and WNT4 signaling. To examine the role of PORCN in WNT4 signaling, here we overexpressed WNT4 or WNT3A in breast cancer, ovarian cancer, and fibrosarcoma cell lines. Conditioned media from these lines and co-culture systems were used to assess the dependence of Wnt secretion and activity on the critical Wnt secretion proteins PORCN and Wnt ligand secretion (WLS) mediator. We observed that WLS is universally required for Wnt secretion and paracrine signaling. In contrast, the dependence of WNT3A secretion and activity on PORCN varied across the cell lines, and WNT4 secretion was PORCN-independent in all models. Surprisingly, WNT4 did not exhibit paracrine activity in any tested context. Absent the expected paracrine activity of secreted WNT4, we identified cell-autonomous Wnt signaling activation by WNT4 and WNT3A, independent of PORCN or Wnt secretion. The PORCN-independent, cell-autonomous Wnt signaling demonstrated here may be critical in WNT4-driven cellular contexts or in those that are considered to have dysfunctional Wnt signaling.

Cell-Intrinsic Wnt4 Influences Conventional Dendritic Cell Fate Determination to Suppress Type 2 Immunity

Whether conventional dendritic cells (cDC) acquire subset identity under direction of Wnt family glycoproteins is unknown. We demonstrate that Wnt4, a β-catenin-independent Wnt ligand, is produced by both hematopoietic and nonhematopoietic cells and is both necessary and sufficient for preconventional DC1/cDC1 maintenance. Whereas bone marrow cDC precursors undergo phosphoJNK/c-Jun activation upon Wnt4 treatment, loss of cDC Wnt4 in CD11c^CreWnt4^flox/flox mice impaired differentiation of CD24⁺, Clec9A⁺, CD103⁺ cDC1 compared with CD11c^Cre controls. Conversely, single-cell RNA sequencing analysis of bone marrow revealed a 2-fold increase in cDC2 gene signature genes, and flow cytometry demonstrated increased numbers of SIRP-α⁺ cDC2 amid lack of Wnt4. Increased cDC2 numbers due to CD11c-restricted Wnt4 deficiency increased IL-5 production, group 2 innate lymphoid cell expansion, and host resistance to the hookworm parasite Nippostrongylus brasiliensis Collectively, these data uncover a novel and unexpected role for Wnt4 in cDC subset differentiation and type 2 immunity.

Colorectal cancer cells suppress CD4+ T cells immunity through canonical Wnt signaling

Understanding how colorectal cancer escapes from immunosurveillance and immune attack is important for developing novel immunotherapies for colorectal cancer. In this study we evaluated the role of canonical Wnt signaling in the regulation of T cell function in a mouse colorectal cancer model. We found that colorectal cancer cells expressed abundant Wnt ligands, and intratumoral T cells expressed various Frizzled proteins. Meanwhile, both active β-catenin and total β-catenin were elevated in intratumoral T cells. In vitro study indicated that colorectal cancer cells suppressed IFN-γ expression and increased IL-17a expression in activated CD4⁺ T cells. However, the cytotoxic activity of CD8⁺ T cells was not altered by colorectal cancer cells. To further evaluate the importance of Wnt signaling for CD4⁺ T cell-mediated cancer immunity, β-catenin expression was enforced in CD4⁺ T cells using lentiviral transduction. In an adoptive transfer model, enforced expression of β-catenin in intratumoral CD4⁺ T cells increased IL-17a expression, enhanced proliferation and inhibited apoptosis of colorectal cancer cells. Taken together, our study disclosed a new mechanism by which colorectal cancer impairs T cell immunity.

Elevated levels of Wnt signaling disrupt thymus morphogenesis and function

All vertebrates possess a thymus, whose epithelial microenvironment is essential for T cell development and maturation. Despite the importance of the thymus for cellular immune defense, many questions surrounding its morphogenesis remain unanswered. Here, we demonstrate that, in contrast to the situation in many other epithelial cell types, differentiation of thymic epithelial cells (TECs) proceeds normally in the absence of canonical Wnt signaling and the classical adhesion molecule E-cadherin. By contrast, TEC-intrinsic activation of β-catenin-dependent Wnt signaling blocks the morphogenesis of the thymus, and overexpression of a secreted Wnt ligand by TECs dominantly modifies the morphogenesis not only of the thymus, but also of the parathyroid and thyroid. These observations indicate that Wnt signaling activity in the thymus needs to be precisely controlled to support normal TEC differentiation, and suggest possible mechanisms underlying anatomical variations of the thymus, parathyroid and thyroid in humans.

Competitive Transplants to Evaluate Hematopoietic Stem Cell Fitness

The gold standard definition of a hematopoietic stem cell (HSC) is a cell that when transferred into an irradiated recipient will have the ability to reestablish blood cell production for the lifespan of the recipient. This protocol explains how to set up a functional assay to compare the HSC capacities of two different populations of cells, such as bone marrow from mice of two different genotypes, and how to analyze the recipient mice by flow cytometry. The protocol uses HSC equivalents rather than cell sorting for standardization and discusses the advantages and disadvantages of both approaches. We further discuss different variations to the basic protocol, including serial transplants, limiting dilution assays, homing assays and non-competitive transplants, including the advantages and preferred uses of these varied approaches. These assays are central for the study of HSC function and could be used not only for the investigation of fundamental HSC intrinsic aspects of biology but also for the development of preclinical assays for bone marrow transplant and HSC expansion in culture.

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.

ADAR1 Activation Drives Leukemia Stem Cell Self-Renewal by Impairing Let-7 Biogenesis

Post-transcriptional adenosine-to-inosine RNA editing mediated by adenosine deaminase acting on RNA1 (ADAR1) promotes cancer progression and therapeutic resistance. However, ADAR1 editase-dependent mechanisms governing leukemia stem cell (LSC) generation have not been elucidated. In blast crisis chronic myeloid leukemia (BC CML), we show that increased JAK2 signaling and BCR-ABL1 amplification activate ADAR1. In a humanized BC CML mouse model, combined JAK2 and BCR-ABL1 inhibition prevents LSC self-renewal commensurate with ADAR1 downregulation. Lentiviral ADAR1 wild-type, but not an editing-defective ADAR1(E912A) mutant, induces self-renewal gene expression and impairs biogenesis of stem cell regulatory let-7 microRNAs. Combined RNA sequencing, qRT-PCR, CLIP-ADAR1, and pri-let-7 mutagenesis data suggest that ADAR1 promotes LSC generation via let-7 pri-microRNA editing and LIN28B upregulation. A small-molecule tool compound antagonizes ADAR1's effect on LSC self-renewal in stromal co-cultures and restores let-7 biogenesis. Thus, ADAR1 activation represents a unique therapeutic vulnerability in LSCs with active JAK2 signaling.

Discrete roles of canonical and non-canonical Wnt signaling in hematopoiesis and lymphopoiesis

The mechanisms that regulate proliferation, fate decisions and differentiation of hematopoietic stem cells (HSC) and thymic stem cells are highly complex. Several signaling pathways including Wnt signaling have important roles during these processes. Both canonical and non-canonical Wnt signaling are important in normal and malignant hematopoiesis and lymphoid development, yet their precise roles are controversial. In a side-by-side comparison, we investigated the roles of the canonical and non-canonical Wnt pathway in hematopoiesis and thymopoiesis. As complete loss-of-function models for non-canonical Wnt signaling are not yet available and highly complex for canonical Wnt signaling, we decided to use a gain-of-function approach. To this end, Wnt3a and Wn5a, two well-known prototypical canonical and non-canonical Wnt ligands were produced in hematopoiesis supporting stromal assays. High levels of Wnt3a signaling blocked T-cell development at early stages, whereas intermediate levels accelerated T-cell development. In contrast, Wnt5a signaling prompted apoptosis in developing thymocytes, without affecting differentiation at a particular stage. To explore the role of Wnt3a and Wnt5a in vivo, we transduced HSCs isolated from fetal liver, transduced with Wnt3a and Wnt5a vectors, and performed reconstitution assays in irradiated C57Bl/6 mice. Wnt3a overexpression led to increased lymphopoiesis, whereas Wnt5a augments myelopoiesis in the bone marrow (BM) and spleen. Thus, the canonical and non-canonical Wnt signaling have discrete roles in hematopoiesis and thymopoiesis, and understanding their right dose of action is crucial for prospective translational applications.

Decline of FOXN1 gene expression in human thymus correlates with age: possible epigenetic regulation

BACKGROUND

Thymic involution is thought to be an important factor of age related immunodeficiency. Understanding the molecular mechanisms of human thymic senescence may lead to the discovery of novel therapeutic approaches aimed at the reestablishment of central and peripheral T cell repertoire.

RESULTS

As an initial approach, here we report that the decline of human thymic FOXN1 transcription correlates with age, while other genes, DLL1, DLL4 and WNT4, essential for thymopoiesis, are constitutively transcribed. Using a human thymic epithelial cell line (hTEC), we show that FOXN1 expression is refractory to signals that induce FOXN1 transcription in primary 3D culture conditions and by stimulation of the canonical WNT signaling pathway. Blockage of FOXN1 induceability in the hTEC line may be mediated by an epigenetic mechanism, the CpG methylation of the FOXN1 gene.

CONCLUSION

We showed a suppression of FOXN1 transcription both in cultured human thymic epithelial cells and in the aging thymus. We hypothesize that the underlying mechanism may be associated with changes of the DNA methylation state of the FOXN1 gene.

Regulation of the Wnt/β-Catenin Signaling Pathway by Human Papillomavirus E6 and E7 Oncoproteins

Cell signaling pathways are the mechanisms by which cells transduce external stimuli, which control the transcription of genes, to regulate diverse biological effects. In cancer, distinct signaling pathways, such as the Wnt/β-catenin pathway, have been implicated in the deregulation of critical molecular processes that affect cell proliferation and differentiation. For example, changes in β-catenin localization have been identified in Human Papillomavirus (HPV)-related cancers as the lesion progresses. Specifically, β-catenin relocates from the membrane/cytoplasm to the nucleus, suggesting that this transcription regulator participates in cervical carcinogenesis. The E6 and E7 oncoproteins are responsible for the transforming activity of HPV, and some studies have implicated these viral oncoproteins in the regulation of the Wnt/β-catenin pathway. Nevertheless, new interactions of HPV oncoproteins with cellular proteins are emerging, and the study of the biological effects of such interactions will help to understand HPV-related carcinogenesis. This review addresses the accumulated evidence of the involvement of the HPV E6 and E7 oncoproteins in the activation of the Wnt/β-catenin pathway.

Frizzled-6 Regulates Hematopoietic Stem/Progenitor Cell Survival and Self-Renewal

Adult hematopoietic stem/progenitor cell (HSPC) numbers remain stable in the absence of external stressors. After bone marrow (BM) transplant, HSPCs need to expand substantially to repopulate the BM and replenish the peripheral blood cell pool. In this study, we show that a noncanonical Wnt receptor, Frizzled-6 (Fzd6), regulates HSPC expansion and survival in a hematopoietic cell-intrinsic manner. Fzd6 deficiency increased the ratio of Flt3(hi) multipotent progenitors to CD150(+) stem cells in the mouse BM, suggesting defective stem cell maintenance. Competitive transplantation experiments demonstrated that Fzd6(-) (/) (-) HSPCs were able to home to the BM but were severely impaired in their capacity to reconstitute a lethally irradiated host. Lack of Fzd6 resulted in a strong activation of caspase-3 and a gradual loss of donor HSPCs and peripheral blood granulocytes. Fzd6 was also necessary for the efficient HSPC expansion during emergency hematopoiesis. Mechanistically, Fzd6 is a negative regulator of Cdc42 clustering in polarized cells. Furthermore, β-catenin-dependent signaling may be disinhibited in Fzd6(-) (/) (-) HSPCs. Collectively, our data reveal that Fzd6 has an essential role in HSPC maintenance and survival. Noncanonical Wnt-Fzd6 signaling pathway could thus present an interesting target for promoting HSPC expansion and multilineage hematopoietic recovery after transplant.

Wnts are dispensable for differentiation and self-renewal of adult murine hematopoietic stem cells

Wnt signaling controls early embryonic hematopoiesis and dysregulated β-catenin is implicated in leukemia. However, the role of Wnts and their source in adult hematopoiesis is still unclear, and is clinically important as upstream Wnt inhibitors enter clinical trials. We blocked Wnt secretion in hematopoietic lineages by targeting Porcn, a membrane-bound O-acyltransferase that is indispensable for the activity and secretion of all vertebrate Wnts. Surprisingly, deletion of Porcn in Rosa-CreER(T2)/Porcn(Del), MX1-Cre/Porcn(Del), and Vav-Cre/Porcn(Del) mice had no effects on proliferation, differentiation, or self-renewal of adult hematopoietic stem cells. Targeting Wnt secretion in the bone marrow niche by treatment with a PORCN inhibitor, C59, similarly had no effect on hematopoiesis. These results exclude a role for hematopoietic PORCN-dependent Wnts in adult hematopoiesis. Clinical use of upstream Wnt inhibitors is not likely to be limited by effects on hematopoiesis.

Adult thymic epithelium contains nonsenescent label-retaining cells

Progress in our understanding of thymic epithelial cell (TEC) renewal and homeostasis is hindered by the lack of markers for TEC progenitors. Stem and progenitor cell populations display remarkable diversity in their proliferative behavior. In some but not all tissues, stemness is associated with quiescence. The primary goal of our study was to discover whether quiescent cells were present in neonatal and adult TECs. To this end, we used a transgenic label-retaining cell (LRC) assay in which a histone H2B-GFP fusion protein is expressed under the control of the reverse tetracycline-controlled transactivator and the tetracycline operator minimal promoter. In adult mice, we found that both cortical and medullary TECs (cTECs and mTECs) proliferated more actively in females than males. Moreover, we observed three main differences between neonatal and adult TECs: 1) neonatal TECs proliferated more actively than adult TECs; 2) whereas cTECs and mTECs had similar turnover rates in young mice, the turnover of mTECs was more rapid than that of cTECs in adults; and 3) although no LRCs could be detected in young mice, LRCs were detectable after a 16-wk chase in adults. In female mice, LRCs were found almost exclusively among cTECs and expressed relatively low levels of p16INK4a, p19ARF, and Serpine1, and high levels of Bmi1, Foxn1, Trp63, and Wnt4. We conclude that LRCs in adult TECs are not senescent postmitotic cells and may represent the elusive progenitors responsible for TEC maintenance in the adult thymus.

Transcriptome sequencing of neonatal thymic epithelial cells

In order to gain novel insights into thymus biology, we analysed the whole transcriptome of cortical and medullary thymic epithelial cells (cTECs and mTECs) and of skin epithelial cells (ECs). Consistent with their ability to express ectopic genes, mTECs expressed more genes than other cell populations. Out of a total of 15,069 genes expressed in TECs, 25% were differentially expressed by at least 5-fold in cTECs vs. mTECs. Genes expressed at higher levels in cTECs than mTECs regulate numerous cell functions including cell differentiation, cell movement and microtubule dynamics. Many positive regulators of the cell cycle were overexpressed in skin ECs relative to TECs. Our RNA-seq data provide novel systems-level insights into the transcriptional landscape of TECs, highlight substantial divergences in the transcriptome of TEC subsets and suggest that cell cycle progression is differentially regulated in TECs and skin ECs.

Expression profile and function of Wnt signaling mechanisms in malignant mesothelioma cells

Malignant mesothelioma (MM) is an uncommon and particularly aggressive cancer associated with asbestos exposure, which currently presents an intractable clinical challenge. Wnt signaling has been reported to play a role in the neoplastic properties of mesothelioma cells but has not been investigated in detail in this cancer. We surveyed expression of Wnts, their receptors, and other key molecules in this pathway in well established in vitro mesothelioma models in comparison with primary mesothelial cultures. We also tested the biological response of MM cell lines to exogenous Wnt and secreted regulators, as well as targeting β-catenin. We detected frequent expression of Wnt3 and Wnt5a, as well as Fzd 2, 4 and 6. The mRNA of Wnt4, Fzd3, sFRP4, APC and axin2 were downregulated in MM relative to mesothelial cells while LEF1 was overexpressed in MM. Functionally, we observed that Wnt3a stimulated MM proliferation while sFRP4 was inhibitory. Furthermore, directly targeting β-catenin expression could sensitise MM cells to cytotoxic drugs. These results provide evidence for altered expression of a number of Wnt/Fzd signaling molecules in MM. Modulation of Wnt signaling in MM may prove a means of targeting proliferation and drug resistance in this cancer.

The nuclear effector of Wnt-signaling, Tcf1, functions as a T-cell-specific tumor suppressor for development of lymphomas

Tcf1 is known to function as a transcriptional activator of Wnt-induced proliferation during T cell development in the thymus. Evidence for an additional contrasting role for Tcf1 as a T-cell specific tumor suppressor gene is now presented.

The HMG-box factor Tcf1 is required during T-cell development in the thymus and mediates the nuclear response to Wnt signals. Tcf1^−/− mice have previously been characterized and show developmental blocks at the CD4−CD8− double negative (DN) to CD4+CD8+ double positive transition. Due to the blocks in T-cell development, Tcf1^−/− mice normally have a very small thymus. Unexpectedly, a large proportion of Tcf1^−/− mice spontaneously develop thymic lymphomas with 50% of mice developing a thymic lymphoma/leukemia at the age of 16 wk. These lymphomas are clonal, highly metastatic, and paradoxically show high Wnt signaling when crossed with Wnt reporter mice and have high expression of Wnt target genes Lef1 and Axin2. In wild-type thymocytes, Tcf1 is higher expressed than Lef1, with a predominance of Wnt inhibitory isoforms. Loss of Tcf1 as repressor of Lef1 leads to high Wnt activity and is the initiating event in lymphoma development, which is exacerbated by activating Notch1 mutations. Thus, Notch1 and loss of Tcf1 functionally act as collaborating oncogenic events. Tcf1 deficiency predisposes to the development of thymic lymphomas by ectopic up-regulation of Lef1 due to lack of Tcf1 repressive isoforms and frequently by cooperating activating mutations in Notch1. Tcf1 therefore functions as a T-cell–specific tumor suppressor gene, besides its established role as a Wnt responsive transcription factor. Thus, Tcf1 acts as a molecular switch between proliferative and repressive signals during T-lymphocyte development in the thymus.

Cancers often develop as a consequence of deregulated expression of key factors that operate during normal development. T-cell factor 1 (Tcf1) has an established role in the nuclear response to Wnt signaling during normal T-cell development in the thymus. Here we show in mice that the absence of Tcf1 can trigger tumorigenesis. As expected from previous work, lack of Tcf1 results in a small thymus with several partial blocks in T-cell development in the thymus. Surprisingly, we observe that a large proportion of Tcf1^−/− mice spontaneously develop thymic lymphomas. Thorough investigation of these thymic-derived tumors revealed that the mechanism underlying these lymphomas is, paradoxically, increased levels of Wnt-signaling. We propose that Wnt-signaling in these tumors is mediated by up-regulated expression of the Tcf1-homologue, Lef1, and specifically its long isoform. Furthermore, we have evidence to propose that in a normal thymus, short isoforms of Tcf1 that cannot respond to Wnt signals act as repressors of Lef1-mediated Wnt-signaling. Thus, we propose that Tcf1 has a dual function developing T cells in mice: it functions as a T-cell–specific tumor suppressor gene in addition to its established role as a transcriptional activator of Wnt-induced proliferation. Whether loss of function of Tcf-1 as a tumor suppressor gene actually occurs in human T-cell lymphoblastic leukemias is currently under investigation.

Noncanonical Wnt signaling maintains hematopoietic stem cells in the niche

Wnt signaling is involved in self-renewal and maintenance of hematopoietic stem cells (HSCs); however, the particular role of noncanonical Wnt signaling in regulating HSCs in vivo is largely unknown. Here, we show Flamingo (Fmi) and Frizzled (Fz) 8, members of noncanonical Wnt signaling, both express in and functionally maintain quiescent long-term HSCs. Fmi regulates Fz8 distribution at the interface between HSCs and N-cadherin(+) osteoblasts (N-cad(+)OBs that enrich osteoprogenitors) in the niche. We further found that N-cad(+)OBs predominantly express noncanonical Wnt ligands and inhibitors of canonical Wnt signaling under homeostasis. Under stress, noncanonical Wnt signaling is attenuated and canonical Wnt signaling is enhanced in activation of HSCs. Mechanistically, noncanonical Wnt signaling mediated by Fz8 suppresses the Ca(2+)-NFAT- IFNγ pathway, directly or indirectly through the CDC42-CK1α complex and also antagonizes canonical Wnt signaling in HSCs. Taken together, our findings demonstrate that noncanonical Wnt signaling maintains quiescent long-term HSCs through Fmi and Fz8 interaction in the niche.

Normal hematopoiesis and hematologic malignancies: role of canonical Wnt signaling pathway and stromal microenvironment

Wnts are a family of evolutionary-conserved secreted signaling molecules critically involved in a variety of developmental processes and in cell fate determination. A growing body of evidence suggests that Wnt signaling plays a crucial role in the influence of bone marrow stromal microenvironment on the balance between hematopoietic stem cell self-renewal and differentiation. Emerging clinical and experimental evidence also indicates Wnt signaling involvement in the disruption of the latter balance in hematologic malignancies, where the stromal microenvironment favors the homing of cancer cells to the bone marrow, as well as leukemia stem cell development and chemoresistance. In the present review, we summarize and discuss the role of the canonical Wnt signaling pathway in normal hematopoiesis and hematologic malignancies, with regard to recent findings on the stromal microenvironment involvement in these process and diseases.

The transcription factor Lyl-1 regulates lymphoid specification and the maintenance of early T lineage progenitors

Thymopoiesis depends on the recruitment and expansion of bone marrow-derived progenitor populations; tight regulation of these processes is required for maintenance of the homeostasis of the T lineage. Lyl-1, a transcription factor that regulates hematopoietic progenitors, is expressed in thymocyte progenitors until T cell commitment. Here we demonstrate a requirement for Lyl-1 in lymphoid specification and the maintenance of early T lineage progenitors (ETPs). Lyl-1 deficiency resulted in profound defects in the generation of lymphoid-primed multipotent progenitors (LMPPs), common lymphoid progenitors (CLPs) and ETPs. Lyl-1-deficient ETPs and thymocyte progenitors at the CD4(-)CD8(-) double-negative 2 (DN2) stage showed more apoptosis, blocked differentiation and impaired population expansion. We identified Gfi1 as a critical transcriptional target of Lyl-1-mediated lymphopoiesis of T cells. Thus, Lyl-1 is a pivotal component of a transcriptional program that controls the lymphoid specification and maintenance of ETPs.

Regulation of mature T cell responses by the Wnt signaling pathway

The canonical Wnt signaling pathway is evolutionarily conserved and plays key roles during development of many organ systems. This pathway utilizes TCF/LEF transcription factors, β-catenin coactivator, and TLE/GRG corepressors to achieve balanced regulation of its downstream gene expression. It is well established that several Wnt ligands and their effector proteins are crucial for normal T cell development. Recent studies have also revealed critical requirements for TCF-1 in generation and persistence of functional memory CD8(+) T cells, and in promoting Th2-differentiation and suppressing Th17-differentiation of activated CD4(+) T cells. Activation of β-catenin facilitated CD8(+) memory T cell formation, with enhanced protective capacity and extended survival of CD4(+) CD25(+) regulatory T cells. Upregulation of Wnt ligands was observed in Drosophila in response to Toll signaling as well as in mammalian dendritic cells and macrophages upon microbial stimulation. These new findings suggest that modulating the activity of Wnt pathway may be a powerful approach to enhance protective immunity and treat autoimmune diseases.

Wnt signaling strength regulates normal hematopoiesis and its deregulation is involved in leukemia development

A strict balance between self-renewal and differentiation of hematopoietic stem cells (HSCs) is required in order to maintain homeostasis, as well as to efficiently respond to injury and infections. Numbers and fate decisions made by progenitors derived from HSC must also be carefully regulated to sustain large-scale production of blood cells. The complex Wnt family of molecules generally is thought to be important to these processes, delivering critical signals to HSC and progenitors as they reside in specialized niches. Wnt proteins have also been extensively studied in connection with malignancies and are causatively involved in the development of several types of leukemias. However, studies with experimental animal models have produced contradictory findings regarding the importance of Wnt signals for normal hematopoiesis and lymphopoiesis. Here, we will argue that dose dependency of signaling via particular Wnt pathways accounts for much, if not all of this controversy. We conclude that there seems little doubt that Wnt proteins are required to sustain normal hematopoiesis, but are likely to be presented in carefully controlled gradients in a tissue-specific manner.

Wnt4 regulates thymic cellularity through the expansion of thymic epithelial cells and early thymic progenitors

Thymus atrophy is the most common immunopathology in humans, and its occurrence is hastened by several factors that coalesce in patients receiving chemotherapy and most of all in recipients of hematopoietic cell transplantation. We have shown previously that posthematopoietic cell transplantation thymic function was improved by retroviral overexpression of Wnt4 in donor hematopoietic cells. Here, by using both conventional and conditional null mutant mice, we show that Wnt4 regulates steady-state thymic cellularity by a thymic epithelial cell (TEC)-dependent mechanism. The absence of Wnt4 suppressed fetal and postnatal thymic expansion and resulted in decreased TEC numbers, an alteration of the medullary-to-cortical TEC ratio, and a disproportionate loss of the most immature cKit(hi) thymocyte precursors. Wnt4 also is implicated in the maintenance of adult thymopoiesis, although the impact of its deletion once thymic involution has been initiated is more subtle. Together, our results show that Wnt4 controls thymic size by modulating TEC expansion and the earliest, TEC-dependent steps of thymocyte development both in the fetal and postnatal thymus. Wnt4 and its downstream signaling pathways could thus represent interesting candidates to improve thymic output in subjects with thymic atrophy.

Development and function of innate polyclonal TCRalphabeta+ CD8+ thymocytes

Innate CD8 T cells are found in mutant mouse models, but whether they are produced in a normal thymus remains controversial. Using the RAG2p-GFP mouse model, we found that ∼10% of TCRαβ(+) CD4(-)CD8(+) thymocytes were innate polyclonal T cells (GFP(+)CD44(hi)). Relative to conventional T cells, innate CD8 thymocytes displayed increased cell surface amounts of B7-H1, CD2, CD5, CD38, IL-2Rβ, and IL-4Rα and downmodulation of TCRβ. Moreover, they overexpressed several transcripts, including T-bet, Id3, Klf2, and, most of all, Eomes. Innate CD8 thymocytes were positively selected, mainly by nonhematopoietic MHCIa(+) cells. They rapidly produced high levels of IFN-γ upon stimulation and readily proliferated in response to IL-2 and IL-4. Furthermore, low numbers of innate CD8 thymocytes were sufficient to help conventional CD8 T cells expand and secrete cytokine following Ag recognition. This helper effect depended on CD44-mediated interactions between innate and conventional CD8 T cells. We concluded that innate TCRαβ(+) CD8 T cells represent a sizeable proportion of normal thymocytes whose development and function differ in many ways from those of conventional CD8 T cells.

T-cell lineage determination

T cells originate from hematopoietic stem cells (HSCs) in the bone marrow but complete their development in the thymus. HSCs give rise to a variety of non-renewing hematopoietic progenitors, among which a rare subset migrates to the thymus via the bloodstream. The earliest T-cell progenitors identified in the thymus are not T-lineage restricted but possess the ability to give rise to cells of many different lineages. Alternative lineage potentials are gradually lost as progenitors progress toward later developmental stages. Here, we review the early developmental events that might be involved in T-cell lineage fate determination, including the properties of possible thymus-settling progenitors, their homing into the thymus, and their T-cell lineage specification and commitment.

Distinct roles for Wnt-4 and Wnt-11 during retinoic acid-induced neuronal differentiation

Retinoic acid and Wnt/β-catenin signals play important roles during neuronal differentiation but less is known about noncanonical Wnt signals in this context. We examined retinoic acid and Wnt signaling in two human embryonal carcinoma cell lines, NTERA-2 (clone D1), which undergoes neuronal differentiation in response to retinoic acid, and 2102Ep, which does not. Retinoic acid treatment inhibited β-catenin/Tcf activity in NTERA-2 cells but not in 2102Ep cells. Inhibition occurred downstream of β-catenin but did not involve competition between retinoic acid receptors and β-catenin for binding to p300 or Tcf-4. Ectopic expression of FZD1 partially restored inhibition in 2102Ep cells, suggesting the involvement of Wnt ligands. Retinoic acid treatment of NTERA-2 cells induced the expression of Wnt-4 and Wnt-11, both of which were able to inhibit β-catenin/Tcf activity. Wnt-4 and Wnt-11 were found at cell borders in islands of cells that expressed OCT4 and GFAP and were predominantly negative for Nestin, PAX6, and GATA6. Gene silencing of Wnt-4, but not Wnt-11, reduced retinoic acid downregulation of OCT4 and Nanog and upregulation of PAX6, ASCL1, HOXC5, and NEUROD1, suggesting that Wnt-4 promotes early neuronal differentiation. Gene expression analysis of NTERA-2 cells stably overexpressing Wnt-11 suggested that Wnt-11 potentiates retinoic acid induction of early neurogenesis. Consistent with this, overexpression of Wnt-11 maintained a population of proliferating progenitor cells in cultures treated with retinoic acid for several weeks. These observations highlight the distinct roles of two noncanonical Wnts during the early stages of retinoic acid-induced neuronal differentiation.

Wnt4 enhances murine hematopoietic progenitor cell expansion through a planar cell polarity-like pathway

BACKGROUND

While the role of canonical (β-catenin-mediated) Wnt signaling in hematolymphopoiesis has been studied extensively, little is known of the potential importance of non-canonical Wnt signals in hematopoietic cells. Wnt4 is one of the Wnt proteins that can elicit non-canonical pathways. We have previously shown that retroviral overexpression of Wnt4 by hematopoietic cells increased thymic cellularity as well as the frequency of early thymic progenitors and bone marrow hematopoietic progenitor cells (HPCs). However, the molecular pathways responsible for its effect in HPCs are not known.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that Wnt4 stimulation resulted in the activation of the small GTPase Rac1 as well as Jnk kinases in an HPC cell line. Jnk activity was necessary, while β-catenin was dispensable, for the Wnt4-mediated expansion of primary fetal liver HPCs in culture. Furthermore, Jnk2-deficient and Wnt4 hemizygous mice presented lower numbers of HPCs in their bone marrow, and Jnk2-deficient HPCs showed increased rates of apoptosis. Wnt4 also improved HPC activity in a competitive reconstitution model in a cell-autonomous, Jnk2-dependent manner. Lastly, we identified Fz6 as a receptor for Wnt4 in immature HPCs and showed that the absence of Wnt4 led to a decreased expression of four polarity complex genes.

CONCLUSIONS/SIGNIFICANCE

Our results establish a functional role for non-canonical Wnt signaling in hematopoiesis through a pathway involving Wnt4, Fz6, Rac1 and Jnk kinases.

Noncanonical Wnt signaling in vertebrate development, stem cells, and diseases

Wnt signaling regulates many aspects of vertebrate development and adult stem cells. Deregulation of Wnt signaling causes development defect and cancer. The signaling is categorized in two pathways: canonical and noncanonical. Both pathways are initiated by Wnt ligands and Frizzled receptors. Canonical pathway leads to β-catenin:T-cell factor/lymphoid enhancer factor-mediated gene expression, which regulates proliferation and differentiation of cells. Noncanonical Wnt signaling is mediated by intracellular calcium ion and JNK. This signaling leads to NFAT, a key transcriptional factor regulating gene expression. In addition, β-catenin:T-cell factor/lymphoid enhancer factor-mediated gene expression is downregulated by CaMKII-TAK1-NLK. Cellular polarity and motility are the main outcomes of the signaling. During development, noncanonical Wnt signaling is required for tissue formation. Recent studies have shown that atypical cadherin Flamingo contributes to noncanonical Wnt signaling by directing the migration of cells. Also, noncanonical Wnt signaling is required for maintenance of adult stem cells. In the field of cancer research, noncanonical Wnt signaling has been considered a tumor suppressor; however, recent evidence has shown that the signaling also enhances cancer progression in the later stages of disease. In this review, we describe and discuss components of noncanonical Wnt signaling, diseases caused by deregulation of the signaling, regulation of adult stem cells by the signaling, and implications in cancer biology.

Non-canonical Wnt signaling pathways in hematopoiesis

Hematopoietic stem cells (HSCs) are a rare population of cells that are responsible for life-long generation of blood cells of all lineages. In order to maintain their numbers, HSCs must establish a balance between the opposing cell fates of self-renewal and initiation of hematopoietic differentiation. Multiple signaling pathways have been implicated in the regulation of HSC cell fate. One such set of pathways are those activated by the Wnt family of ligands. The function of the canonical Wnt signaling pathway, which utilizes beta-catenin to regulate gene expression, has been extensively studied in hematopoiesis. However, there is a growing body of evidence that the other Wnt signaling pathways, termed non-canonical, also play an important role. In this review, we will discuss the regulation of hematopoiesis by the Wnt signaling pathways, focusing on the potential functions of non-canonical Wnt signaling pathways.

The Apc(min) mouse has altered hematopoietic stem cell function and provides a model for MPD/MDS

Apc, a negative regulator of the canonical Wnt signaling pathway, is a bona-fide tumor suppressor whose loss of function results in intestinal polyposis. APC is located in a commonly deleted region on human chromosome 5q, associated with myelodysplastic syndrome (MDS), suggesting that haploinsufficiency of APC contributes to the MDS phenotype. Analysis of the hematopoietic system of mice with the Apc(min) allele that results in a premature stop codon and loss of function showed no abnormality in steady state hematopoiesis. Bone marrow derived from Apc(min) mice showed enhanced repopulation potential, indicating a cell intrinsic gain of function in the long-term hematopoietic stem cell (HSC) population. However, Apc(min) bone marrow was unable to repopulate secondary recipients because of loss of the quiescent HSC population. Apc(min) mice developed a MDS/myeloproliferative phenotype. Our data indicate that Wnt activation through haploinsufficiency of Apc causes insidious loss of HSC function that is only evident in serial transplantation strategies. These data provide a cautionary note for HSC-expansion strategies through Wnt pathway activation, provide evidence that cell extrinsic factors can contribute to the development of myeloid disease, and indicate that loss of function of APC may contribute to the phenotype observed in patients with MDS and del(5q).

Generation of an allele to inactivate Wnt4 gene function conditionally in the mouse

The Wnt family member Wnt4 is critical for the development of several organs, including the kidney, gonad, and adrenal, mammary, and pituitary glands. To study its potential postnatal functions, we generated a floxed Wnt4 allele. A single loxP site was targeted to the second intron, while a loxP-Neo-loxP cassette was placed 3' from Exon 5. The floxed Neo cassette was subsequently removed by crossing with MeuCre40 transgenic mice. The Wnt4 gene was specifically inactivated with CAGCre and another Wnt4 allele, Wnt4(EGFPCre), in which the Cre is driven by the endogenous Wnt4 promoter. Deletion of Wnt4 gene function with CAGCre impaired kidney development, as is the case with the conventional knockout. Similarly, the Wnt4(EGFPCre)-mediated inactivation of Wnt4 function considerably reduced the amount of Wnt4 transcripts, led to a severe defect in kidney development, and caused the female embryos to undergo partial sex reversal to males. All in all, the floxed Wnt4 allele serves as a useful tool for studying the roles of Wnt4 signaling during the life cycle.

WNT signaling in ovarian follicle biology and tumorigenesis

The WNTS are an expansive family of glycoprotein signaling molecules known mostly for the roles they play in embryonic development. WNT signaling first caught the attention of ovarian biologists when it was reported that the inactivation of Wnt4 in mice results in partial female-to-male sex reversal and oocyte depletion. More recently, studies using loss- and gain-of-function transgenic mouse models demonstrated the requirement for Wnt4, Fzd4 and Ctnnb1, components of the WNT pathway, for normal folliculogenesis, luteogenesis and steroidogenesis, and showed that dysregulated WNT signaling can cause granulosa cell tumor development. This review covers our current knowledge of WNT signaling in ovarian follicles, highlighting both the great promise and the many unresolved questions of this emerging field of research.

Wnt4 is not sufficient to induce lobuloalveolar mammary development

BACKGROUND

Brisken et al (2000) showed that Wnt4 null mammary glands were deficient in early lobuloalveolar mammary outgrowth during pregnancy, and implicated Wnt4 as an effector for the progesterone-induced mammary growth program. Though ectopic Wnt1 signaling is known to be mitogenic and oncogenic, no endogenously expressed Wnt ligands have ever been directly implicated in mammary growth and morphogenesis. Therefore, we generated conditional transgenic mice to test whether Wnt4 can stimulate mammary epithelial cell growth.

RESULTS

We found that despite pregnancy-associated expression levels of Wnt4, mammary glands did not display the side-branching typical of early pregnancy. Control experiments designed to test the Wnt4 construct in zebrafish reproduced other studies that demonstrated Wnt4-specific phenotypes distinct from Wnt1-induced phenotypes. Indeed, using qPCR-based array analyses, we found that a specific transcriptional target of Wnt4, namely Wnt16, was induced in Wnt4-expressing transgenic glands, to levels equivalent to that of early pregnant glands.

CONCLUSION

Taken together, we propose that Wnt4 is necessary, but not sufficient, to induce side-branch development.

WNT proteins: environmental factors regulating HSC fate in the niche

The Wnt signaling pathway has been implicated in regulation of hematopoiesis through a plethora of studies from many different laboratories. However, different inducible gain- and loss-of-function approaches retrieved controversial and sometimes contradictory results. Different levels of activation of the pathway, dosages of Wnt signaling required, and the interference by other signals in the context of Wnt activation collectively explain these controversies. Gain-of-function or in vitro exposure to WNT proteins and more specifically WNT3a was shown to enhance hematopoietic stem cell (HSC) activity, but its exact role was still not completely understood. In a recent study we analyzed the hematopoietic system of mice deficient for this specific Wnt gene. Wnt3a deficiency results in early embryonic lethality around embryonic day 12.5 (E12.5), precluding analysis in adult mice, but allowing hematopoiesis to be studied in fetal liver (FL) and in the just colonized thymic rudiment. Notably, we showed that long-term HSCs and multipotent progenitors are reduced in FL and have severely reduced long-term reconstitution capacity as observed in serial transplantation assays. Of interest, deficiency in Wnt3a leads to complete abolition of canonical Wnt signaling in FL hematopoietic stem and progenitor cells. This HSC deficiency is not explained by altered cell cycle or survival and is irreversible, since it cannot be restored by transplantation into Wnt3a-competent mice. In addition, Wnt3a deficiency differentially affects myeloid and B-lymphoid lineages, with myeloid cells being affected at the progenitor level, while B lymphopoiesis is apparently unaffected. Immature thymocytes, however, were reduced in cell numbers due to lack of Wnt3a production by the thymic microenvironment. Our results show that while in the thymus Wnt3a provides cytokine-like, proliferative stimuli to developing thymocyte Wnt3a regulates cell fate decisions of FL HSC in a nonredundant way.

Untangling the T branch of the hematopoiesis tree

T cells develop in the thymus. Previous work suggested an early separation of lymphoid from myeloerythroid lineages during hematopoiesis and hypothesized the thymus was settled exclusively by lymphoid-restricted hematopoietic progenitors. Recent data have instead established the existence of lymphoid-myeloid progenitors, which possess lymphoid and myeloid lineage potentials but lack erythroid potential. Myeloid and lymphoid potentials are present at the clonal level in early thymic progenitors, confirming that progenitors settling the thymus include lymphoid-myeloid progenitors. These results revise our view of the T lineage branch of hematopoiesis and focus attention on the generation, circulation, and homing of lymphoid-myeloid progenitors to the thymus.

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.