WNT and beta-catenin signalling: diseases and therapies

WNT signalling has been studied primarily in developing embryos, in which cells respond to WNTs in a context-dependent manner through changes in survival and proliferation, cell fate and movement. But WNTs also have important functions in adults, and aberrant signalling by WNT pathways is linked to a range of diseases, most notably cancer. What is the full range of diseases that involve WNT pathways? Can inhibition of WNT signalling form the basis of an effective therapy for some cancers? Could activation of WNT signalling provide new therapies for other clinical conditions? Finally, on the basis of recent experiments, might WNTs normally participate in self-renewal, proliferation or differentiation of stem cells? If so, altering WNT signalling might be beneficial to the use of stem cells for therapeutic means.     

WNT signalling in the immune system: WNT is spreading its wings

WNT proteins are secreted morphogens that are required for basic developmental processes, such as cell-fate specification, progenitor-cell proliferation and the control of asymmetric cell division, in many different species and organs. In blood and immune cells, WNT signalling controls the proliferation of progenitor cells and might also affect the cell-fate decisions of stem cells. Recent studies indicate that WNT proteins also regulate effector T-cell development, regulatory T-cell activation and dendritic-cell maturation. WNT signalling seems to function as a universal mechanism in leukocytes to establish a pool of undifferentiated cells for further selection, effector-cell maturation and terminal differentiation. WNT signalling is therefore subject to strict molecular control, and dysregulated WNT signalling is implicated in the development of haematological malignancies.     

WNT secretion and signalling in human disease

Wnt signalling, a key pathway involved in various aspects of embryonic development, also underlies many human diseases, in particular, cancer. Research focused on signal transduction within signal-receiving cells led to the discovery of many Wnt pathway components, but study of the secretion of Wnt ligands themselves was neglected until recently. Attention was drawn to this highly regulated process by the association of aberrant Wnt levels with an increasing number of diseases. Studying the biogenesis and processing of active Wnt ligands will open new avenues for generating therapeutics to specifically target aberrant Wnt signalling. Here we review the proteins required for Wnt secretion and signalling at the plasma membrane, ending with a discussion on potential therapeutic approaches to treat Wnt-induced diseases.     

Principal signalling complexes in haematopoiesis: structural aspects and mimetic discovery

Blood production is a highly regulated process involving multiple inhibitory and stimulatory cytokines present in the haematopoietic stem cell niche. Small molecules mimics of these signalling molecules have substantial potential as drugs and in the development of bioreactors to generate blood products. We review the structural biology of the extracellular signalling domains of five of the most important cytokines, analyze their structure-property relationships, and summarize the progress in developing small molecule mimics using the molecular information from structural biology and mutation studies.     

Connective-tissue growth factor modulates WNT signalling and interacts with the WNT receptor complex

Connective-tissue growth factor (CTGF) is a member of the CCN family of secreted proteins. CCN family members contain four characteristic domains and exhibit multiple activities: they associate with the extracellular matrix, they can mediate cell adhesion, cell migration and chemotaxis, and they can modulate the activities of peptide growth factors. Many of the effects of CTGF are thought to be mediated by binding to integrins, whereas others may be because of its recently identified ability to interact with BMP4 and TGF beta. We demonstrate, using Xenopus embryos, that CTGF also regulates signalling through the Wnt pathway, in accord with its ability to bind to the Wnt co-receptor LDL receptor-related protein 6 (LRP6). This interaction is likely to occur through the C-terminal (CT) domain of CTGF, which is distinct from the BMP- and TGF beta-interacting domain. Our results define new activities of CTGF and add to the variety of routes through which cells regulate growth factor activity in development, disease and tissue homeostasis.     

Transmembrane adaptor protein WBP1L regulates CXCR4 signalling and murine haematopoiesis

WW domain binding protein 1‐like (WBP1L), also known as outcome predictor of acute leukaemia 1 (OPAL1), is a transmembrane adaptor protein, expression of which correlates with ETV6‐RUNX1 (t(12;21)(p13;q22)) translocation and favourable prognosis in childhood leukaemia. It has a broad expression pattern in haematopoietic and in non‐haematopoietic cells. However, its physiological function has been unknown. Here, we show that WBP1L negatively regulates signalling through a critical chemokine receptor CXCR4 in multiple leucocyte subsets and cell lines. We also show that WBP1L interacts with NEDD4‐family ubiquitin ligases and regulates CXCR4 ubiquitination and expression. Moreover, analysis of Wbp1l‐deficient mice revealed alterations in B cell development and enhanced efficiency of bone marrow cell transplantation. Collectively, our data show that WBP1L is a novel regulator of CXCR4 signalling and haematopoiesis.     

Functional interactions between anthrax toxin receptors and the WNT signalling protein LRP6

To exert its activity, anthrax toxin must be endocytosed and its enzymatic toxic subunits delivered to the cytoplasm. It has been proposed that, in addition to the anthrax toxin receptors (ATRs), lipoprotein-receptor-related protein 6 (LRP6), known for its role in Wnt signalling, is also required for toxin endocytosis. These findings have however been challenged. We show that LRP6 can indeed form a complex with ATRs, and that this interaction plays a role both in Wnt signalling and in anthrax toxin endocytosis. We found that ATRs control the levels of LRP6 in cells, and thus the Wnt signalling capacity. RNAi against ATRs indeed led to a drastic decrease in LRP6 levels and a subsequent drop in Wnt signalling. Conversely, LRP6 plays a role in anthrax toxin endocytosis, but is not essential. We indeed found that toxin binding triggered tyrosine phosphorylation of LRP6, induced its redistribution into detergent-resistant domains, and its subsequent endocytosis. RNAis against LRP6 strongly delayed toxin endocytosis. As the physiological role of ATRs is probably to interact with the extracellular matrix, our findings raise the interesting possibility that, through the ATR-LRP6 interaction, adhesion to the extracellular matrix could locally control Wnt signalling.     

Beta-arrestin and casein kinase 1/2 define distinct branches of non-canonical WNT signalling pathways

Recent advances in understanding beta-catenin-independent WNT (non-canonical) signalling suggest an increasing complexity, raising the question of how individual non-canonical pathways are induced and regulated. Here, we examine whether intracellular signalling components such as beta-arrestin (beta-arr) and casein kinases 1 and 2 (CK1 and CK2) can contribute to determining signalling specificity in beta-catenin-independent WNT signalling to the small GTPase RAC-1. Our findings indicate that beta-arr is sufficient and required for WNT/RAC-1 signalling, and that casein kinases act as a switch that prevents the activation of RAC-1 and promotes other non-canonical WNT pathways through the phosphorylation of dishevelled (DVL, xDSH in Xenopus). Thus, our results indicate that the balance between beta-arr and CK1/2 determines whether WNT/RAC-1 or other non-canonical WNT pathways are activated.     

WNT unrelated activities in commercially available preparations of recombinant WNT3a

WNT signaling pathways play an important role in both development and disease. By analyzing the signaling capabilities of commercially available WNT3a preparations towards the PI3K/AKT/GSK3 signaling pathway, we discovered unexpected inconsistencies from lot to lot of recombinant WNT3a. We provide evidence that: (1) The ability to trigger AKT/GSK3 signaling varies dramatically between different lots of WNT3a, without any variation in their ability to activate the canonical WNT/β‐catenin signaling. (2) sFRP1, a WNT signaling inhibitor, is unable to interfere with the activation of AKT/GSK3 signaling induced by some of the WNT3a lots. (3) Pharmacological inhibition of AKT/GSK3 phosphorylation by PI3K inhibitors fails to affect the stabilization of β‐catenin, the central effector of the canonical WNT/β‐catenin signaling pathway. In summary, while all tested lots of recombinant WNT3a activated WNT/β‐catenin pathway, our results suggest that individual lots of recombinant WNT3a activate the PI3K/AKT/GSK3 pathway in a WNT‐independent manner, hampering thus the analysis of regulation of PI3K/AKT/GSK3 by WNT ligand. J. Cell. Biochem. 111: 1077–1079, 2010. © 2010 Wiley‐Liss, Inc.     

A balance of FGF, BMP and WNT signalling positions the future placode territory in the head

The sensory nervous system in the vertebrate head arises from two different cell populations: neural crest and placodal cells. By contrast, in the trunk it originates from neural crest only. How do placode precursors become restricted exclusively to the head and how do multipotent ectodermal cells make the decision to become placodes or neural crest? At neural plate stages, future placode cells are confined to a narrow band in the head ectoderm, the pre-placodal region (PPR). Here, we identify the head mesoderm as the source of PPR inducing signals, reinforced by factors from the neural plate. We show that several independent signals are needed: attenuation of BMP and WNT is required for PPR formation. Together with activation of the FGF pathway, BMP and WNT antagonists can induce the PPR in na?ve ectoderm. We also show that WNT signalling plays a crucial role in restricting placode formation to the head. Finally, we demonstrate that the decision of multipotent cells to become placode or neural crest precursors is mediated by WNT proteins: activation of the WNT pathway promotes the generation of neural crest at the expense of placodes. This mechanism explains how the placode territory becomes confined to the head, and how neural crest and placode fates diversify.     

WNT1 and WNT3a promote expansion of melanocytes through distinct modes of action

Summary WNT1 and WNT3a have been described as having redundant roles in promoting the development of neural crest-derived melanocytes (NC-Ms). We used cell lineage restricted retroviral infections to examine the effects of WNT signaling on defined cell types in neural crest cultures. RCAS retroviral infections were targeted to melanoblasts (NC-M precursor cells) derived from transgenic mice that express the virus receptor, TVA, under the control of a melanoblast promoter (DCT). As expected, over 90% of DCT-TVA+ cells expressed early melanoblast markers MITF and KIT. However, by following the fate of infected cells in standard culture conditions, we find that only 5% of descendents were NC-Ms. The majority of the descendents were not NC-Ms, but expressed smooth muscle cell markers, demonstrating that mammalian melanoblasts are not committed to the NC-M lineage. RCAS infection of DCT-TVA+ cells demonstrated that overexpression of canonical WNT signaling genes (betaCAT, WNT3a or WNT1) can increase NC-M numbers in an endothelin dependent manner. However, WNT1 and WNT3a have different modes of action with respect to melanoblast fate. Intrinsic over-expression of betaCAT or WNT3a can increase NC-M numbers by biasing the fate of DCT-TVA+ cells to NC-Ms. In contrast, the DCT-TVA+ melanoblasts cannot respond to WNT1 signaling and do not alter their fate towards NC-M. Instead, WNT1 only increases NC-M numbers through paracrine signaling on melanoblast precursors to increase the numbers of neural crest cells that become NC-Ms.