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The WNT/β‐catenin dependent transcription: A tissue‐specific business

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Abstract β‐catenin‐mediated Wnt signaling is an ancient cell‐communication pathway in which β‐catenin drives the expression of certain genes as a consequence of the trigger given by extracellular WNT molecules. The events occurring from signal to transcription are evolutionarily conserved, and their final output orchestrates countless processes during embryonic development and tissue homeostasis. Importantly, a dysfunctional Wnt/β‐catenin pathway causes developmental malformations, and its aberrant activation is the root of several types of cancer. A rich literature describes the multitude of nuclear players that cooperate with β‐catenin to generate a transcriptional program. However, a unified theory of how β‐catenin drives target gene expression is still missing. We will discuss two types of β‐catenin interactors: transcription factors that allow β‐catenin to localize at target regions on the DNA, and transcriptional co‐factors that ultimately activate gene expression. In contrast to the presumed universality of β‐catenin´s action, the ensemble of available evidence suggests a view in which β‐catenin drives a complex system of responses in different cells and tissues. A malleable armamentarium of players might interact with β‐catenin in order to activate the right “canonical” targets in each tissue, developmental stage, or disease context. Discovering the mechanism by which each tissue‐specific β‐catenin response is executed will be crucial to comprehend how a seemingly universal pathway fosters a wide spectrum of processes during development and homeostasis. Perhaps more importantly, this could ultimately inform us about which are the tumor‐specific components that need to be targeted to dampen the activity of oncogenic β‐catenin. This article is categorized under: Cancer > Molecular and Cellular Physiology Cancer > Genetics/Genomics/Epigenetics Cancer > Stem Cells and Development
β‐catenin is a docking station for a plethora of nuclear proteins. β‐catenin is represented in the center of the figure, and its partners are listed below (transcription factors) and above (transcriptional regulators). For each interacting protein, a gray bar spans its binding surface along the corresponding domains of β‐catenin. On the right of each interactor, the relevant reference describing binding information. The main activity of each interactor (or group of interactors) is pinpointed on the right of the figure. The numbers within β‐catenin list the armadillo repeats. The white bar in armadillo repeat 10 indicates the 22 amino acid residues insertion causing the deviation from the armadillo repeat structure (Huber, Nelson, & Weis, 1997). For sake of simplicity, this is not represented in the other figures of this article. CTD, C‐terminal domain. H3K4m2/3, di‐ and tri‐methylated lysine 4 on histone 3 tail; HAT, histone acetyltransferase; HMT, histone methyl transferase; NTD, N‐terminal domain; RNAPII, RNA polymerase II
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Developmental requirement of β‐catenin, BCL9/9L and PYGO1/2 as deduced from the stage in which lethality is induced by mutations in their genes during murine development. While Ctnnb1 (encoding for β‐catenin) mutant embryos die during gastrulation at ca. 6.5 days post coitum (dpc), the lethality induced by mutations in Bcl9/9l or Pygo1/2 occurs later, at ca. 10.5 and 13.5 dpc, respectively. This proves that β‐catenin can sustain the Wnt‐dependent transcription in the absence of the BCL9‐PYGO cooperation until 10.5 dpc, a stage in which the body axes have been established and organogenesis initiated. Analogously, Pygo1/2 mutants die after 13.5 dpc, emphasizing that there must exist processes for which BCL9/9l are required but PYGO1/2 are not. Note that the lethality induced at 10.5 dpc by mutations in Bcl9/9l does not imply that all β‐catenin outputs must require BCL9/9L from this moment of development onward. It is in principle possible that only a single process required for survival is affected by loss of these genes
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The Wnt/β‐catenin transcriptional complex, referred to as Wnt enhanceosome, as proposed by van Tienen and colleagues. Colored small balls indicate the physical interactions that glue each component within the complex. Green balls represent the interactions that only occur in the Wnt‐ON state, while all others (in red) appear to be constitutive. The long protein BCL9 or its paralogue BCL9L (BCL9/9L; orthologues of the Drosophila Lgs) run through the complex and keep it together via three fundamental interactions: with the PHD (Plant Homology Domain) of PYGO1/2 via the HD1 (Homology Domain 1); with the ChiLS complex via HD3; with TLE/Groucho via its C‐terminus (on the right side of the complex). The interaction between BCL9 and β‐catenin is mediated by the HD2 domain of BCL9/9L that contacts the N‐terminal portion of β‐catenin. Vertebrate BCL9 proteins possess additional homology domains (HD4‐6, in light gray), absent in the Drosophila Lgs, whose function is largely unknown. The NHD (N‐terminal Homology Domain) of PYGO1/2 contains the NPF tripeptide that interacts with the ChiLS complex. H3K4me2/3 stands for di‐ and trimethylated lysine 4 on the histone 3 tails, chromatin marks associated with active regulatory regions and promoters. WRE, Wnt Responsive Element
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Alternative transcription factors (TFs) that team up with β‐catenin. The TFs are arbitrarily divided in two categories: those that compete with TCF/LEF for β‐catenin association (left column) and those that cooperate with TCF/LEF and β‐catenin in a ternary complex (right column). Each model is individually enclosed in a gray box to schematically reflect the discovery of its mechanism. Within each box we indicate the name of the TF and the main tissue/cellular model used for the discovery. Below each model we provide the reference of the relevant article. Activation events are indicated by a green arrow, while inhibition is represented with red arrows lacking arrowheads. Phosphorylation is displayed as a circled “P” connected to a target protein. WRE, Wnt responsive element
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The two‐fold handicap of β‐catenin. β‐catenin is considered central for the activation of Wnt target genes. Yet, it lacks both DNA‐binding ability and a transcription trans‐activating domain. Great efforts have been devoted in the last two decades to understand how β‐catenin solves these shortcomings so that its action results in exclusive association to WRE (Wnt responsive elements) and specific Wnt target gene expression (left panel). The solution must rely on the existence of transcription factors (TFs) that permit specific association of β‐catenin on select regulatory elements (RE), and the recruitment of co‐factors (Co‐Fs) capable of open the chromatin and assist RNA polymerase II–mediated transcription (right panel; Box 1). As it will be discussed, the rigid scaffold structure of β‐catenin allows simultaneous anchorage of multiple protein complexes. In theory, different combinations of TF/Co‐F (e.g., from the figure: A1, A2, A3, B1, B2 et cetera) could complex with β‐catenin to stimulate tissue‐ or cell‐specific gene expression programs. As we will see, the TCF/LEF TFs are the main players conferring β‐catenin its functions (Section 3). However, β‐catenin is found to engage with several other TFs and Co‐Fs in various contexts, and this might contribute in explaining the tissue‐specific outcomes downstream of the Wnt signaling
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Schematic representation of the canonical Wnt signaling pathway. Key events along the pathway are numbered within the figure and described in the legend. The different types of relation between the pathway components are listed in the legend in the top‐right corner, and are represented in the diagram in red if they occur in the OFF state (when WNT ligands are absent), and in green if they take place in the ON state (when WNT binds to the receptor). Briefly, WNT proteins are produced and secreted by a cell. Upon reaching target cell, the WNT proteins trigger a cascade of intracellular events when they contact the Frizzled receptor and LRP co‐receptor located at the membrane surface. The receptor complex recruits the cytosolic protein Disheveled (DSH), which plays an important role in the inhibition of the β‐catenin destruction complex, composed by GSK3, AXIN, CK1, and APC. Free cytosolic β‐catenin is commonly phosphorylated by the destruction complex and thus marked for degradation. When the destruction complex is inhibited, increased levels of β‐catenin causes its translocation to the nucleus. Here, β‐catenin physically binds to the TCF/LEF family of transcription factors and is thought to displace the TLE/Groucho co‐repressors via not completely understood mechanisms. β‐catenin subsequently recruits a series of transcriptional co‐factors required for the activation of Wnt target genes
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There is a steady and rapid increase in the number of published articles in the Wg/Wnt field (red curve, left panel). This is of no surprise considering the rapid exponential growth of the biomedical literature in every sector (Khare, Leaman, & Lu, 2014). What is surprising is the plateau phase reached by the subset of articles in the field that focus on nuclear components (blue curve, left panel). After their discovery as main transcription factors assisting β‐catenin in 1996, TCF/LEF have been mentioned in an exponentially growing number of articles for the subsequent decade (see the more detailed central panel). It appears however that the interest in nuclear TCF/β‐catenin co‐factors has not been growing in a manner similar to the rest of the field. The central and right charts represent the contribution of each of the β‐catenin co‐factors to the total number of nuclear Wg/Wnt signaling papers (each factor is listed in a color‐coded legend table on the right)
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