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- 1 Wnt/β-catenin Signalling Pathway
- 1.1 Introduction
- 1.2 History
- 1.3 Mechanism of action
- 1.4 Diseases associated with Wnt/β-catenin signalling
- 1.5 Key players in Wnt/β-catenin Signalling
- 1.6 Glossary
- 1.7 External links
- 1.8 Reference List
Wnt/β-catenin Signalling Pathway
2012 marks the 30th anniversary of the identification of Wnt-1, the first component of the Wnt signaling pathway to be discovered. Since this breakthrough, researchers from around the globe have extensively studied and pieced together the multiple components that form the Wnt signaling pathway as it is known and accepted today.
The classical pathway of Wnt signalling that is best understood in current research is the Wnt/β-catenin signalling pathway which features heavily in development, regeneration, stem cell regulation and cellular processes such as proliferation and migration.
Wnt proteins encompass a network of secreted glycolipoproteins, forming the basis of this highly conserved pathway. The various elements, agonists and antagonists of this pathway will be discussed, with key emphasis on the structure and function.
Furthermore, mutations in components of this pathway have been associated with cancers, developmental defects and other diseases which will be discussed in further detail.
It is important to note that the Wnt/β-catenin signalling pathway is becoming increasingly complex with the addition of new molecules being discovered in recent years. Areas of current research and future directions will also be outlined.
1973: A mutant of Drosophila melanogaster which had no wings was characterised as Wingless (Wg) 
1982: Roel Nusse and Harold Varmus found that mouse mammary tumour virus (MMTV) stimulated tumour formation when the Int1 (integration 1) gene was activated.
1987: Rijsewijk et al identifies the mammalian homolog of the Drosophila Wingless gene as Int1.
1989: Screening of zygotic lethal mutations in Drosophila identified new Wnt signalling molecules e.g. Dishevelled and Shaggy.
1990: Riggleman et al reports on how the accumulation of Armadillo (mammalian homolog is beta-catenin) in Drosophila is posttranscriptionally controlled by the Wingless gene.
1991: The nomenclature of Wingless and Int-1 was combined to form the mnemonic, Wnt.
1992: Wingless signaling was discovered to be regulated by zeste-white 3 (also called Shaggy), which is the Drosophila homolog of mammalian Glycogen Synthase Kinase-3 (GSK-3).
1994: Dishevelled was identified as an essential element in the Wnt pathway 
1995: APC was found to regulate Β-catenin stability 
1996: β-catenin was found to directly interact with LEF-TCF transcription factors 
1996: Frizzled, a seven span transmembrane receptor, was identified as the cell surface receptor of Wnt ligands 
1996: Porcupine, a multi-transmembrane protein, was found to process Wnt ligands 
1996: Nuclear accumulation of β-catenin was found in colorectal cancers 
1997: The three-dimensional structure of β-catenin was determined 
1997: Phosphorylation targets β-catenin to ubiquitylation, involving interaction with the E3 ligase B-TrCP, and to proteasome dependent degradation 
1997: Identification of the homeotic gene Ubx as the first Wnt target gene 
1999: Casein kinase 1 (CK1) was found to regulate β-catenin function 
2000: The three-dimensional structure of the β-catenin–TCF complex was determined 
2000: Arrow, LRP5 and LRP6 were identified as coreceptors of Frizzled 
2001: LRP5 was found to transduce Wnt signals by recruitment of axin to the plasma membrane 
2003: The tyrosine kinase receptor Derailed in Drosophila (RYK in mammals) was identified as an alternative Wnt receptor 
2006: LEF1 mutations were associated with sebaceous gland tumours in humans, showing that Wnt–β-catenin signalling is inhibited in these tumours 
2007: Dishevelled was found to polymerize at the plasma membrane and to recruit axin upon Wnt stimulation 
Mechanism of action
- Wnt proteins encompass a network of secreted glycolipoproteins 
- Wnt signalling best known for playing a variety roles in embryogenesis, control of cellular proliferation and the resulting birth defects, cancers and other diseases arising from mutations in the pathway 
- Beta-catenin commonly exists as a subunit making up the cadherin protein complex 
- Cadherin proteins play an integral role in the formation of adhesion junctions between cells 
- Beta-catenin also acts to help anchor the actin cytoskeleton within the cell 
- The Beta-catenin gene can be said to function as an oncogene given that the promotion of transcription of Wnt target genes by beta-catenin has been shown to be involved in the development of an array of cancers including basal cell carcinoma, colorectal cancer and breast cancer.
In the absence of Wnt (“off state”)
- No extracellular "Wnt signal"
- Beta-catenin targeted for proteosomal degradation by its incorporation into a “destruction complex” comprising:
- Intracellular Axin, GSK-3β (glycogen synthase kinase β) and APC (Adenomatous polyposis coli – coded for by the APC tumour suppressor gene)
- Leads to phosphorylation of the N-terminal of β-catenin by the coordinated action of CK1 and GSK-3β 
- B-catenin is recognised by β-Trcp, ubiquinated and subsequently degraded by the 26S proteosome  
- Level of Beta-catenin in the cytosol is low, and therefore does not reach the nucleus at a level sufficient enough to affect transcription
- No transcription of Wnt target genes
In the presence of Wnt (“on state”)
- Extracellular “Wnt signal” binds to a cell surface G-protein coupled receptor of the “Frizzled” (FRZ) family
- Low-density lipoprotein receptor-related protein (LRP) family functions as a transmembrane co-receptor for FRZ
- Activation (phosphorylation and poly-ubiquination) of proteins of the “Dishevelled” (DSH) family
- Implicated in the inactivation of the “destruction complex” by the recruitment of GSK-3β away from the complex
- With the complex now interrupted, beta-catenin resists ubiquination and reaches the nucleus in increased levels where it functions as a transcriptional enhancer alongside TCF 
- Transcription of Wnt target genes and synthesis of their encoded proteins
Diseases associated with Wnt/β-catenin signalling
|Disease||Gene affected||Mutation||Normal function||Effects of mutation|
|Familiar Adenomatous Polyposis (FAP)||APC||Loss of function (LOF) mutation. One defective APC allele is inherited.||Facilitates B-catenin degradation and acts as a tumour suppressor||Large numbers of colon adenomas or polyps (benign out growths of epithelial cells) are developed. Inevitably, some of these polyps progress into malignant adenocarnicoma.|
Wnt/β-Catenin Signaling: Components, Mechanisms, and Diseases 
- Signaling by the Wnt family of secreted glycolipoproteins is one of the fundamental mechanisms that direct cell proliferation, cell polarity, and cell fate determination during embryonic development and tissue homeostasis (Logan and Nusse, 2004). As a result, mutations in the Wnt pathway are often linked to human birth defects, cancer, and other diseases (Clevers, 2006). A critical and heavily studied Wnt pathway is the canonical Wnt pathway, which functions by regulating the amount of the transcriptional coactivator β-catenin, which controls key developmental gene expression programs.
- Given the critical roles of Wnt/b-catenin signaling in development and homeostasis, it is no surprise that mutations of the Wnt pathway components are associated with many hereditary disorders, cancer, and other diseases (Table 1).
- Association of deregulated Wnt/β-catenin signaling with cancer has been well documented, particularly with colorectal cancer (Polakis, 2007) (Table 1). Constitutively activated β-catenin signaling, due to APC deficiency or β-catenin mutations that prevent its degradation, leads to excessive stem cell renewal/proliferation that predisposes cells to tumorigenesis.
- Mutations of β-catenin at and surrounding these serine and threonine residues are frequently found in cancers, generating mutant β-catenin that escapes phosphorylation and degradation (Table 1).
Caught up in a Wnt storm: Wnt signaling in cancer 
- The Wnt signaling pathway, named for its most upstream ligands, the Wnts, is involved in various differentiation events during embryonic development and leads to tumor formation when aberrantly activated. Molecular studies have pinpointed activating mutations of the Wnt signaling pathway as the cause of approximately 90% of colorectal cancer (CRC), and somewhat less frequently in cancers at other sites, such as hepatocellular carcinoma (HCC).
- Greater than 90% of all CRCs will have an activating mutation of the canonical Wnt signaling pathway, ultimately leading to the stabilization and accumulation of β-catenin in the nucleus of a cell.
- Fig. Schematic representation of a colon crypt and proposed model for polyp formation. At the bottom third of the crypt, the progenitor proliferating cells accumulate nuclear β-catenin. Consequently, they express β-catenin/TCF target genes. An uncharacterized source of WNT factors likely resides in the mesenchymal cells surrounding the bottom of the crypt, depicted in red. As the cells reach the mid-crypt region, β-catenin/TCF activity is downregulated and this results in cell cycle arrest and differentiation. Cells undergoing mutation in APC or β-catenin become independent of the physiological signals controlling β-catenin/TCF activity. As a consequence, they continue to behave as crypt progenitor cells in the surface epithelium giving rise to aberrant crypt foci.
Key players in Wnt/β-catenin Signalling
Figure 4. Structure of beta-catenin <refname="PMID11806834"/>
|Initiates Wnt signalling when it binds as a ligand to the Fz receptor with LRP  and destabilises the β-catenin degradation complex, dephosphorylating β-catenin and subsequently enabling its migration and accumulation in the cell nucleus for gene transcription .||
|LRP-5/6||Associates with the Wnt ligand binds to the Fz receptor in the transduction of the Wnt signal [in mammals] [PMID 11029008]. Causes translocation and association of axin to its intracellular tail, thus destabilising its β-catenin binding [PMID 11336703 ]||
|Dsh||Prevents phosphorylatory activity of GSK-3β [PMID: 7744250], thus preventing b-catenin ubiquitination and degradation, to promote translocation into the nucleus.||
|Axin||In the absence of Wnt signalling, it constitutes the β-catenin ubiquitination complex along with APC and GSK-3β [PMID 9312064 ] and its role as a scaffolding protein enhances GSK-3β phosphorylation of β-catenin [PMID: 9482734].||
|GSK-3β||Constitutes the β-catenin ubiquitination complex along with axin and APC [PMID 9312064 ]; its primary role in the Wnt/β-catenin signalling pathway is inhibition of β-catenin nuclear translocation through phosphorylation of three amino acids at the N-terminus [PMID: 12027456] after priming by CK-1 [PMID: 12000790], for subsequent ubiquitination and degradation by the proteasome. This activity is enhanced by GSK-3β phosphorylation of axin [PMID: 9482734] and APC [PMID: 8638126] which appears to promote phosphorylation of β-catenin by GSK-3β in the complex.||
|Diversin||Primes b-catenin for degradation by recruiting CK-I to phosphorylate serine 45 on the N-terminus before subsequent phosphorylation of threonine 41, serine 37 and serine 33 by GSK-b and degradation by E3 ubiquitin ligase β-TrCP [PMID: 12000790]|
|APC||A large 312 kDa protein||Posses a numerous and varied set of roles ranging from cell migration and adhesion, cell cycle regulation and chromosome stability [PMID: 11978510]. In the Wnt/b-catenin signalling pathway, APC constitutes the β-catenin ubiquitination complex along with axin and GSK-3β [PMID 9312064 ]|
- transcription cofactor with TCF and LEF PMID 17693601
- may play an important role in cell-cell adhesion by coordinating rearrangement of the actin cytoskeleton through binding dynein at the "plus end" of actin filaments, close to the cytoskeleton
- β-catenin undergoes ubiquitination by phosphorylation through the serine/threonine kinases casein kinase I (CKI) and glycogen synthase-3-β(GSK-3-β) and degradation by the 26S proteosome PMID 9312064
- excess E-cadherin inhibits translocation of β-catenin into the nucleusCite error: Closing
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- How do the Fz receptors work?
- What is Dsh doing to transduce the signal?
- What is the relationship between APC and Wnt signaling?
- How does Arm/β-catenin get into the nucleus?
List of Abbreviations Used
- 7TM: seven transmembrane receptor
- APC: adenomatous polyposis coli
- CamKII: Ca2+/calmodulin-dependent protein kinase II
- CK-I: casein kinase I
- CRD: cysteine rich domain
- Dsh: Dishevelled
- Fz: Frizzled
- GSK-3β: glycogen synthase kinase-3β
- HMG: high mobility group
- LEF: lymphoid enhancer-binding factor
- LRP: lipoprotein receptor–related protein
- Nkd: naked cuticle
- PKC: protein kinase C
- SAMP: Ser-Ala-Met-Pro repeated amino acid segment
- Stbm: strabismus* TCF: T cell factor
List of Terminology
- canonical: standard and well accepted
- differentiation: the process by which cells become mature and specialised in structure and function
- glycolipoprotein: a protein with attached lipid and carbohydrate groups
- homeotic gene: genes involved in embryonic development, specifically controlled the anterior-posterior axis
- oncogene: genes which transform normal cells into cancerous cells
- pluripotency: the ability of stem cells to differentiate into ectoderm, mesoderm and endoderm
- PDZ domain: a common structural domain of 80-90 amino acids of signalling proteins, whose name is derived from the first three letters of the first three proteins to be discovered to share the domain: post synaptic density protein, Drosophila disc large tumor suppressor, zonula occludens-1 protein
- proteasome: a large intracellular particle which degrades proteins
- self renewal: the capability to undergo numerous cell divisions and maintain undifferentiation
- ubiquitylation: the process of adding ubiquitin
- Hubrecht Institute: Wnt signaling and cancer
- Children's Hospital Boston: Wnt signaling in development
- University of Pennsylvania: Wnt in developmental biology
- Stanford School of Medicine: Stem Cells, Wnt signaling and Tissue Repair
- University of Nebraska: Convergence of Wnt and PKA signaling
- Centre for Cancer Research: The Role of Wnts in Vertebrate Development and Cancer
- Sharma RP Wingless, a new mutant in D.melanogaster. Drosoph. Inf. Serv: 1973, 50;134
- Cite error: Invalid
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