Difference between revisions of "2012 Group 5 Project"

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Revision as of 12:57, 8 May 2012


2012 Projects: Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6 | Group 7 | Group 8 | Group 9

Wnt/β-catenin Signalling Pathway

Introduction

History

Figure 1. Overall structure of the β-catenin/XAxin-CBD complex

1973: A Drosophila melanogaster mutant lacking wings, Wingless (Wg), was described [1]

1982: Roel Nusse and Harold Varmus found that Int1, a mouse protooncogene was associated with MMTV-induced mammary gland tumours [2]

1987: Int1 was found to be the mammalian homologue of Wingless in Drosophila [3]

1989: New Wnt pathway components were found in a screen of lethal mutations in Drosophila [4]

1991: β-catenin was molecularly cloned [5]

1992: Shaggy was found to be cytoplasmic mediator of Wnt signalling [6]

1993: APC was found to directly interact with β-catenin [7][8]

1994: Dishevelled was identified as an essential element in the Wnt pathway [9]

1995: APC was found to regulate Β-catenin stability [10]

1996: β-catenin was found to directly interact with LEF-TCF transcription factors [11]

1996: Frizzled, a seven span transmembrane receptor, was identified as the cell surface receptor of Wnt ligands [12]

1996: Porcupine, a multi-transmembrane protein, was found to process Wnt ligands [13]

1996: Nuclear accumulation of β-catenin was found in colorectal cancers [14]

1997: The three-dimensional structure of β-catenin was determined [15]

1997: Phosphorylation targets β-catenin to ubiquitylation, involving interaction with the E3 ligase B-TrCP, and to proteasome dependent degradation [16]

1997: Identification of the homeotic gene Ubx as the first Wnt target gene [17]

1998: Axin 1 and axin 2 were found to interact with β-catenin, GSK3β and APC and to promote GSK3β - dependent phosphorylation and degradation of β-catenin [18][19]

1999: Casein kinase 1 (CK1) was found to regulate β-catenin function [20]

1999: Protein phosphatase 2A (PP2A) interacts with the β-catenin destruction complex and modulates GSK3 β (Glycogen synthase kinase 3β) function [21][22]

2000: The three-dimensional structure of the β-catenin–TCF complex was determined [23]

2000: Arrow, LRP5 and LRP6 were identified as coreceptors of Frizzled [24]

2001: LRP5 was found to transduce Wnt signals by recruitment of axin to the plasma membrane [25]

2003: The tyrosine kinase receptor Derailed in Drosophila (RYK in mammals) was identified as an alternative Wnt receptor [26]

2006: LEF1 mutations were associated with sebaceous gland tumours in humans, showing that Wnt–β-catenin signalling is inhibited in these tumours [27]

2007: Dishevelled was found to polymerize at the plasma membrane and to recruit axin upon Wnt stimulation [28]

Mechanism of action

Figure 2. "Off" and "On" Sates of the Wnt/Beta-catenin Signalling Pathway
  • Wnt proteins encompass a network of secreted glycolipoproteins [29]
  • 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 [29]
  • Beta-catenin commonly exists as a subunit making up the cadherin protein complex [30]
  • Cadherin proteins play an integral role in the formation of adhesion junctions between cells [30]
  • Beta-catenin also acts to help anchor the actin cytoskeleton within the cell [30]
  • Beta-catenin plays a vital role in the Wnt signalling pathway by directly affecting the control of protein synthesis within the cell. The interaction of beta-catenin with the TCF/LEF family of transcription factors on the template DNA strand converts them from repressors to activators, triggering downstream transcription of Wnt target genes and synthesis of their encoded proteins. [31]
  • This interaction is reliant on an intracellular cascade which ultimately dictates the amount of Beta-catenin present in the cytoplasm which is then able to reach the nucleus and effect transcription. [32]
  • In the absence of Wnt (“off state”), beta-catenin is targeted for proteasomal degradation. Intracellular Axin, GSK-3β (glycogen synthase kinase 3β) and APC (Adenomatous polyposis coli – coded for by the APC tumour suppressor gene) form a “destruction complex” leading to phosphorylation of the N-terminal of β-catenin by the coordinated action of CK1 and GSK-3β. With B-catenin now recognisable by β-Trcp, it is ubiquinated and subsequently degraded by the 26S proteosome phosphorylation. [33] [34]
  • In the presence of Wnt (“on state”) an extracellular “Wnt signal” binds to a cell surface G-protein coupled receptor of the “Frizzled” (FRZ) family. This results in the 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, an increased amount of beta-catenin is able to reach the nucleus where it can promote transcription of Wnt target genes. [32]
  • Low-density lipoprotein receptor-related protein (LRP) family functions as a transmembrane co-receptor for Frizzled. It functions to recruit axin to the membrane, leading to axin degradation upon the initiation of the Wnt signalling cascade and the dissociation of beta-catenin into the cytoplasm without degradation by the “destruction complex”. [35]
  • Histone Deacetylase (HDAC) represses transcription via chromatin remodelling in the form of histone deacetylation [36]
  • 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 basal cell carcinoma, colorectal cancer and breast cancer.[37][38] [39]

Diseases associated with Wnt/β-catenin signalling

File:Table 1. Human Diseases Associated with Mutations of the Wnt Signaling Components.png
Table 1. Human Diseases Associated with Mutations of the Wnt Signaling Components [31]
File:Figure. Schematic representation of a colon crypt and proposed model for polyp formation.png
Figure 3. Schematic_representation_of_a_colon_crypt_and_proposed_model_for_polyp_formation [40]
Disease Causes Clinical manifestations Treatment Image
Colorectal Cancer *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.
  • 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.
Disease 2
Disease 3
Disease 4
Disease 5

Notes

Wnt/β-Catenin Signaling: Components, Mechanisms, and Diseases [31]

  • 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 [40]

  • 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 [41]
Protein (Gene) Structure Function Activators Inhibitors
Wnt () Serves as a ligand to the Fz receptor. *GSK-3β
Fz
LRP
Dsh
Axin
GSK-3β
APC
β-catenin
TCF/LEF
Groucho
Pygopus
  • Frizzled receptor
    • seven transmembrane receptor-like protein with an extracellular amino terminal rich in cysteine residues PMID 9407023
  • Wnt proteins
    • secreted lipid-modified signaling proteins that influence multiple processes in animal development
    • Evolutionarily conserved across species PMID 9407023
    • Wnt interacts with Frizzled proteins and LRPs to initiate signal transduction PMID 9407023
    • 19 proteins have thus far been identified in this protein family PMID 9407023
  • β-catenin
    • transcription cofactor with TCF and LEF PMID 17693601
    • may play an important role in cell-cell adhesion by coordinating rearrangement of the actin cytoskeleton[42] through binding dynein at the "plus end" of actin filaments, close to the cytoskeleton[43]
    • 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 nucleus[44][45]
  • Cadherin protein
    • Prevents β-catenin transmigrating into the cell, by binding it at the cell membrane PMID 15001769
  • Frizzled proteins

Embryonic development

The Wnt/β-catenin signaling pathway has been implicated as an important pathway in human fetal development. Through immunohistochemical staining, Eberhart and Argani (2001)localised nuclear beta catenin in fetal lung, placenta, kidney, cartilage, capillaries, adrenal glands and skin. This indicates that Wnt signaling regulates the development of specific set of organs and tissues.[46] For example, Wnt genes such as Wnt4 regulate the conversion of mesenchyme to epithelial cells in kidney morphogenesis.

In addition, Wnt/β-catenin signaling is also involved in maintaining the pluripotency of human embryonic stem cells (hESCs). Wnt3a promotes the reprogramming of somatic cells to pluripotency in conjunction with the classical transcription factors, Oct4, Sox2 and Nanog.[47] For example, Oct4 has the effect of repressing Wnt/β-catenin signaling in self renewing hESCs and is depressed during hESC differentiation.[48] Hence, this suggests that Wnt/β-catenin signaling is involved in differentiation rather than self renewal.

Current research

Future research

Taken from Wnt signaling: a common theme in animal development [49] - the research outlined will be explored in the current literature

  • 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?

Glossary

List of Abbreviations Used

  • APC: adenomatous polyposis coli
  • Dsh: Dishevelled
  • Fz: Frizzled
  • GSK-3β: glycogen synthase kinase-3β
  • LEF: lymphoid enhancer-binding factor
  • LRP: lipoprotein receptor–related protein
  • 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
  • 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

External links

Reference List

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