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Overview of signal transduction pathways. On the upper right hand side of the cell, a Wnt signaling protein is shown to bind to a frizzled receptor.

The Wnt signaling pathway describes a network of proteins most well known for their roles in embryogenesis and cancer, but also involved in normal physiological processes in adult animals.[1]

Contents

Discovery

The name Wnt was coined as a combination of Wg (wingless) and Int [2] and can be pronounced as 'wint'. The wingless gene had originally been identified as a recessive mutation affecting wing and haltere development in Drosophila melanogaster[3]. It was subsequently characterized as segment polarity gene in Drosophila melanogaster that functions during embryogenesis[4] and also during adult limb formation during metamorphosis.[5] The INT genes were originally identified as vertebrate genes near several integration sites of mouse mammary tumor virus (MMTV).[6] The Int-1 gene and the wingless gene were found to be homologous, with a common evolutionary origin evidenced by similar amino acid sequences of their encoded proteins.

Mutations of the wingless gene in the fruit fly were found in wingless flies, while tumors caused by MMTV were found to have copies of the virus integrated into the genome forcing overproduction of one of several Wnt genes. The ensuing effort to understand how similar genes produce such different effects has revealed that Wnts are a major class of secreted morphogenic ligands of profound importance in establishing the pattern of development in the bodies of all multicellular organisms studied.

Members

The following is a list of human genes that encode WNT signaling proteins:[7]

Mechanism

Figure 2. Wnt binds to (activates) the receptor. Axin is removed from the "destruction complex." β-Cat moves into the nucleus, binds to a transcription factor on DNA, and activates transcription of a protein. "P" represents phosphate.
Figure 1. Wnt doesn't bind to the receptor. Axin, GSK and APC form a "destruction complex," and β-Cat is destroyed. Compare to Figure 2. See the article main text for details.

The Wnt pathway involves a large number of proteins that can regulate the production of Wnt signaling molecules, their interactions with receptors on target cells and the physiological responses of target cells that result from the exposure of cells to the extracellular Wnt ligands. Although the presence and strength of any given effect depends on the Wnt ligand, cell type, and organism, some components of the signaling pathway are remarkably conserved in a wide variety of organisms, from Caenorhabditis elegans to humans. Protein homology suggests that several distinct Wnt ligands were present in the common ancestor of all bilaterian life, and certain aspects of Wnt signaling are present in sponges and even in slime molds.

The canonical Wnt pathway describes a series of events that occur when Wnt proteins bind to cell-surface receptors of the Frizzled family, causing the receptors to activate Dishevelled family proteins and ultimately resulting in a change in the amount of β-catenin that reaches the nucleus (Figure 2). Dishevelled (DSH) is a key component of a membrane-associated Wnt receptor complex (Figure 2) which, when activated by Wnt binding, inhibits a second complex of proteins that includes axin, GSK-3, and the protein APC (Figure 1). The axin/GSK-3/APC complex normally promotes the proteolytic degradation of the β-catenin intracellular signaling molecule. After this "β-catenin destruction complex" is inhibited, a pool of cytoplasmic β-catenin stabilizes, and some β-catenin is able to enter the nucleus and interact with TCF/LEF family transcription factors to promote specific gene expression (interaction 2, Figure 2). Some additional details of the pathway are described below.

Cell surface Frizzled (FRZ) proteins usually interact with a transmembrane protein called LRP (Figure 2).[8] LRP binds Frizzled, Wnt and axin and may stabilize a Wnt/Frizzled/LRP/Dishevelled/axin complex at the cell surface ("receptor complex" in Figure 2).

In vertebrates, several secreted proteins have been described that can modulate Wnt signaling by either binding to Wnts[9] or binding to a Wnt receptor protein. For example, Sclerostin (not shown in a figure) can bind to LRP and inhibit Wnt signaling.[10]

The part of the pathway linking the cell surface Wnt-activated Wnt receptor complex to the prevention of β-catenin degradation is still under investigation. There is evidence that trimeric G proteins (G in Figure 2) can function downstream from Frizzled.[11] It has been suggested that Wnt-activated G proteins participate in the disassembly of the axin/GSK3 complex.[12]

Several protein kinases and protein phosphatases have been associated with the ability of the cell surface Wnt-activated Wnt receptor complex to bind axin and disassemble the axin/GSK3 complex.[13] Phosphorylation of the cytoplasmic domain of LRP by CK1 and GSK3 can regulate axin binding to LRP (interaction 1 in Figure 2). The protein kinase activity of GSK3 appears to be important for both the formation of the membrane-associated Wnt/FRZ/LRP/DSH/Axin complex and the function of the Axin/APC/GSK3/β-catenin complex. Phosphorylation of β-catenin by GSK3 leads to the destruction of β-catenin (Figure 1).

Ligands which act on Wnt signaling

  • 2-amino-4-[3,4-(methylenedioxy)benzyl-amino]-6-(3-methoxyphenyl)pyrimidine as an agonist of Wnt signaling.[14]
  • The signalling molecule Cerberus inhibits Wnt, thus repressing the inhibition of βCatenin on SoxB1 family members.This enables the specification of Neuroepithelium in Drosophila neural induction.

Wnt-induced cell responses

Several important effects of the canonical Wnt pathway include:

  • Cancers. Alterations of Wnts, APC, axin, and TCFs are all associated with carcinogenesis.
  • Body axis specification. Injection of Xenopus eggs with Wnt inhibitors is involved in the development of a second head. Wnt is extensively involved in formation of the posterior nervous system and are released by tail "organizers".
  • Morphogenic signaling. Wnts produced from specific sites, such as the edge of the developing fly wing or the ventral edge of the neural tube of the developing vertebrate, are distributed throughout adjacent tissues in a gradient fashion. The Wnt pathway becomes activated to different degrees in cells of these tissues depending on how close they are to the production site, leading to subtle but crucial differences in the level of genes regulated by the Wnt pathway.

Non-canonical Wnt signaling is associated with other activities, such as:

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Planar cell polarity

An example of the control of planar cell polarity in insects like Drosophila is determining which direction the tiny hairs on the wings of a fly are aligned.[15] Planar cell polarity is distinct from and perpendicular to apical/basal polarity. The signaling pathway that is involved in planar cell polarity includes frizzled and dishevelled but not the axin complex proteins. The non-classical cadherins Fat, Dachsous and Flamingo can apparently modulate frizzled function. Other proteins including prickle, strabismus, rhoA and rho-kinase act downstream of frizzled and dishevelled to regulate the cytoskeleton and planar cell polarity.[15][16]

Some of the proteins involved in planar cell patterning of the Drosophila wing are used in vertebrates during regulation of cell movements during events such as gastrulation. A common feature of both hair patterning in Drosophila and cell movements such as vertebrate gastrulation is control of actin filaments by G proteins such as Rho and Rac.[17]

Axon Guidance

Wnt has some diverse roles in axon guidance. For example, the Wnt Receptor Ryk is required for Wnt mediated axon guidance on the controlateral side of the corpus callosum.[18] Another example is in the growing spinal cord commissural neurons: after their extending axons cross the midplate of the spinal cord, they are guided by a Wnt gradient , which is active through the Frizzled receptors in this case.[19]

Stem cells

Traditionally, it is assumed that Wnt proteins can act as Stem Cell Growth Factors, promoting the maintenance and proliferation of stem cells.[20]

However, a recent study conducted by the Stanford University School of Medicine revealed that Wnt appears to block proper communication, with the Wnt signaling pathway having a negative effect on stem cell function. Thus, in the case of muscle tissue, the misdirected stem cells, instead of generating new muscle cells (myoblasts), differentiated into scar-tissue-producing cells called fibroblasts. The stem cells failed to respond to instructions, actually creating wrong cell types.[21]

Understanding the mechanisms by which pluripotency, self-renewal and subsequent differentiation are controlled in embryonic stem cells is crucial to utilizing them therapeutically. Additionally, control of Wnt signaling may allow for minimizing the use of animal products, which can introduce unwanted pathogens, in stem cell cultures.[22] Wnt signaling was first identified as a potential component to differentiation because of its established role in development. Recent research has supported this hypothesis. There are data to suggest that Wnt signaling induces differentiation of pluripotent stem cells into mesoderm and endoderm progenitor cells.

There are several pieces of evidence to suggest that Wnt signaling is important in stem cell differentiation.[22] TCF3, a transcription factor regulated by Wnt signaling, has been shown to repress nanog, a gene required for stem cell pluripotency and self-renewal.[23] Over expression of another gene associated with pluripotency, OCT4 leads to increased beta-catenin activity, suggesting Wnt involvement.[24]

Studies of embryoid bodies (see embryoid body) have led to new insights regarding the role of Wnt signaling in human embryonic stem cells. Researchers at Stanford School of Medicine observed that embryoid bodies spontaneously begin gastrulation.[25] They determined that gastrulation in embryoid bodies mimics the in vivo process in human embryos; in vivo gastrulation has been previously linked to the Wnt pathway. Formation of the primitive streak in particular was associated with localized Wnt activation in the embryoid bodies. Once the Wnt pathway is activated, it is self-reinforcing. It is unclear, however, what induces the initial Wnt signaling that begins gastrulation.

Research published in the Journal of Biological Chemistry has suggested that activation of the Wnt pathway in mouse embryonic stem cells induces differentiation into multipotent mesoderm and endoderm cells.[26] This study showed that upon inducing Wnt signaling in mono-layer embryonic stem cell cultures, the cells express high levels of markers associated with mesoderm development, particularly T-brachyury and Flk-1. The cells also expressed high levels of Foxa2, Lhx1, and AFP, which are associated with endoderm development. The progenitor cells created via Wnt activation seemed to have particularly high potential to differentiate into bone and cartilage. The researchers suggested that beta-catenin plays an important role in skeletal development. They demonstrated that the progenitor cells could also develop into endothelial, cardiac, and vascular smooth muscle lineages.

A publication from the American Society of Hematology extended the previous study to human embryonic stem cells (hESCs) by demonstrating that Wnt signaling can induce hematoendothelial cell development from hESCs.[27] This study showed that Wnt3 leads to mesoderm committed cells with hematopoietic potential. Over expression of Wnt1 led to faster, more efficient hematoendothelial differentiation than Wnt3 over expression. Wnt1 has also been shown to antagonize neural differentiation; this observation suggests a variety of roles for the Wnt pathway in stem cell activity. In contrast to Wnt3, which is associated with mesoderm and endoderm differentiation, Wnt1 serves the opposite function in neural stem cells. Wnt1 appears to be a major factor in self-renewal of neural stem cells. Wnt stimulation is also associated with regeneration of nervous system cells, which is further evidence of a role in promoting neural stem cell proliferation.[22]

Environmental enrichment

Changes in Wnt signaling mimic in adult mice the effects of environmental enrichment upon synapses in the hippocampus in regard to reversible increase in their numbers, and spine plus synapse densities at large mossy fiber terminals[28] It seems that Wnt signaling might be part of the means by which experience regulates synapse numbers and hippocampal network structure.[28]

Wnt Pathway in Cancer Stem Cells

The Wnt Pathway (also the Hedgehog and Notch pathways) are thought to be involved in the occurrence of Cancer stem cell (CSC). sFRP1 (Secreted Frizzled Protein) is a regulator of Wnt. When Wnt binds to sFRP, it cannot activate the Wnt pathway. Beachy et al. (2004, nature review) found that sFRP is lost in colorectal and breast cancer.

See also

External links

References

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