In cellular biology, the Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that Cell signaling into a cell through cell surface receptors. The name Wnt, pronounced "wint", is a portmanteau created from the names Wingless and Int-1.[
]
Three Wnt signaling pathways have been characterized: the canonical Wnt pathway, the noncanonical planar cell polarity pathway, and the noncanonical Wnt/calcium pathway. All three pathways are activated by the binding of a Wnt-protein ligand to a Frizzled family receptor, which passes the biological signal to the Dishevelled inside the cell. The canonical Wnt pathway leads to regulation of gene transcription, and is thought to be negatively regulated in part by the SPATS1 gene.
Wnt signaling was first identified for its role in carcinogenesis, then for its function in embryonic development. The embryonic processes it controls include body axis patterning, cell fate specification, cell proliferation and cell migration. These processes are necessary for proper formation of important tissues including bone, heart and muscle. Its role in embryonic development was discovered when genetic mutations in Wnt pathway proteins produced abnormal fruit fly embryos. Later research found that the genes responsible for these abnormalities also influenced breast cancer development in mice. Wnt signaling also controls tissue regeneration in adult bone marrow, skin and intestine.
This pathway's clinical importance was demonstrated by mutations that lead to various diseases, including breast cancer and prostate cancer, glioblastoma, type II diabetes and others.
History and etymology
The discovery of Wnt signaling was influenced by research on oncogenic (cancer-causing) retroviruses. In 1982, Roel Nusse and Harold Varmus infected mice with mouse mammary tumor virus in order to mutate mouse genes to see which mutated genes could cause breast tumors. They identified a new mouse proto-oncogene that they named int1 (integration 1).
Int1 is highly conserved across multiple species, including humans and Drosophila. In 1987, researchers discovered that the int1 gene in Drosophila was actually the already known and characterized Drosophila gene known as Wingless (Wg).[ Since previous research by Christiane Nüsslein-Volhard and Eric Wieschaus (which won them the Nobel Prize in Physiology or Medicine in 1995) had already established the function of Wg as a segment polarity gene involved in the formation of the body axis during embryonic development, researchers determined that the mammalian int1 discovered in mice is also involved in embryonic development.]
Continued research led to the discovery of further int1-related genes; however, because those genes were not identified in the same manner as int1, the int gene nomenclature was inadequate. Thus, the int/Wingless family became the Wnt family and int1 became Wnt1. The name Wnt is a portmanteau of int and Wg and stands for "Wingless-related integration site".
Proteins
Wnt comprises a diverse family of secreted lipid-modified signaling glycoproteins that are 350–400 amino acids in length.
These proteins are highly conserved across species.[ They can be found in mice, humans, Xenopus, zebrafish, Drosophila and many others.]|
|
| WNT1, WNT2, WNT2B, WNT3, WNT3A, WNT4, WNT5A, WNT5B, WNT6, WNT7A, WNT7B, WNT8A, WNT8B, WNT9A, WNT9B, WNT10A, WNT10B, WNT11, WNT16 |
| Wnt1, Wnt2, Wnt2B, Wnt3, Wnt3A, Wnt4, Wnt5A, Wnt5B, Wnt6, Wnt7A, Wnt7B, Wnt8A, Wnt8B, Wnt9A, Wnt9B, Wnt10A, Wnt10B, Wnt11, Wnt16 |
| Wnt1, Wnt2, Wnt2B, Wnt3, Wnt3A, Wnt4, Wnt5A, Wnt5B, Wnt7A, Wnt7B, Wnt8A, Wnt8B, Wnt10A, Wnt10B, Wnt11, Wnt11R |
|
| Wnt1, Wnt2, Wnt2B, Wnt3, Wnt3A, Wnt4, Wnt5A, Wnt5B, Wnt6, Wnt7A, Wnt7B, Wnt8A, Wnt8B, Wnt10A, Wnt10B, Wnt11, Wnt16 |
| Wg, DWnt2, DWnt3/5, DWnt 4, DWnt6, WntD/DWnt8, DWnt10 |
| hywnt1, hywnt5a, hywnt8, hywnt7, hywnt9/10a, hywnt9/10b, hywnt9/10c, hywnt11, hywnt16 |
mom-2, lin-44, egl-20, cwn-1, cwn-2 |
Mechanism
Foundation
Wnt signaling begins when a Wnt protein binds to the N-terminal extra-cellular cysteine-rich domain of a Frizzled (Fz) family receptor.[ These receptors span the plasma membrane seven times and constitute a distinct family of G-protein coupled receptors (GPCRs).]
Canonical and noncanonical pathways
The three best characterized Wnt signaling pathways are the canonical Wnt pathway, the noncanonical planar cell polarity pathway, and the noncanonical Wnt/calcium pathway. As their names suggest, these pathways belong to one of two categories: canonical or noncanonical. The difference between the categories is that a canonical pathway involves the protein beta-catenin (β-catenin) while a noncanonical pathway operates independently of it.
Canonical pathway
The canonical Wnt pathway (or Wnt/β-catenin pathway) is the Wnt pathway that causes an accumulation of Beta-catenin in the cytoplasm and its eventual translocation into the Cell nucleus to act as a transcriptional coactivator of transcription factors that belong to the TCF/LEF family. Without Wnt, β-catenin would not accumulate in the cytoplasm since a destruction complex would normally degrade it. This destruction complex includes the following proteins: Axin, adenomatosis polyposis coli (APC), protein phosphatase 2A (PP2A), glycogen synthase kinase 3 (GSK3) and casein kinase 1α (CK1α).
Noncanonical pathways
The noncanonical planar cell polarity (PCP) pathway does not involve β-catenin. It does not use LRP-5/6 as its co-receptor and is thought to use NRH1, Ryk, PTK7 or ROR2. The PCP pathway is activated via the binding of Wnt to Fz and its co-receptor. The receptor then recruits Dishevelled, which uses its PDZ and DIX domains to form a complex with Dishevelled-associated activator of morphogenesis 1 (DAAM1). Daam1 then activates the small G-protein Rho through a guanine exchange factor. Rho activates Rho-associated kinase (ROCK), which is one of the major regulators of the cytoskeleton. Dsh also forms a complex with rac1 and mediates profilin binding to actin. Rac1 activates JNK and can also lead to actin polymerization. Profilin binding to actin can result in restructuring of the cytoskeleton and gastrulation.
The noncanonical Wnt/calcium pathway also does not involve β-catenin. Its role is to help regulate calcium release from the endoplasmic reticulum (ER) in order to control intracellular calcium levels. Like other Wnt pathways, upon ligand binding, the activated Fz receptor directly interacts with Dsh and activates specific Dsh-protein domains. The domains involved in Wnt/calcium signaling are the PDZ and DEP domains.
Integrated Wnt Pathway
The binary distinction of canonical and non-canonical Wnt signaling pathways has come under scrutiny and an integrated, convergent Wnt pathway has been proposed.
Other pathways
Wnt signaling also regulates a number of other signaling pathways that have not been as extensively elucidated. One such pathway includes the interaction between Wnt and GSK3. During cell growth, Wnt can inhibit GSK3 in order to activate mTOR in the absence of β-catenin. However, Wnt can also serve as a negative regulator of mTOR via activation of the tumor suppressor TSC2, which is upregulated via Dsh and GSK3 interaction.
Regulation
In order to ensure proper functioning, Wnt signaling is constantly regulated at several points along its signaling pathways.
Upon secretion, the ligand can be prevented from reaching its receptor through the binding of proteins such as the stabilizers Dally and glypican 3 (GPC3), which inhibit diffusion. In cancer cells, both the heparan sulfate chains
At the Fz receptor, the binding of proteins other than Wnt can antagonize signaling. Specific antagonists include Dickkopf (Dkk), Wnt inhibitory factor 1 (WIF-1),
Interactions between Wnt signaling pathways also regulate Wnt signaling. As previously mentioned, the Wnt/calcium pathway can inhibit TCF/β-catenin, preventing canonical Wnt pathway signaling.
Induced cell responses
Embryonic development
Wnt signaling plays a critical role in embryonic development. It operates in both vertebrates and invertebrates, including humans, frogs, zebrafish, C. elegans, Drosophila and others. It was first found in the segment polarity of Drosophila, where it helps to establish anterior and posterior polarities. It is implicated in other developmental processes. As its function in Drosophila suggests, it plays a key role in body axis formation, particularly the formation of the anterior and dorsoventral axes. It is involved in the induction of cell differentiation to prompt formation of important organs such as lungs and ovaries. Wnt further ensures the development of these tissues through proper regulation of cell proliferation and Cell migration. Wnt signaling functions can be divided into axis patterning, cell fate specification, cell proliferation and cell migration.
Axis patterning
In early embryo development, the formation of the primary body axes is a crucial step in establishing the organism's overall body plan. The axes include the anteroposterior axis, dorsoventral axis, and right-left axis. Wnt signaling is implicated in the formation of the anteroposterior and dorsoventral (DV) axes. Wnt signaling activity in anterior-posterior development can be seen in mammals, fish and frogs. In mammals, the primitive streak and other surrounding tissues produce the morphogenic compounds Wnts, BMPs, FGFs, NODAL and retinoic acid to establish the posterior region during late gastrula. These proteins form concentration gradients. Areas of highest concentration establish the posterior region while areas of lowest concentration indicate the anterior region. In fish and frogs, β-catenin produced by canonical Wnt signaling causes the formation of organizing centers, which, alongside BMPs, elicit posterior formation. Wnt involvement in DV axis formation can be seen in the activity of the formation of the Spemann organizer, which establishes the dorsal region. Canonical Wnt signaling β-catenin production induces the formation of this organizer via the activation of the genes twin and siamois.[ Similarly, in avian gastrulation, cells of the Koller's sickle express different mesodermal marker genes that allow for the differential movement of cells during the formation of the primitive streak. Wnt signaling activated by FGFs is responsible for this movement.]
Wnt signaling is also involved in the axis formation of specific body parts and organ systems later in development. In vertebrates, sonic hedgehog (Shh) and Wnt morphogenetic signaling gradients establish the dorsoventral axis of the central nervous system during neural tube axial patterning. High Wnt signaling establishes the dorsal region while high Shh signaling indicates the ventral region.[
]
In the embryonic differentiation waves model of development Wnt plays a critical role as part a signalling complex in competent cells ready to differentiate. Wnt reacts to the activity of the cytoskeleton, stabilizing the initial change created by a passing wave of contraction or expansion and simultaneously signals the nucleus through the use of its different signalling pathways as to which wave the individual cell has participated in. Wnt activity thereby amplifies mechanical signalling that occurs during development.
Cell fate specification
Cell fate specification or cell differentiation is a process where undifferentiated cells can become a more specialized cell type. Wnt signaling induces differentiation of pluripotent stem cells into mesoderm and endoderm progenitor cells.[
] Wnt1 antagonizes neural differentiation and is a major factor in self-renewal of neural stem cells. This allows for regeneration of nervous system cells, which is further evidence of a role in promoting neural stem cell proliferation.
Cell proliferation
In order to have the mass differentiation of cells needed to form the specified cell tissues of different organisms, proliferation and growth of embryonic stem cells must take place. This process is mediated through canonical Wnt signaling, which increases nuclear and cytoplasmic β-catenin. Increased β-catenin can initiate transcriptional activation of proteins such as cyclin D1 and c-myc, which control the G1 to S phase transition in the cell cycle. Entry into the S phase causes DNA replication and ultimately mitosis, which are responsible for cell proliferation.
The biochemistry of cancer stem cells is subtly different from that of other tumor cells. These so-called Wnt-addicted cells hijack and depend on constant stimulation of the Wnt pathway to promote their uncontrolled growth, survival and migration. In cancer, Wnt signaling can become independent of regular stimuli, through mutations in downstream oncogenes and tumor suppressor genes that become permanently activated even though the normal receptor has not received a signal. β-catenin binds to transcription factors such as the protein TCF4 and in combination the molecules activate the necessary genes. LF3 strongly inhibits this binding in vitro, in cell lines and reduced tumor growth in mouse models. It prevented replication and reduced their ability to migrate, all without affecting healthy cells. No cancer stem cells remained after treatment. The discovery was the product of "Drug design", involving AlphaScreens and ELISA technologies.
Cell migration
Cell migration during embryonic development allows for the establishment of body axes, tissue formation, limb induction and several other processes. Wnt signaling helps mediate this process, particularly during convergent extension. Signaling from both the Wnt PCP pathway and canonical Wnt pathway is required for proper convergent extension during gastrulation. Convergent extension is further regulated by the Wnt/calcium pathway, which blocks convergent extension when activated. Wnt signaling also induces cell migration in later stages of development through the control of the migration behavior of , neural crest cells, , and tracheal cells.
Wnt signaling is involved in another key migration process known as the epithelial-mesenchymal transition (EMT). This process allows epithelial cells to transform into mesenchymal cells so that they are no longer held in place at the laminin. It involves cadherin down-regulation so that cells can detach from laminin and migrate. Wnt signaling is an inducer of EMT, particularly in mammary development.
Insulin sensitivity
Insulin is a peptide hormone involved in glucose homeostasis within certain organisms. Specifically, it leads to upregulation of glucose transporters in the cell membrane in order to increase glucose uptake from the bloodstream. This process is partially mediated by activation of Wnt/β-catenin signaling, which can increase a cell's insulin sensitivity. In particular, Wnt10b is a Wnt protein that increases this sensitivity in skeletal muscle cells.
Clinical implications
Cancer
Since its initial discovery, Wnt signaling has had an association with cancer. When Wnt1 was discovered, it was first identified as a proto-oncogene in a mouse model for breast cancer. The fact that Wnt1 is a homolog of Wg shows that it is involved in embryonic development, which often calls for rapid cell division and migration. Misregulation of these processes can lead to tumor development via excess cell proliferation.
Canonical Wnt pathway activity is involved in the development of Benign tumor and
/ref> Its presence is revealed by elevated levels of β-catenin in the nucleus and/or cytoplasm, which can be detected with immunohistochemical staining and Western blotting. Increased β-catenin expression is correlated with poor prognosis in breast cancer patients. This accumulation may be due to factors such as mutations in β-catenin, deficiencies in the β-catenin destruction complex, most frequently by mutations in structurally disordered regions of APC, overexpression of Wnt ligands, loss of inhibitors and/or decreased activity of regulatory pathways (such as the Wnt/calcium pathway).
Wnt signaling has been implicated in the development of other cancers as well as in desmoid fibromatosis.
The link between PGE2 and Wnt suggests that a chronic inflammation-related increase of PGE2 may lead to activation of the Wnt pathway in different tissues, resulting in carcinogenesis.
Type II diabetes
Diabetes mellitus type 2 is a common disease that causes reduced insulin secretion and increased insulin resistance in the periphery. It results in increased blood glucose levels, or hyperglycemia, which can be fatal if untreated. Since Wnt signaling is involved in insulin sensitivity, malfunctioning of its pathway could be involved. Overexpression of Wnt5b, for instance, may increase susceptibility due to its role in adipogenesis, since obesity and type II diabetes have high comorbidity.
See also
-
AXIN1
-
GSK-3
-
Management of hair loss
-
Wingless localisation element 3 (WLE3)
-
WNT1-inducible-signaling pathway protein 1 (WISP1)
-
WNT1-inducible-signaling pathway protein 2 (WISP2)
-
WNT1-inducible-signaling pathway protein 3 (WISP3)
Further reading
External links
