Wnt Signaling

The name Wnt is a portmanteau of int and Wg and means “Wingless-related integration site“. Wnt has discharged elements that manage cell development, motility, and differentiation during embryonic development. Wnt acts in a paracrine design by enacting different signaling fountains inside the objective cells. 

Wnt Gene Family

The Wnt family consists of a number of highly conserved genes that regulate gene expression, cell behavior, cell adhesion, and cell polarity, including 19 genes in humans and mice, 7 in Drosophila, and 5 in C. elegans. Wnt-1 is one member of a gene family whose additional members were isolated either as a target for MMTV insertion (Wnt-3, Wnt-3A was subsequently isolated by homology to Wnt-3), fortuitously from a chromosomal walk directed around the cystic fibrosis gene (Wnt-2), or from mouse embryo RNA using the polymerase chain reaction (Wnt-4, -5A, -5B, -6, -7A, and -7B).

•  There are now at least 10 known members of the Wnt family in the mouse; all of which are expressed during development, many in the developing nervous system with some expressed in the adult brain as well.
• In addition, five members of the Wnt family are expressed in the normal mammary gland in the mouse and are differentially regulated during pregnancy and lactation. 
• This family has been remarkably well conserved throughout evolution, with homologs present in both invertebrates and vertebrates. 
• In addition to the predicted amino acid sequence similarities among family members, a role in cell signaling has also been documented for several Wnt family members.
•  For example, wingless, the Drosophila homologue of Wnt-1, is necessary for proper segmental patterning of the embryo and is proposed to function locally via cell-cell interactions.

Wnt Signaling Pathways

The Wnt signaling pathway is a conserved pathway. The Wnt family of signaling proteins participates in multiple developmental events during embryogenesis and has also been implicated in adult tissue homeostasis. Wnt signals are pleiotropic, with effects that include mitogenic stimulation, cell fate specification, and differentiation.

• The Wnt signaling pathway is an ancient and evolutionarily conserved pathway that regulates crucial aspects of cell fate determination, cell migration, cell polarity, neural patterning, and organogenesis during embryonic development.
• The Wnts are secreted glycoproteins and comprise a large family of nineteen proteins in humans hinting at the daunting complexity of signaling regulation, function, and biological output.
• Wnt proteins are secreted glycoproteins that bind to the N-terminal extra-cellular cysteine-rich domain of the Frizzled (Fz) receptor family of which there is ten Fz (frizzled) in humans. 
• The extra-cellular Wnt signal stimulates several intracellular signal transduction cascades. 
• understanding the mechanisms of Wnt signaling, which is divided into two major branches: the canonical pathway and the noncanonical pathway. The canonical pathway is also called the Wnt/β-catenin pathway. There are two major non-canonical pathways: the Wnt-planar cell polarity pathway (Wnt-PCP pathway) and the Wnt-calcium pathway (Wnt-Ca²⁺ pathway).

Wnt signaling pathways are divided into two types 

• Canonical Pathway
• Noncanonical Pathway

Canonical Wnt Pathway 

The canonical Wnt pathway (or Wnt/β-catenin pathway) is the Wnt pathway that causes an accumulation of β-catenin in the cytoplasm and its eventual translocation into the 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, adenomatous polyposis coli (APC), Protein Phosphatase 2A (PP2A), glycogen synthase kinase 3 (GSK3), and casein kinase 1α (CK1α). 
• It degrades β-catenin by targeting it for ubiquitination, which subsequently sends it to the proteasome to be digested.
•  However, as soon as Wnt binds Fz(frizzled) and LRP5/6 (Low-density lipoprotein receptor-related protein 5) the destruction complex function becomes disrupted. This is due to Wnt causing the translocation of the negative Wnt regulator, Axin, and the destruction complex to the plasma membrane.
• Phosphorylation by other proteins in the destruction complex subsequently binds Axin to the cytoplasmic tail of LRP5/6. Axin becomes de-phosphorylated and its stability levels decrease.  
• However, a unified theory of how β‐catenin drives target gene expression is still missing, and tissue-specific players might assist β‐catenin to define its target genes. 
• The extensivity of the β-catenin interacting proteins complicates our understanding: β-catenin may be directly phosphorylated at Ser552 by Akt, which causes its disassociation from cell-cell contacts and accumulation in the cytosol, thereafter it interacts with β-catenin (pSer552) and enhances its nuclear translocation.
•  BCL9 and Pygopus have been reported, in fact, to possess several β-catenin-independent functions (therefore, likely, Wnt signaling-independent).

Non-Canonical Wnt Pathways

Intracellular signaling of the Wnt pathway diversifies into at least three branches: 

• The β-catenin pathway (canonical Wnt pathway), which activates target genes in the nucleus; 
• The planar cell polarity pathway, which involves jun N-terminal kinase (JNK); 
• The Wnt/Ca²⁺ pathway. 

The last two kinds can be classified into the non-canonical Wnt pathways. In the planar cell polarity pathway, frizzled activates JNK and directs the asymmetric cytoskeletal organization and coordinated polarization of cells within the plane of epithelial sheets. This pathway involves the Fmi, Knyand Stbm The Wnt/Ca²⁺ pathway leads to the release of intracellular Ca²⁺, possibly via G-proteins. This pathway involves the activation of PLC and PKC. Elevated Ca²⁺ can activate the phosphatase calcineurin, which leads to dephosphorylation of the transcription factor NF-AT and its accumulation in the nucleus.

• 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 PTK7 (Tyrosine-protein kinase-like 7 also known as colon carcinoma kinase 4 (CCK4)) or ROR2 (Tyrosine-protein kinase transmembrane receptor ROR2, also known as neurotrophic tyrosine kinase,). 
• The PCP pathway is activated via the binding of Wnt to Fz(frizzled) and its co-receptor. The receptor then recruits Dsh(Dishevelled), which uses its PDZ and DIX domains to form a complex with the Dishevelled-associated activator of morphogenesis 1 (DAAM1)(Disheveled-associated activator of morphogenesis 1). 
• Daam1 then activates the small G-protein Rho through a guanine exchange factor. Rho(Ras-related C3 botulinum toxin substrate 1) 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 the 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(frizzled) receptor directly interacts with Dsh and activates specific Dsh-protein domains. The domains involved in Wnt/calcium signaling are the PDZ and DEP domains.
• However, if PDE(phosphodiesterase) is activated, calcium release from the ER is inhibited. PDE mediates this through the inhibition of PKG, which subsequently causes the inhibition of calcium release.

Wnt Pathway Regulation 

One key level of regulation of Wnt signaling occurs in the extra-cellular milieu with the presence of a diverse number of secreted Wnt antagonists. After binding of Wnt to the receptor complex, the signal is transduced to cytoplasmic phosphoprotein Dishevelled (Dsh/Dvl), and studies have uncovered that Dsh can directly interact with Fz(frizzled). At the level of Dsh(Dishevelled), the Wnt signal branches into at least three major cascades, canonical, Planar Cell Polarity, and Wnt/Ca²⁺. Dsh (Dishevelled) is an important downstream component of this transduction pathway and is the first cytoplasmic protein that is pivotally involved in all three major branches of Wnt signaling. The Wnt ligands are secreted glycoproteins that are heavily modified prior to transport and release into the extra-cellular milieu.

• The porcupine protein has been shown to play an important role in the palmitoylation of the Wnt proteins, and their secretion is regulated by the wntless or evenness-interrupted proteins and the retromer complex. In the extra-cellular matrix, the Wnt proteins may be bound to and stabilized by heparan sulfate proteoglycans including Dally and glypican 3 which further limits their diffusion and modulate their signaling abilities.
• In the extra-cellular matrix, a number of secreted proteins that bind to Wnts and prevent their interaction with either Fz(Dishevelled) or LRP5/6(lipoprotein receptor-related protein) to antagonize Wnt signaling have been identified. 
• These include Dickkopf (Dkk) proteins, Wnt-inhibitor protein (WIF), soluble Frizzled-related proteins (SFRP), Cerberus, Frzb(frizzled-related protein) and the context-dependent Wnt inhibitor Wise. 
• Each of these secreted inhibitors is tightly regulated during embryogenesis and serves to limit or likely create a gradient of Wnt signaling for pattern formation. 
• An interesting recent finding is the identification of factors including Norrin and R-Spondin2, which can bind to the LRP5/6(lipoprotein receptor-related protein) receptor and may activate Wnt signaling independent of a Wnt ligand. 
• The SOST(Sclerostin) protein can also bind to LRP5/6 (lipoprotein receptor-related protein) where it can antagonize Wnt signaling.

Functions of Wnt Proteins in Polarity Orientation

• At least for some cells, Wnts can instruct the polarity orientation by functioning as a positional signal.
• Recent genetic analyses have suggested that Wnt can also control cell polarity from a distance. For example, CWN-1 and CWN-2 (Common Wire Nail – CWN) are respectively expressed posteriorly and anteriorly with regard to the seam cells and control the seam cells’ polarity orientation.
• In addition to Wnts, mutants of some Wnt receptors cause polarity reversal with some frequency, consistent with the notion that they mediate the functions of Wnts. For example, the polarity of the V1 seam cell is reversed in cam-1/Ror (a non-Fz type Wnt receptor with a tyrosine kinase domain)
• The T cell polarity can be reversed in lin-17/Fz mutants, although the loss of polarity is a more frequent outcome.
• In addition, Wnt receptors are essential for cell polarization itself, since the polarity of embryonic cells and postembryonic seam cells are lost in mom-5( (Frizzled type Wnt receptor)) single and lin-17 mom-5( (Frizzled type Wnt receptor)) cam-1 triple mutant, respectively.
• Although Wnts are involved in many asymmetric divisions, those of the SGPs (somatic gonadal precursors; Z1 and Z4 cells) appear to be Wnt independent, since their polarity is not affected in quintuple Wnt mutants.

FAQs on Wnt Signaling 

Question 1: Define Wnt Signaling.

Answer: 

The name Wnt is a portmanteau of int and Wg and means “Wingless-related integration site”.

Question 2: What Is Wnt Signaling Pathway?

Answer:

The Wnt signaling pathway is an ancient and evolutionarily conserved pathway that regulates crucial aspects of cell fate determination, cell migration, cell polarity, neural patterning, and organogenesis during embryonic development.

Question 3: What do Wnt Genes do?

Answer:

Wnt genes are highly conserved between vertebrate species sharing overall sequence identity and gene structure and are slightly less conserved between vertebrates and invertebrates.

Question 4: What are the roles of Wnt Genes?

Answer:

During development, Wnts have diverse roles in governing cell fate, proliferation, migration, polarity, and death.

Question 5: What activates Wnt Pathway?

Answer:

The Wnt/β-catenin pathway is activated when a Wnt ligand binds to a seven-pass transmembrane Frizzled (Fz) receptor and its co-receptor, low-density lipoprotein receptor-related protein 6 (LRP6) or its close relative LRP5.

Question 6: Is Wnt a Growth Factor?

Answer:

The Wnt family of secreted growth factors plays key roles in directing cell patterning both during development and in adult tissues.