Wnt Family

Beta-Catenin-dependent Wnt Signaling Pathways

Wnt Ligands

Wnt proteins are a large family of secreted, lipid-modified proteins that regulate embryonic development and adult tissue homeostasis. The Wnt family consists of 19 different proteins in vertebrates. These proteins share 27-83% amino acid sequence identity in humans and orthologues in different vertebrate species have a high degree of sequence similarity with their human counterparts. All Wnt proteins have an N-terminal signal sequence and 22 highly conserved cysteine residues with similar spacing. The first of these conserved cysteine residues is palmitoylated and this lipid modification is necessary for both Wnt protein secretion and activity. Additionally, Wnt protein palmitoylation is required for targeting Wnts to different membranes and for their N-linked glycosylation. As a result of their lipid modification, Wnts are highly hydrophobic and are typically associated with the cell membrane or extracellular matrix.

Wnt Receptors

The Frizzled seven transmembrane, G protein-coupled receptors are the primary receptors for Wnt family proteins. The Frizzled family consists of at least 10 different receptors that are widely expressed. These receptors, along with the co-receptors, low density lipoprotein receptor-related proteins-5/6 (LRP-5/6), mediate canonical beta-catenin-dependent Wnt signaling. Additionally, Frizzled receptors can activate non-canonical, beta-catenin-independent Wnt signaling by interacting with the co-receptors, receptor tyrosine kinase-like orphan receptor-1 (ROR1), ROR2, Ryk, or PTK7. All Frizzled receptors contain a highly conserved, extracellular N-terminal cysteine-rich domain (CRD) that is required for Wnt binding, followed by a hydrophilic linker region and seven hydrophobic domains that are predicted to form transmembrane alpha helices. The intracellular C-terminal ends of Frizzled receptors contain a membrane-proximal KTXXXW motif that has been suggested to mediate their interaction with the intracellular signaling protein, Dishevelled. Aside from this motif, the C-terminal ends of Frizzled receptors are variable in length and are not well-conserved among family members. It is currently unknown whether some Frizzled proteins specifically activate canonical Wnt signaling, while others are involved in activating non-canonical Wnt signaling.

Wnt Signaling

Wnt signaling is an evolutionarily conserved signaling pathway that plays a critical role in cell fate determination, cell survival, cell proliferation, cell motility, tissue patterning, and organogenesis during embryonic development. Additionally, Wnt signaling is involved in the maintenance of adult tissue homeostasis and contributes to the pathogenesis of a variety of diseases. Wnt proteins can activate three established signaling pathways, the beta-catenin-dependent Wnt signaling pathway, the Wnt/Ca²⁺ pathway, and the planar cell polarity (PCP) pathway. The beta-catenin-dependent Wnt signaling pathway is activated by recruitment of the Axin protein complex to the receptor following Wnt binding to the Frizzled receptor. This leads to internalization of the Wnt-receptor-Axin protein complex into endosomes and sequesters two kinases that typically phosphorylate beta-catenin, glycogen synthase kinase-3 beta (GSK-3 beta) and casein kinase 1 (CK1), away from the beta-catenin protein. In the absence of these two proteins, the unphosphorylated form of beta-catenin accumulates and enters the nucleus, where it activates gene expression through its association with TCF/LEF family transcription factors and co-activators, such as Bcl-9 and Pygopus (Pygo). In contrast, when Wnt is absent, GSK-3 beta and CK1 phosphorylate beta-catenin, creating a docking site for an E3 ubiquitin ligase known as beta-TrCP, which promotes the ubiquitination and proteasomal degradation of beta-catenin.

The signal transduction cascades involved in the non-canonical, beta-catenin-independent pathways of Wnt signaling are not as well understood. In the Wnt-Ca²⁺ pathway, Wnt binds to Frizzled receptors and activates a G protein-coupled signaling pathway that results in activation of phospholipase C-beta (PLC-beta). This subsequently leads to protein kinase C (PKC) activation and an IP3-dependent calcium flux that activates both calcium/calmodulin-dependent protein kinase type II (CaMKII) and calcineurin phosphatase. These two molecules activate multiple transcription factors, including NF-kappaB, CREB, and NFAT, and promote the expression of genes that regulate cell fate and cell migration.

In the PCP pathway, Wnt binds to Frizzled or a co-receptor such as ROR2 or Ryk, which may or may not require Frizzled or another Wnt receptor. This binding activates Dishevelled, which then recruits RhoA and Rac GTPases to activate Rho kinases (ROCK) and JNK, respectively. Activation of ROCK leads to actin polymerization, cell migration, and polarity, while JNK activates the transcription factor, AP-1. In addition to Wnt-ROR2 and Wnt-Ryk signaling, the PCP pathway has also been shown to be regulated in different tissues by MuSK and VANGL2 during vertebrate development.

Misregulation of Wnt signaling is of great interest as it has been linked to developmental defects and multiple pathological conditions, including bone and kidney diseases, type II diabetes, and cancer. Wnt signaling is a key regulator of skeletal development and is required for postnatal bone homeostasis. Loss of function mutations in LRP5 and LRP6 are associated with disorders that are characterized in part by osteoporosis caused by decreased bone formation. Genetic variations in several components of the Wnt signaling pathway are also strongly associated with metabolic disorders and susceptibility to type II diabetes. Furthermore, Wnt signaling plays a central role in kidney development and diseases. While Wnt signaling has been shown to be required for tubular repair and regeneration following acute kidney injury, sustained activation of Wnt signaling leads to chronic kidney diseases and renal fibrosis. Additionally, mutations in components of the Wnt signaling pathway, or high levels of Wnt pathway activation have also been linked to many different types of cancer, including leukemia, multiple myeloma, colorectal, breast, and pancreatic cancers.