The Wnts

Abstract

The Wnt genes encode a large family of secreted protein growth factors that have been identified in animals from hydra to humans. In humans, 19 WNT proteins have been identified that share 27% to 83% amino-acid sequence identity and a conserved pattern of 23 or 24 cysteine residues. Wnt genes are highly conserved between vertebrate species sharing overall sequence identity and gene structure, and are slightly less conserved between vertebrates and invertebrates. During development, Wnts have diverse roles in governing cell fate, proliferation, migration, polarity, and death. In adults, Wnts function in homeostasis, and inappropriate activation of the Wnt pathway is implicated in a variety of cancers.

Figure 1

(a) Structures of selected members of the human WNT gene family. Exons are shown as boxes and introns as lines. For each gene, 'RNA' represents the portion of the gene that is transcribed and 'CDS' represents the portion that encodes protein. WNT8a/d is an example of a gene with 3' alternative splicing and WNT16 is an example of a gene with alternatively used 5' exons. (b) Structural features of the Wnt protein. The amino terminus contains a signal sequence (S). All Wnts contain 23 or 24 conserved cysteine residues (C) with similar spacing, suggesting that the folding of Wnt proteins depends on the formation of multiple intramolecular disulfide bonds.

Figure 2

Predicted evolutionary relationships between members of the Wnt gene family. (a) Predicted relationships between 18 of the 19 known human WNT protein sequences; WNT15 was omitted because only a partial sequence is available. (b) Predicted evolutionary relationships between selected human WNT proteins (representing each large grouping shown in (a)) and Wnt proteins from mouse, Xenopus, Drosophila, and Caenorhabditis elegans. Sequences were aligned using the ClustalW program; trees were constructed from the alignments using the neighbor-joining method and are diagrammed using midpoint rooting. Numbers indicate branch lengths.

Figure 3

The known Wnt signaling pathways. (a) In the Wnt/β-catenin pathway, Wnt signaling depends on the steady-state levels of the multi-functional protein β-catenin. In the absence of Wnt signal, a multi-protein destruction complex that includes the adenomatous polyposis coli protein (APC) and a member of the Axin family facilitates the phosphorylation of β-catenin by glycogen synthase kinase 3 (GSK3). GSK3 substrates also include APC and Axin; phosphorylation of each of these proteins leads to enhanced binding of β-catenin. Phosphorylated β-catenin is bound by the F-box protein β-TrCP, a component of an E3 ubiquitin ligase complex, and is ubiquitinated; the ubiquitin tag marks β-catenin for destruction by the proteasome. When a cell is exposed to a Wnt, the Wnt interacts with its coreceptors Frizzled and LRP. Activation of Frizzled and LRP leads to the phosphorylation of Dishevelled (Dsh), a cytoplasmic scaffold protein, perhaps through stimulation of casein kinase Iε (CKIε) and/or casein kinase II (CKII). Dsh then functions through its interaction with Axin to antagonize GSK3, preventing the phosphorylation and ubiquitination of β-catenin. In vertebrates, inhibition of GSK3 may involve the activity of GSK3 binding protein (GBP/Frat), which binds to both Dsh and GSK3 and can promote dissociation of GSK3 from the destruction complex. Unphosphorylated β-catenin escapes degradation, accumulates in the cell, and enters the nucleus, where it interacts with members of the TCF/LEF family of HMG-domain transcription factors to stimulate expression of target genes. In addition to the components of the Wnt/β-catenin pathway described here, many additional proteins with potential roles in regulating Wnt/β-catenin signaling have been reported including the phosphatase PP2A and the kinases Akt/protein kinase B, integrin-linked kinase (ILK), and PKC. (b) Signaling through the Wnt/Ca²⁺ pathway appears to involve activation of the two pertussis-toxin-sensitive G proteins, G_αo and G_.αt, in combination with G_β2 [34,35]. G-protein activation then leads to an increase in intracellular Ca²⁺ and the subsequent stimulation of Ca²⁺/calmodulin-dependent kinase II (CamKII) [37]. Activation of the Wnt/Ca²⁺ pathway also results in stimulation of PKC activity in the form of the translocation of PKC to the plasma membrane [34]. Downstream targets of the Wnt/Ca2⁺ pathway have not been identified. (c) The Wnt/polarity pathway, which regulates cytoskeletal organization; the Drosophila Wnt/polarity pathway that regulates the polarity of trichomes in the wing is shown as an example. In this case, the nature of the polarity signal is not known.