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There are 19 human WNT genes, several of which encode additional alternatively spliced isoforms (Miller 2001; Kawano and Kypta 2003). Wnt proteins initiate signaling through binding Frizzled receptors, which contain seven transmembrane spanning domains. Ten Frizzled receptors have been identified in humans. Signal specificity may be achieved through cell specific expression of different Frizzled receptors, which are capable of forming homo- and hetero-oligomers or through the association of Frizzled receptors with different co-receptors (Kohn and Moon 2005).
In the absence of Wnt, the signaling pool of beta-catenin is maintained at low levels through degradation (Dale 1998; Huelsken and Behrens 2002; Nusse 2005; The Wnt homepage). Beta-catenin is targeted for ubiquitination by the beta-transducin repeat containing protein (bTrCP) and is then degraded by the proteosome. Beta-catenin is phosphorylated by the serine/threonine kinases casein kinase 1 (CK1) and glycogen synthase 3b (GSK3β). Phosphorylation of beta-catenin occurs in a multi-protein complex (the destruction complex), which comprises axin, adenomatous polyposis coli (APC) protein and diversin. Upon receipt of a Wnt signal, Dishevelled prevents degradation of beta-catenin, possibly through the recruitment of GBP/Frat-1, which in turn displaces GSK3β from the destruction complex. LRP5/6, which are members of the low-density lipoprotein receptor-related protein family serve as co-receptors for beta-catenin-dependent Wnt signaling. Frodo and beta-arrestin act synergistically with Dishevelled, whilst Dapper has been identified as an antagonist of Dishevelled.
Stabilised beta-catenin enters the nucleus and associates with T cell factor (TCF)/lymphoid enhancer factor (LEF) transcription factors, which leads to the transcription of Wnt target genes such as cyclin D1, PPARd and twin. In the absence of a Wnt signal TCF/LEF family members interact with transcriptional inhibitors such as Groucho, which serve to repress Wnt signaling. The repressing effect of Groucho is mediated by interactions with histone deactylases (HDAC), which are thought to make DNA refractory to transcriptional activation. Negative regulators of beta-catenin signaling include ICAT and duplin, which directly bind to beta-catenin preventing it interacting with TCF/LEF (Kikuchi et al., 2006). In addition to its role in Wnt signaling, beta-catenin plays a role in cell adhesion through binding to the cytoplasmic domain of type 1 cadherins and linking them through alpha-catenin to the actin cytoskeleton.
In the planar cell polarity pathway, Wnt signaling through Frizzled receptors mediates asymmetric cytoskeletal organization and the polarization of cells by inducing modifications to the actin cytoskeleton. Two independent pathways that are initiated by Dishevelled trigger the activation of the small GTPases Rho and Rac. Activation of Rho requires Daam-1 and leads in turn to the activation of the Rho-associated kinase ROCK. Rac activation is independent of Daam-1 and stimulates Jun Kinase (JNK) activity (Huelsken and Behrens 2002; Habas and Dawid 2005).
Wnt signaling via Frizzled receptors can also lead to the release of intracellular calcium. Frizzled co-receptors, involved in this pathway include Knypek and Ror2. Other intracellular second messengers associated with this pathway include heterotrimeric G-proteins, phospholipase C (PLC) and protein kinase C (PKC). The exact genes activated by the Wnt/Ca2+ pathway are unknown, but NFAT appears to be involved, which is a transcription factor regulated by the calcium/calmodulin-dependent protein phosphatase, calcineurin. The Wnt/Ca2+ pathway is important for cell adhesion and cell movements during gastrulation (Habas and Dawid 2005; Kohn and Moon 2005).
Wnt antagonists can be divided into two functional classes, the secreted Frizzled related proteins (sFRP class) and the Dickkopf (Dkk) class (Kawano and Kypta 2003). Members of the sFRP class include the sFRP family (sFRP1-5), Wnt inhibitory factor-1 (WIF-1) and Cerberus. These antagonists bind directly to Wnt and alter their ability to bind to the Wnt receptor complex. In contrast, members of the Dickkopf class (Dkk1-4) inhibit Wnt signaling by binding to LRP5/LRP6. Kremen, which binds to Dkk is thought to trigger the internalization and clearing of the Dkk–LRP complex from the cell surface. The segregation and internalization of LRP thereby renders Wnt unable to activate intracellular signaling (Mao et al., 2002; Rothbächer and Lemaire 2002). Thus, in theory, antagonists of the sFRP class inhibit both canonical and noncanonical Wnt signaling pathways, whereas those of the Dickkopf class specifically inhibit beta-catenin-dependent canonical Wnt signaling.