|Figure 1. The atypical receptor tyrosine kinase, Ryk, acts as a Wnt co-receptor. It may also mediate the interaction between Frizzled and Dishevelled in the canonical Wnt pathway and regulate neurite outgrowth. [Note: figure adapted from Moon, R.T. et al. (2002) Science 296:1644.]|
The Wnts make up a large family of secreted proteins with roles in pattern formation, cell fate decisions, axon guidance, and tumor formation. The most widely studied Wnt signaling pathway, the canonical pathway, has been reviewed in depth and involves the regulation of cellular ß-Catenin levels.1,2 Briefly, in the absence of Wnt, ß-Catenin is recruited to a multimolecular protein complex that includes Axin, APC, and the kinase GSK-3ß. ß-Catenin is phosphorylated by GSK-3ß, and subsequently ubiquitinated and targeted for proteasomal degradation. Wnt binding to its receptor Frizzled, and potential co-receptor LRP-5/6, suppresses GSK-3ß phosphorylation of ß-Catenin. ß-Catenin accumulates and binds to LEF/TCF transcription factors resulting in the activation of Wnt target genes. The adaptor protein Dishevelled lies downstream of the Frizzled receptor and is critical for Wnt-mediated effects on cellular ß-Catenin levels. The exact mechanism leading to the activation of Dishevelled remains unclear, although it may involve Ryk (related to tyrosine kinase), a catalytically inactive, atypical receptor tyrosine kinase (Figure 1).3
Ryk is a mammalian ortholog of Drosophila Derailed and C. elegans Lin-18, proteins that bind Wnts and have roles in axon guidance and pattern formation.4,5 Ryk has previously been shown to associate with members of the Eph family.6,7 Like other receptor tyrosine kinases, Ryk is a type I, single pass, transmembrane protein.3 The extracellular region contains a WIF module related to a similar region in WIF-1 (Wnt inhibitory factor 1), a secreted protein known to bind and modulate Wnt activities.8 This suggests that Ryk might be a Wnt receptor. A recent study by Lu et al. shows that Ryk does directly bind Wnt-1 and Wnt-3a.9 In addition, TCF activation by Wnt-3a is doubled in cells expressing Ryk, while knockdown of Ryk using siRNA suppresses TCF activation by Wnt-1. In addition to binding Wnt, Ryk also directly interacts with Frizzled-8 suggesting that Ryk may act as a Wnt co-receptor. Earlier studies have indicated that the intracellular C-terminus of Frizzled exhibits a direct, although weak, interaction with the PDZ domain of Dishevelled.10 Lu et al. provide evidence that the association between Frizzled and Dishevelled may also occur indirectly through Ryk.9 Ryk and Dishevelled co-immunoprecipitate from mouse brain and their co-expression in cultured 293T cells leads to a significant increase in TCF activation.9 The interaction between Dishevelled and Ryk is eliminated by mutating the Dishevelled PDZ domain, and blocking endogenous Dishevelled using siRNA suppresses Wnt-3a/Ryk-mediated TCF activation. How might Ryk affect the functional activities of Wnt? In vitro, Wnt-3a-stimulated axon outgrowth from dorsal root ganglion (DRG) explants is inhibited by suppressing Ryk using siRNA. In vivo, Ryk siRNA mice (E10-10.5) exhibit fasciculation and projection defects in craniofacial motor axons, and glassopharyngeal, opthalmic, and vagus nerves.
An increasing array of Wnt receptors, co-receptors, and binding proteins continue to be identified. This might be expected given the intricate processes that Wnts regulate. The addition of Ryk as a Wnt co-receptor adds yet another level of complexity in the attempt to understand the mechanisms underlying Wnt functions.