A Wnt survival guide: from flies to human disease

Abstract

It has been two decades since investigators discovered the link between the Drosophila wingless (Wg) gene and the vertebrate oncogene int-1, thus establishing the family of signaling proteins known as Wnts. Since the inception of the Wnt signaling field, there have been 19 Wnt isoforms identified in humans. These secreted glycoproteins can activate at least two distinct signaling pathways in vertebrate cells, leading to cellular changes that regulate a vast array of biological processes, including embryonic development, cell fate, cell proliferation, cell migration, stem cell maintenance, tumor suppression, and oncogenesis. In certain contexts, one subset of Wnt isoforms activates the canonical Wnt/beta-catenin pathway that is characterized by the activation of certain beta-catenin-responsive target genes in response to the binding of Wnt ligand to its cognate receptors. Similarly, a second subset of Wnt isoforms activates beta-catenin-independent pathways, including the Wnt/calcium (Wnt/Ca) pathway and the Wnt/planar cell polarity (Wnt/PCP) pathway, in certain cellular contexts. In addition, research has identified several secreted proteins known to regulate Wnt signaling, including the Dickkopf (DKK) family, secreted Frizzled-related proteins (sFRPs), and Wnt inhibitory factor-1 (WIF-1). The advent of technologies that can provide genome-wide expression data continues to implicate Wnts and proteins that regulate Wnt signaling pathways in a growing number of disease processes. The aim of this review is to provide a context on the Wnt field that will facilitate the interpretation and study of Wnt signaling in the context of human disease.

Figure 1. A simplified representation of Wnt/β-catenin and β-catenin-independent signaling pathways

In the absence of Wnt ligand (far left), the “destruction complex” composed of the core proteins Axin, adenomatous polyposis coli (APC), and glycogen synthase kinase-3 (GSK3) rapidly phosphorylates (P) cytosolic β-catenin, targeting it for subsequent ubiquitination (Ub) and proteasome-mediated destruction. Binding of Wnt to Frizzled (Fzd) and low-density lipoprotein receptor-related protein 5/6 (LRP5/6) activates the cytosolic protein Dishevelled (DVL), leading to inhibition of the destruction complex. The resulting accumulated β-catenin can then translocate to the nucleus to activate Wnt-responsive target genes regulated by TCF and LEF family transcription factors, leading to various cellular effects. Activation of the small GTPases Rho and Rac can result in cytoskeletal rearrangements that affect cellular motility upon Wnt stimulation. Binding of Wnt isoforms to either Fzd or receptors such as receptor tyrosine kinase-like orphan receptor 2 (Ror2, a receptor for Wnt-5a), can trigger β-catenin-independent downstream signaling events, including the inhibition of Wnt/β-catenin signaling. The mechanisms underlying β-catenin-independent Wnt signaling are not well defined, and may be largely determined by cellular context. The secreted inhibitor Dickkopf (DKK) can antagonize Wnt signaling by competitively binding to LRP5/6. Secreted Fzd-related proteins (SFRPs) and Wnt inhibitory factor (WIF) are thought to antagonize Wnt signaling by sequestering Wnt ligand in the extracellular space.

Figure 2. The use of Wnt reporters can provide important information on the status of Wnt/β-catenin signaling in animal and cell models

(a) Transgenic BATGAL mice (Maretto et al., 2003) contain the lacZ reporter gene downstream of an optimized β-catenin/TCF promoter, allowing both the temporal and spatial visualization of active Wnt/β-catenin signaling (blue areas) in a mouse embryo (E11.5) after a β-galactosidase reaction. The boxed area is magnified to show that Wnt/β-catenin activation in individual cells can be readily assessed. Bar = 1mm. (b) Cultured H1 human embryonic stem cells (hESCs) stably expressing GFP downstream of an optimized β-catenin/TCF promoter allow visualization of endogenous Wnt/β-catenin pathway activation in embryoid bodies. This reporter allows ready visualization of asymmetric endogenous Wnt/β-catenin signaling in cells within a single embryoid body by comparing the light micrograph (left panel) to the GFP image (right panel). Bar = 100 µm.