Wnt signaling is an evolutionarily conserved metazoan cell communication pathway required for proper animal development. Of the myriad of signaling events that have been ascribed to cellular activation by Wnt ligands, the canonical Wnt/β-catenin pathway has been the most studied and best understood. Misregulation of Wnt/β-catenin signaling has been implicated in developmental defects in the embryo and major diseases in the adult. Despite the latter, no drugs that inhibit the Wnt/β-catenin pathway have been approved by the FDA. In this review, we explore the least understood step in the Wnt/β-catenin pathway—nuclear regulation of Wnt target gene transcription. We initially describe our current understanding of the importation of β-catenin into the nucleus. We then focus on the mechanism of action of the major nuclear proteins implicated in driving gene transcription. Finally, we explore the concept of a nuclear Wnt enhanceosome and propose a modified model that describes the necessary components for the transcription of Wnt target genes.

Intestinal stem cell (ISC) self-renewal and proliferation are directed by Wnt/β-catenin signaling in mammals, whereas aberrant Wnt pathway activation in ISCs triggers the development of human colorectal carcinoma. Herein, we have utilized the Drosophila midgut, a powerful model for ISC regulation, to elucidate the mechanisms by which Wingless (Wg)/Wnt regulates intestinal homeostasis and development. We provide evidence that the Wg signaling pathway, activation of which peaks at each of the major compartment boundaries of the adult intestine, has essential functions. Wg pathway activation in the intestinal epithelium is required not only to specify cell fate near compartment boundaries during development, but also to control ISC proliferation within compartments during homeostasis. Further, in contrast with the previous focus on Wg pathway activation within ISCs, we demonstrate that the primary mechanism by which Wg signaling regulates ISC proliferation during homeostasis is non-autonomous. Activation of the Wg pathway in absorptive enterocytes is required to suppress JAK-STAT signaling in neighboring ISCs, and thereby their proliferation. We conclude that Wg signaling gradients have essential roles during homeostasis and development of the adult intestine, non-autonomously controlling stem cell proliferation inside compartments, and autonomously specifying cell fate near compartment boundaries. The highly conserved Wingless/Wnt signal transduction pathway directs many cellular processes in metazoans and its deregulation underlies numerous human congenital diseases and cancers. Most notably, more than 80% of colon cancers arise from aberrant activation of the Wnt pathway. A better understanding of how Wnt signaling functions in the intestinal stem cells (ISCs) during homeostasis and in disease states is thus critical. The Drosophila digestive tract provides a powerful genetic model and an entry point to study these questions. Here, we find that the Wg ligand and pathway activation are enriched at Drosophila intestinal compartment boundaries and are essential for development and homeostasis of the adult gut. During homeostasis, Wg signaling in enterocytes is required to prevent the overproliferation of ISCs non-autonomously. In addition, during development, Wg signaling ensures proper cell fate specification near compartment boundaries. These findings provide insight into the mechanisms underlying the Wg-dependent regulation of adult intestinal function.

Wnt/β-catenin signalling directs fundamental processes during metazoan development and can be aberrantly activated in cancer. Wnt stimulation induces the recruitment of the scaffold protein Axin from an inhibitory destruction complex to a stimulatory signalosome. Here we analyse the early effects of Wnt on Axin and find that the ADP-ribose polymerase Tankyrase (Tnks)—known to target Axin for proteolysis—regulates Axin’s rapid transition following Wnt stimulation. We demonstrate that the pool of ADP-ribosylated Axin, which is degraded under basal conditions, increases immediately following Wnt stimulation in both Drosophila and human cells. ADP-ribosylation of Axin enhances its interaction with the Wnt co-receptor LRP6, an essential step in signalosome assembly. We suggest that in addition to controlling Axin levels, Tnks-dependent ADP-ribosylation promotes the reprogramming of Axin following Wnt stimulation; and propose that Tnks inhibition blocks Wnt signalling not only by increasing destruction complex activity, but also by impeding signalosome assembly.

Wnt/β–catenin signal transduction directs metazoan development and is deregulated in numerous human congenital disorders and cancers. In the absence of Wnt stimulation, a multi-protein "destruction complex", assembled by the scaffold protein Axin, targets the key transcriptional activator β-catenin for proteolysis. Axin is maintained at very low levels that limit destruction complex activity, a property that is currently being exploited in the development of novel therapeutics for Wnt-driven cancers. Here, we use an in vivo approach in Drosophila to determine how tightly basal Axin levels must be controlled for Wnt/Wingless pathway activation, and how Axin stability is regulated. We find that for nearly all Wingless-driven developmental processes, a three- to four-fold increase in Axin was insufficient to inhibit signaling, setting a lower-limit for the threshold level of Axin in the majority of in vivo contexts. Further, we find that both the tumor suppressor Adenomatous polyposis coli (APC) and the ADP-ribose polymerase Tankyrase (Tnks) have evolutionarily conserved roles in maintaining basal Axin levels below this in vivo threshold, and we define separable domains in Axin that are important for APC- or Tnks-dependent destabilization. Together, these findings reveal that both APC and Tnks maintain basal Axin levels below a critical in vivo threshold to promote robust pathway activation following Wnt stimulation.