Links between signal transduction, transcription and adhesion in epithelial bud development

Abstract

The morphogenesis of organs as diverse as lungs, teeth and hair follicles is initiated by a downgrowth from a layer of epithelial stem cells 1,2 . During follicular morphogenesis, stem cells form this bud structure by changing their polarity and cell-cell contacts. Here we show that this process is achieved through simultaneous receipt of two external signals: a Wnt protein to stabilize β-catenin, and a bone morphogenetic protein (BMP) inhibitor to produce Lef1. β-Catenin then binds to, and activates, Lef1 transcription complexes that appear to act uncharacteristically by downregulating the gene encoding E-cadherin, an important component of polarity and intercellular adhesion. When either signal is missing, functional Lef1 complexes are not made, and E-cadherin downregulation and follicle morphogenesis are impaired. In Drosophila, E-cadherin can influence the plane of cell division and cytoskeletal dynamics 3 . Consistent with this notion, we show that forced elevation of E-cadherin levels block invagination and follicle production. Our findings reveal an intricate molecular programme that links two extracellular signalling pathways to the formation of a nuclear transcription factor that acts on target genes to remodel cellular junctions and permit follicle formation.During skin development, signals from adjacent epithelial and mesenchymal cells instruct select ectodermal cells to form hair follicle buds. In turn, each bud signals to a small group of underlying mesenchymal cells to condense 1,2 . Once the bud proliferates to form a larger bulb (matrix), it encases this dermal condensate (papilla), and further differentiates into the cells of the hair shaft (Fig. 1a) 1 . Recent evidence suggests that Wnt signalling is involved in this process at a time that correlates with bud-specific patterns of upregulation of P-cadherin and downregulation of E-cadherin 4-9 . Cadherins form the transmembrane core of adherens junctions (AJs) by bridging to α-catenin and the cytoskeleton through β-catenin, a protein which on its own is prone to degradation 10,11 . β-Catenin's degradation machinery is transiently suppressed by Wnt signalling. This renders β-catenin a new-found stability and function, binding to and activating members of the Lef1/Tcf family of DNA binding proteins 11-13 . Whether there is underlying functional significance to β-catenin's link between adhesion and transcription is an issue that we now address.Wnts are expressed in ectodermal buds 14,15 , and are prime candidates to stabilize β-catenin at these sites. We confirmed this by testing the ability of the canonical skin Wnt3a to generate nuclear β-catenin in mouse keratinocytes. Keratinocytes exposed to Wnt3a-conditioned media displayed an ~7 times increase in β-catenin, as judged by immunoblot and densitometry analysis (Fig. 1b). This increase was paralleled by accumulation of β-catenin in ~85 ± 5% of the nuclei of treated cells (Fig. 1c).

Role of Pinin in neural crest, dorsal dermis, and axial skeleton development and its involvement in the regulation of Tcf/Lef activity in mice

“…The involvement of PNN in the regulation of Tcf/Lef activity may provide an excellent explanation for a previously proposed ability of PNN to affect the expression of E-cadherin, MMP-7, c-Myc, and p21, because Wnt signaling has been shown to influence the expression of E-cadherin during epithelial bud formation (Jamora et al, 2003) and p21, c-Myc, and MMP-7 in small intestine organogenesis van de Wetering et al, 2002;Okubo and Hogan, 2004;Andreu et al, 2005). Furthermore, ␤-catenin has been recently reported to influence pre-mRNA splicing (Sato et al, 2005;Lee et al, 2006), a process where PNN is also involved (Li et al, 2003;Zimowska et al, 2003).…”

“…Although the signaling pathway or the mechanism orchestrating these coordinated alterations in gene expression are still unknown, PNN was shown to directly interact with CtBP and have an effect on the activity of E-cadherin promoter (Alpatov et al, 2004). Because CtBP has been closely implicated in Wnt signaling (Chinnadurai, 2002;Hamada and Bienz, 2004;Sierra et al, 2006) and expression of E-cadherin is often negatively regulated by Wnt signaling in development and cancer (Jamora et al, 2003;Yook et al, 2005), PNN's ability to interact with CtBP, along with PNNmediated phenotypic changes in adhesion, proliferation, and motility, may suggest possible involvement of PNN in Wnt signaling. In addition, PNN interacts with several pre-mRNA splicing factors by means of its RS domain and plays a role in alternative splicing Zimowska et al, 2003).…”

Wnt inhibitory factor 1 induces apoptosis and inhibits cervical cancer growth, invasion and angiogenesis in vivo

“…In vector-treated tumors, cells showed weak or loss of membranous Ecadherin expression and weak-to-moderate cytoplasmic staining (Figure 5d). Importantly, WIF1 treatment resulted in a marked increase in the expression of Ecadherin (Figure 5d), a direct b-catenin transcriptional target (Jamora et al, 2003). Furthermore, in WIF1-treated tumors, E-cadherin staining was mainly localized to the cell membrane (Figure 5d), a pattern commonly seen in the normal cervical epithelium.…”

Neurogenin3 initiates stepwise delamination of differentiating endocrine cells during pancreas development

“…In other systems, EMT can be triggered by various signaling pathways (Jamora et al, 2003;Nawshad and Hay, 2003;Medici et al, 2006), but these pathways usually converge on zinc finger transcription factors of the Snail family which directly control downstream effectors. In the pancreas, Snail2 may be the transcription factor that induces EMT downstream of Ngn3.…”