cGMP-signalling pathway in development

Wnt signalling in development operates via members of the Frizzleds, G-protein-coupled receptors that bind specific Wnt ligands and mediate signalling via distinct pathways. The Wnt/Ca2+/cGMP pathway mediated by Frizzled-2 was discovered recently. Activation of this pathway leads to increased intracellular concentrations of Ca2+ and decreased intracellular concentrations of cGMP. The nature of the phosphodiesterase responsible for this Frizzled-2-mediated effect on cGMP levels was identified based on three separate criteria: (i) sensitivity to selective enzyme inhibitors, (ii) behaviour on chromatographic separation, and (ii) isolation by two-dimensional gels in tandem with direct mapping by MS of tryptic digests of the activity. On the basis of results from these three analyses, the cGMP-specific phosphodiesterase, PDE6, is demonstrated to be an effector for the Wnt/Ca2+/cGMP signalling pathway of development, which is mediated by Frizzled-2. Canonical signalling by Wnt-Frizzleds The Wnts comprise a group of vertebrate genes encoding 350–380 amino acid secreted signalling ligands, which appear to play diverse and important roles in the generation of cell polarity, embryonic induction, specification of cell fate and in human diseases [1–3]. Wnt proteins operate via receptormediated signalling pathways and their receptors are members of the Frizzled (Fz) gene family [4,5]. Genetic and developmental approaches have outlined a canonical Wnt8Fz1 pathway, in which Fz1 activation by Wnt8 is coupled with the activation of the novel phosphoprotein Dishevelled, which acts in complex with protein phosphatase 2A, the scaffold protein Axin and the product of the adenomatous polyposis coli gene to suppress GSK3-β (glycogen synthase kinase 3-β) activity [1,2,6]. In the absence of Wnt, GSK3 acts unopposed to phosphorylate Axin and β-catenin and this destabilizes β-catenin, directing it to the proteosome for degradation [7]. In the presence of Wnt, Fz1 action inactivates GSK3, stabilizes β-catenin and leads to the accumulation and translocation of β-catenin to the nucleus where it combines with members of the Tcf family of DNAbinding proteins, enabling transcription [8–12]. β-Catenin appears to compete with the transcriptional repressor protein, Groucho, transforming Tcf from a repressor to an activator [11,13].