pygopusencodes a nuclear protein essential for Wingless/Wnt signaling

Abstract

The Wingless (Wg)/Wnt signal transduction pathway regulates many developmental processes through a complex of Armadillo(Arm)/β-catenin and the HMG-box transcription factors of the Tcf family. We report the identification of a new component, Pygopus (Pygo), that plays an essential role in the Wg/Wnt signal transduction pathway. We show that Wg signaling is diminished during embryogenesis and imaginal disc development in the absence of pygo activity. Pygo acts downstream or in parallel with Arm to regulate the nuclear function of Arm protein. pygo encodes a novel and evolutionarily conserved nuclear protein bearing a PHD finger that is essential for its activity. We further show that Pygo can form a complex with Arm in vivo and possesses a transcription activation domain(s). Finally, we have isolated a Xenopus homolog of pygo (Xpygo). Depletion of maternal Xpygo by antisense deoxyoligonucleotides leads to ventralized embryonic defects and a reduction of the expression of Wnt target genes. Together, these findings demonstrate that Pygo is an essential component in the Wg/Wnt signal transduction pathway and is likely to act as a transcription co-activator required for the nuclear function of Arm/β-catenin.

Genome-wide phenotypic RNAi screen in theDrosophilawing: global parameters

“…Phenotypes presenting loss of veins are reminiscent of loss of Dpp/BMP (Decapentaplegic/bone morphogenetic protein) or EGFR (epidermal growth factor receptor) signaling ( de Celis 2003 ). pygopus ( pygo ) encodes a nuclear component of the Wg (Wingless)/Wnt-βcatenin signaling pathway ( Belenkaya et al 2002 ) and its phenotype in the wing margin is compatible with reduced Wg signaling ( Figure 7C ). Knockdown of pygo also causes loss of veins, a phenotype that is not related to Wg signaling ( Figure 7C ).…”

The WNT/β‐catenin dependent transcription: A tissue‐specific business

“…The co‐factor Pygopus (Pygo) was simultaneously identified in four independent genetic screens carried out in Drosophila (Belenkaya et al, 2002 ; Kramps et al, 2002 ; Parker, Jemison, & Cadigan, 2002 ; Thompson, Townsley, Rosin‐Arbesfeld, Musisi, & Bienz, 2002 ). pygo loss‐of‐function caused a segment polarity phenotype reminiscent of strong mutations in arm or wg (Kramps et al, 2002 ; Thompson et al, 2002 ), implying that Pygo was necessary for transducing every canonical Wg/Wnt output.…”

“…Pygo has two vertebrate homologs, PYGO1 and PYGO2, that can potentiate Wnt signaling transcription in HEK293T (Kramps et al, 2002 ) and in CRC human cells (Thompson et al, 2002 ). Their downregulation in Xenopus by antisense deoxyoligonucleotides caused ventralization accompanied by a reduction of Wnt target genes expression (Belenkaya et al, 2002 ). Two evolutionarily conserved domains coordinate Pygo function: a C‐terminal PHD (plant homology domain) zinc‐coordinating finger that, as it occurs in other PHD‐containing proteins, confers affinity to methylated lysines on histone tails (such as di‐ and tri‐methylation of lysine 4 on histone H3, H3K4m2/3), and a N‐terminal homology domain (NHD) (Parker et al, 2002 ).…”

“…Upon their discovery, Pygo and Lgs/BCL9 immediately emerged as the ultimate activators of the Wg/WNT‐dependent transcription: loss of their function in Drosophila caused a complete loss of Wg outputs in all the contexts considered (Belenkaya et al, 2002 ; Kramps et al, 2002 ; Parker et al, 2002 ; Thompson et al, 2002 ). Hence, they were considered necessary for Arm/β‐catenin transcription.…”

Mechanisms of Wnt signaling and control

“…These are required for transmission of the Wg signal throughout development; lgs recruits pygo to the TCF/β-catenin transcriptional complex, where it functions as a transcriptional coactivator (Kramps et al, 2002). In vertebrates, these are similarly required for β-catenin-mediated transcription (Belenkaya et al, 2002;Hoffmans & Basler, 2007;Thompson, Townsley, Rosin-Arbesfeld, Musisi, & Bienz, 2002), though some of these effects are context specific (Song et al, 2007;Sustmann, Flach, Ebert, Eastman, & Grosschedl, 2008). Lgs and Pygo are also implicated in the nuclear localization of β-catenin (Brembeck et al, 2004;Townsley, Cliffe, & Bienz, 2004).…”