The Pygo2-H3K4me2/3 interaction is dispensable for mouse development and Wnt signaling-dependent transcription

Abstract

SUMMARYPygopus has been discovered as a fundamental Wnt signaling component in Drosophila. The mouse genome encodes two Pygopus homologs, Pygo1 and Pygo2. They serve as context-dependent β-catenin coactivators, with Pygo2 playing the more important role. All Pygo proteins share a highly conserved plant homology domain (PHD) that allows them to bind di-and trimethylated lysine 4 of histone H3 (H3K4me2/3). Despite the structural conservation of this domain, the relevance of histone binding for the role of Pygo2 as a Wnt signaling component and as a reader of chromatin modifications remains speculative. Here we generate a knock-in mouse line, homozygous for a Pygo2 mutant defective in chromatin binding. We show that even in the absence of the potentially redundant Pygo1, Pygo2 does not require the H3K4me2/3 binding activity to sustain its function during mouse development. Indeed, during tissue homeostasis, Wnt/β-catenin-dependent transcription is largely unaffected. However, the Pygo2-chromatin interaction is relevant in testes, where, importantly, Pygo2 binds in vivo to the chromatin in a PHD-dependent manner. Its presence on regulatory regions does not affect the transcription of nearby genes; rather, it is important for the recruitment of the histone acetyltransferase Gcn5 to chromatin, consistent with a testis-specific and Wnt-unrelated role for Pygo2 as a chromatin remodeler.

Evolutionary Adaptation of the Fly Pygo PHD Finger toward Recognizing Histone H3 Tail Methylated at Arginine 2

“…published evidence arguing against the physiological relevance of Pygo’s histone binding during mouse development, based on a knock-in of a histone binding-deficient Pygo2 mutant (Cantù et al., 2013). Unfortunately, this mutant (A342E, identical to A356E in hPygo1) retains significant histone binding (K d  = 109 ± 8 μM; Fiedler et al., 2008), with an approximate ten times higher affinity than fly Pygo, which therefore seriously undermines the conclusions drawn by these authors (Cantù et al., 2013). …”

A cytoplasmic role of Wnt/β-catenin transcriptional cofactors Bcl9, Bcl9l, and Pygopus in tooth enamel formation

“…3F-H). Notably, abrogating just the interaction between Bcl9/9l and Pygo1/2 proteins in K14-Cre; Bcl9 flox/∆HD1 ; Bcl9l flox/ ∆HD1 (K14-Bcl9/9l-∆HD1; (17)) is also sufficient to perturb enamel structure (Fig. S4).…”

Pax6-dependent, but β-catenin-independent, function of Bcl9 proteins in mouse lens development

“…b-Catenin-independent function of Bcl9/9l mice, Pygo2 has tissue-specific, Wnt-independent functions in the testis (Nair et al 2008;Cant u et al 2013) and lens ). Also, in Drosophila, Pygo was recently reported to act in a b-cateninindependent manner during heart development (Tang et al 2013(Tang et al , 2014).…”