The PHD domain is required to link Drosophila Pygopus to Legless/β-catenin and not to histone H3

In Drosophila Pygopus (Pygo) and Legless (Lgs)/BCL9 are integral components of the nuclear Wnt/Wg signaling machine. Despite intense research, ideas that account for their mode of action remain speculative. One proposition, based on a recently discovered function of PHD fingers, is that Pygo, through its PHD, may decipher the histone code. We found that human, but not Drosophila, Pygo robustly interacts with a histone-H3 peptide methylated at lysine-4. The different binding behavior is due to a single amino acid change that appears unique to Drosophilidae Pygo proteins. Rescue experiments with predicted histone binding mutants showed that in Drosophila the ability to bind histones is not essential. Further experiments with Pygo–Lgs fusions instead demonstrated that the crucial role of the PHD is to provide an interaction motif to bind Lgs. Our results reveal an interesting evolutionary dichotomy in Pygo structure–function, as well as evidence underpinning the chain of adaptors model.     

Analysis of mPygo2 mutant mice suggests a requirement for mesenchymal Wnt signaling in pancreatic growth and differentiation

Pygopus has recently been identified in Drosophila as an essential component of the nuclear complex required for canonical Wnt signaling. Here, we have investigated the role of the mammalian pygopus ortholog, mPygo2, in pancreas development. We show that a null mutation of mPygo2 in mice causes pancreas hypoplasia due to decreased progenitor cell proliferation after embryonic day (e) 12.5. During the same time window, mPygo2-deficient embryos begin to display a reduction in endocrine progenitors and consequently a decrease in islet endocrine cell mass. Consistent with its function after e12.5, late-developing endocrine cell types, such as beta, delta and PP cells, are specifically reduced, while the earlier-forming alpha cells develop normally. We find canonical Wnt signaling to be predominantly active in the mesenchyme at the time when mPygo2 is required and demonstrate the dependence of Wnt signal transduction on mPygo2. Furthermore, conditional deletion of mPygo2^flox allele in the pancreatic epithelium does not phenocopy the defects in mPygo2-null mutants. Since mPygo2 is expressed in the pancreatic mesenchyme and the role of the mesenchyme in epithelial progenitor cell expansion is well documented, our findings suggest an indirect role for mPygo2 in epithelial growth and differentiation through regulation of mesenchymal signals. Together, our data suggest a previously unappreciated role for mesenchymal Wnt signaling in regulating pancreatic organ growth and cell differentiation.