Identification of putative interaction partners for the Xenopus Polycomb-group protein Xeed

Polycomb repressive complex PRC2 regulates Xenopus retina development downstream of Wnt/β-catenin signaling

The histone methyltransferase complex PRC2 controls key steps in developmental transitions and cell fate choices; however, its roles in vertebrate eye development remain unknown. Here, we report that in Xenopus, PRC2 regulates the progression of retinal progenitors from proliferation to differentiation. We show that the PRC2 core components are enriched in retinal progenitors and downregulated in differentiated cells. Knockdown of the PRC2 core component Ezh2 leads to reduced retinal progenitor proliferation, in part due to upregulation of the Cdk inhibitor p15(Ink4b). In addition, although PRC2 knockdown does not alter eye patterning, retinal progenitor gene expression or expression of the neural competence factor Sox2, it does cause suppression of proneural bHLH gene expression, indicating that PRC2 is crucial for the initiation of neural differentiation in the retina. Consistent with this, knocking down or blocking PRC2 function constrains the generation of most retinal neural cell types and promotes a Müller glial cell fate decision. We also show that Wnt/β-catenin signaling acting through the receptor Frizzled 5, but independent of Sox2, regulates expression of key PRC2 subunits in the developing retina. This is consistent with a role for this pathway in coordinating proliferation and the transition to neurogenesis in the Xenopus retina. Our data establish PRC2 as a regulator of proliferation and differentiation during eye development.

Characterization of the expression pattern of the PRC2 core subunit Suz12 during embryonic development of Xenopus laevis

The Polycomb repressive complex 2 is a multimeric aggregate that mediates silencing of a broad range of genes, and is associated with important biological contexts such as stem cell maintenance and cancer progression. PRC2 mainly trimethylates lysine 27 of histone H3 and is composed of three essential core subunits: EZH2, EED, and SUZ12. The Xenopus orthologs of PRC2 subunits Ezh2 and Eed have been described but Suz12 remained unidentified. Here, we report the cloning of the Xenopus Suz12, and determine its spatiotemporal expression during development. Xsuz12 transcript is provided maternally and continues to be expressed throughout development, particularly in the anterior part of the developing central nervous system. Importantly, comparative analysis of the expression of the PRC2 subunits Xez, Xeed, and Xrbbp4 indicates that their expression largely coincides with Xsuz12 in the nervous system, suggesting that PRC2 may have unexplored functions in the development of the frog central nervous system.

The human polycomb group EED protein interacts with the integrase of human immunodeficiency virus type 1

Human EED, a member of the superfamily of WD-40 repeat proteins and of the Polycomb group proteins, has been identified as a cellular partner of the human immunodeficiency virus type 1 (HIV-1) matrix (MA) protein (R. Peytavi et al., J. Biol. Chem. 274:1635-1645, 1999). In the present study, EED was found to interact with HIV-1 integrase (IN) both in vitro and in vivo in yeast. In vitro, data from mutagenesis studies, pull-down assays, and phage biopanning suggested that EED-binding site(s) are located in the C-terminal domain of IN, between residues 212 and 264. In EED, two putative discrete IN-binding sites were mapped to its N-terminal moiety, at a distance from the MA-binding site, but EED-IN interaction also required the integrity of the EED last two WD repeats. EED showed an apparent positive effect on IN-mediated DNA integration reaction in vitro, in a dose-dependent manner. In situ analysis by immunoelectron microscopy (IEM) of cellular distribution of IN and EED in HIV-1-infected cells (HeLa CD4(+) cells or MT4 lymphoid cells) showed that IN and EED colocalized in the nucleus and near nuclear pores, with maximum colocalization events occurring at 6 h postinfection (p.i.). Triple colocalizations of IN, EED, and MA were also observed in the nucleoplasm of infected cells at 6 h p.i., suggesting the ocurrence of multiprotein complexes involving these three proteins at early steps of the HIV-1 virus life cycle. Such IEM patterns were not observed with a noninfectious, envelope deletion mutant of HIV-1.

Polycomb group genes as epigenetic regulators of normal and leukemic hemopoiesis

Epigenetic modification of chromatin structure underlies the differentiation of pluripotent hemopoietic stem cells (HSCs) into their committed/differentiated progeny. Compelling evidence indicates that Polycomb group (PcG) genes play a key role in normal and leukemic hemopoiesis through epigenetic regulation of HSC self-renewal/proliferation and commitment. The PcG proteins are constituents of evolutionary highly conserved molecular pathways regulating cell fate in several other tissues through diverse mechanisms, including 1) regulation of self-renewal/proliferation, 2) regulation of senescence/immortalization, 3) interaction with the initiation transcription machinery, 4) interaction with chromatin-condensation proteins, 5) modification of histones, 6) inactivation of paternal X chromosome, and 7) regulation of cell death. It is therefore not surprising that PcG genes lead to pleiotropic phenotypes when mutated and have been associated with malignancies in several systems in both mice and humans. Although much remains to be learned regarding the PcG mechanism(s) of action, advances in identifying the functional domains and enzymatic activities of these multimeric protein complexes have provided insights into how PcG proteins accomplish such processes. Some of the new insights into a role for the PcG cellular memory system in regulating normal and leukemic hemopoiesis are reviewed here, with special emphasis on their potential involvement in epigenetic regulation of gene expression through modification of chromatin structure.

Memory by modification: the influence of chromatin structure on gene expression during vertebrate development

Multicellular development is programmed by regulated interactions between transcription factors and target genes. Target genes function as nucleosomal arrays whose higher order structure, composition and accessibility to transcription machinery are strictly and dynamically controlled. Several classes of chromatin-associated proteins generate or remove localized, covalent chromatin modifications that signify gene expression status, whereas others modulate nucleosome organization and so regulate template availability for transcription. In vertebrates, covalent modification of the DNA template itself also has dramatic impacts on gene expression and development. Here I review recent discoveries that improve our understanding of the influence of chromatin structure on gene expression and I discuss their relevance to mechanisms of vertebrate development.