Mi-2 chromatin remodeling factor functions in sensory organ development through proneural gene repression in Drosophila

Mi-2, the central component of the nucleosome remodeling and histone deacetylation (NuRD) complex, is known as an SNF2-type ATP-dependent nucleosome remodeling factor. No morphological mutant phenotype of Drosophila Mi-2 (dMi-2) had been reported previously; however, we found that rare escapers develop into adult flies showing an extra bristle phenotype. The dMi-2 enhanced the phenotype of ac(Hw49c), which is a dominant gain-of-function allele of achaete (ac) and produces extra bristles. Consistent with these observations, the ac-expressing proneural clusters were expanded, and extra sensory organ precursors (SOP) were formed in the dMi-2 mutant wing discs. Immunostaining of polytene chromosomes showed that dMi-2 binds to the ac locus, and dMi-2 and acetylated hisotones distribute on polytene chromosomes in a mutually exclusive manner. The chromatin immunoprecipitation assay of the wing imaginal disc also demonstrated a binding of dMi-2 on the ac locus. These results suggest that the Drosophila Mi-2/NuRD complex functions in neuronal differentiation through the repression of proneural gene expression by chromatin remodeling and histone deacetylation.     

PICKLE is a CHD subfamily II ATP-dependent chromatin remodeling factor

PICKLE plays a critical role in repression of genes that regulate development identity in Arabidopsis thaliana. PICKLE codes for a putative ATP-dependent chromatin remodeler that exhibits sequence similarity to members of subfamily II of animal CHD remodelers, which includes remodelers such as CHD3/Mi-2 that also restrict expression of developmental regulators. Whereas animal CHD3 remodelers are a component of the Mi-2/NuRD complex that promotes histone deacetylation, PICKLE promotes trimethylation of histone H3 lysine 27 suggesting that it acts via a distinct epigenetic pathway. Here, we examine whether PICKLE is also a member of a multisubunit complex and characterize the biochemical properties of recombinant PICKLE protein. Phylogenetic analysis indicates that PICKLE-related proteins in plants share a common ancestor with members of subfamily II of animal CHD remodelers. Biochemical characterization of PICKLE in planta, however, reveals that PICKLE primarily exists as a monomer. Recombinant PICKLE protein is an ATPase that is stimulated by ssDNA and mononucleosomes and binds to both naked DNA and mononucleosomes. Furthermore, recombinant PICKLE exhibits ATP-dependent chromatin remodeling activity. These studies demonstrate that subfamily II CHD proteins in plants, such as PICKLE, retain ATP-dependent chromatin remodeling activity but act through a mechanism that does not involve the ubiquitous Mi-2/NuRD complex.     

Open, repair and close again: chromatin dynamics and the response to UV-induced DNA damage

Due to its link with human pathologies, including cancer, the mechanism of Nucleotide Excision Repair (NER) has been extensively studied. Most of the pathway and players have been defined using in vitro reconstitution experiments. However, in vivo, the NER machinery must deal with the presence of organized chromatin, which in some regions, such as heterochromatin, is highly condensed but still susceptible to DNA damage. A series of events involving different chromatin-remodeling factors and histone-modifying enzymes target chromatin regions that contain DNA lesions. CPDs change the structure of the nucleosome, allowing access to factors that can recognize the lesion. Next, DDB1-DDB2 protein complexes, which mono-ubiquitinate histones H2A, H3, and H4, recognize nucleosomes containing DNA lesions. The ubiquitinated nucleosome facilitates the recruitment of ATP-dependent chromatin-remodeling factors and the XPC-HR23B-Centrin 2 complex to the target region. Different ATP-dependent chromatin-remodeling factors, such as SWI/SNF and INO80, have been identified as having roles in the UV irradiation response prior to the action of the NER machinery. Subsequently, remodeling of the nucleosome allows enzymatic reactions by histone-modifying factors that may acetylate, methylate or demethylate specific histone residues. Intriguingly, some of these histone modifications are dependent on p53. These histone modifications and the remodeling of the nucleosome allow the entrance of TFIIH, XPC and other NER factors that remove the damaged strand; then, gap-filling DNA synthesis and ligation reactions are carried out after excision of the oligonucleotide with the lesion. Finally, after DNA repair, the initial chromatin structure has to be reestablished. Therefore, factors that modulate chromatin dynamics contribute to the NER mechanism, and they are significant in the future design of treatments for human pathologies related to genome instability and the appearance of drug-resistant tumors.     

Molecular association between ATR and two components of the nucleosome remodeling and deacetylating complex, HDAC2 and CHD4.

Ataxia telangiectasia mutated (ATM)- and Rad3-related protein (ATR) is a phosphatidylinositol-kinase (PIK)-related kinase that has been implicated in the response of human cells to multiple forms of DNA damage and may play a role in the DNA replication checkpoint. The purification of an ATR complex allowed identification of chromodomain-helicase-DNA-binding protein 4 (CHD4) as an ATR-associated protein by tandem mass spectrometric sequencing. CHD4 (also called Mi-2beta) is a component of a histone-deacetylase-2 (HDAC2)-containing complex, the nucleosome remodeling and deacetylating (NRD) complex. Endogenous ATR, CHD4, and HDAC2 are shown to coimmunoprecipitate, and ATR and HDAC2 coelute through two biochemical purification steps. Other members of the NRD complex, HDAC1, MTA1, and MTA2, are also detectable in ATR immunoprecipitates. ATR's association with CHD4 and HDAC2 suggests that there may be a linkage between ATR's role in mediating checkpoints induced by DNA damage and chromatin modulation via remodeling and deacetylation.     

Cell cycle regulation of chromatin at an origin of DNA replication

Selection and licensing of mammalian DNA replication origins may be regulated by epigenetic changes in chromatin structure. The Epstein-Barr virus (EBV) origin of plasmid replication (OriP) uses the cellular licensing machinery to regulate replication during latent infection of human cells. We found that the minimal replicator sequence of OriP, referred to as the dyad symmetry (DS), is flanked by nucleosomes. These nucleosomes were subject to cell cycle-dependent chromatin remodeling and histone modifications. Restriction enzyme accessibility assay indicated that the DS-bounded nucleosomes were remodeled in late G1. Remarkably, histone H3 acetylation of DS-bounded nucleosomes decreased during late G1, coinciding with nucleosome remodeling and MCM3 loading, and preceding the onset of DNA replication. The ATP-dependent chromatin-remodeling factor SNF2h was also recruited to DS in late G1, and formed a stable complex with HDAC2 at DS. siRNA depletion of SNF2h reduced G1-specific nucleosome remodeling, histone deacetylation, and MCM3 loading at DS. We conclude that an SNF2h-HDAC1/2 complex coordinates G1-specific chromatin remodeling and histone deacetylation with the DNA replication initiation process at OriP.     

The Mi-2/NuRD complex associates with pericentromeric heterochromatin during S phase in rapidly proliferating lymphoid cells

Chromosomal replication results in the duplication not only of DNA sequence but also of the patterns of histone modification, DNA methylation, and nucleoprotein structure that constitute epigenetic information. Pericentromeric heterochromatin in human cells is characterized by unique patterns of histone and DNA modification. Here, we describe association of the Mi-2/NuRD complex with specific segments of pericentromeric heterochromatin consisting of Satellite II/III DNA located on human chromosomes 1, 9, and 16 in some but not all cell types. This association is linked in part to DNA replication and chromatin assembly and may suggest a role in these processes. Mi-2/NuRD accumulation is independent of Polycomb association and is characterized by a unique pattern of histone modification. We propose that Mi-2/NuRD constitutes an enzymatic component of a pathway for assembly and maturation of chromatin utilized by rapidly proliferating lymphoid cells for replication of constitutive heterochromatin. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.