The end adjusts the means: Heterochromatin remodelling during terminal cell differentiation

All cells that constitute mature tissues in an eukaryotic organism undergo a multistep process of cell differentiation. At the terminal stage of this process, cells either cease to proliferate forever or rest for a very long period of time. During terminal differentiation, most of the genes that are required for cell ‘housekeeping’ functions, such as proto-oncogenes and other cell-cycle and cell proliferation genes, become stably repressed. At the same time, nuclear chromatin undergoes dramatic morphological and structural changes at the higher-order levels of chromatin organization. These changes involve both constitutively inactive chromosomal regions (constitutive heterochromatin) and the formerly active genes that become silenced and structurally modified to form facultative heterochromatin. Here we approach terminal cell differentiation as a unique system that allows us to combine biochemical, ultrastructural and molecular genetic techniques to study the relationship between the hierarchy of chromatin higher-order structures in the nucleus and its function(s) in dynamic packing of genetic material in a form that remains amenable to regulation of gene activity and other DNA-dependent cellular processes.     

Engineered human dicentric chromosomes show centromere plasticity

The centromere is essential for the faithful distribution of a cell's genetic material to subsequent generations. Despite intense scrutiny, the precise genetic and epigenetic basis for centromere function is still unknown. Here, we have used engineered dicentric human chromosomes to investigate mammalian centromere structure and function. We describe three classes of dicentric chromosomes isolated in different cell lines: functionally monocentric chromosomes, in which one of the two genetically identical centromeres is consistently inactivated; functionally dicentric chromosomes, in which both centromeres are consistently active; and dicentric chromosomes heterogeneous with respect to centromere activity. A study of serial single cell clones from heterogeneous cell lines revealed that while centromere activity is usually clonal, the centromere state (i.e. functionally monocentric or dicentric) in some lines can switch within a growing population of cells. Because pulsed field gel analysis indicated that the DNA at the centromeres of these chromosomes did not change detectably, this switching of the centromere state is most likely due to epigenetic changes. Inactivation of one of the two active centromeres in a functionally dicentric chromosome was observed in a percentage of cells after treatment with Trichostatin A, an inhibitor of histone deacetylation. This study provides evidence that the activity of human centromeres, while largely stable, can be subject to dynamic change, most likely due to epigenetic modification.     

The kinase haspin is required for mitotic histone H3 Thr 3 phosphorylation and normal metaphase chromosome alignment

Post-translational modifications of conserved N-terminal tail residues in histones regulate many aspects of chromosome activity. Thr 3 of histone H3 is highly conserved, but the significance of its phosphorylation is unclear, and the identity of the corresponding kinase unknown. Immunostaining with phospho-specific antibodies in mammalian cells reveals mitotic phosphorylation of H3 Thr 3 in prophase and its dephosphorylation during anaphase. Furthermore we find that haspin, a member of a distinctive group of protein kinases present in diverse eukaryotes, phosphorylates H3 at Thr 3 in vitro. Importantly, depletion of haspin by RNA interference reveals that this kinase is required for H3 Thr 3 phosphorylation in mitotic cells. In addition to its chromosomal association, haspin is found at the centrosomes and spindle during mitosis. Haspin RNA interference causes misalignment of metaphase chromosomes, and overexpression delays progression through early mitosis. This work reveals a new kinase involved in composing the histone code and adds haspin to the select group of kinases that integrate regulation of chromosome and spindle function during mitosis and meiosis.     

New Zealand white rabbits immunized with RNA-complexed total histones develop an autoimmune-like response.

The antibody response of rabbits immunized with a total histone mixture containing randomly coiled H1/H5, H2A, H2B, H3 and H4 devoid of DNA was investigated in direct and competitive ELISA. The antisera were tested with isolated histones and chromatin and with a series of overlapping synthetic peptides covering the entire sequences of the four core histones and two peptides of H1. It was found that the New Zealand (NZ) white rabbits immunized with the total histone (TH) mixture complexed with RNA produced IgG antibodies reacting with histones and with a number of histone peptides but not with chromatin. The antisera also contained IgG antibodies which bound components that correspond to common target antigens in autoimmune diseases such as native dsDNA, peptides of Sm-D antigen, ubiquitin, branched peptides of ubiquitinated H2A and poly(ADP-ribose). By competition experiments, it was shown that these antibodies corresponded to non-crossreacting antibody populations. New Zealand rabbits immunized with TH in the absence of RNA or random outbred rabbits immunized with the RNA-complexed histone fraction produced antibodies reacting with histone, chromatin and very few histone peptides, while no activity with non-related antigens was observed. The pattern of reactivity of antisera raised in NZ rabbits with RNA-complexed TH was found to be very similar to that observed in sera of patients with systemic lupus erythematosus while, in contrast, the antibody response was very different in NZ or outbred rabbits immunized with various native nuclear particles and with individual histones. Altered nucleosome particles rather than native nucleosomes may represent the antigenic stimulus giving rise to autoantibodies.     

Mucosal bromodomain-containing protein 4 mediates aeroallergen-induced inflammation and remodeling

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Effects of aging on rat cortical presynaptic cholinergic processes

We studied the effects of aging on the [³H]-choline uptake, acetylation, [³H]-ACh release and muscarinic modulation of [³H]-ACh release in cortical synaptosomes prepared from Fischer 344 male rats. Our results indicate that 6 and 24 month old rats take up and acetylate [³H]-choline to a similar extent, but that the older animals release significantly less [³H]-ACh in response to K⁺-depolarization than the young adults do. This difference in K⁺-induced release is not due to a difference in presynaptic muscarinic receptor inhibitory activity since the older animals appear to be, if anything, slightly less sensitive to oxotremorine than the younger animals are. Atropine (1 μM) had no effect on ACh-release but blocked oxotremorine-induced modulation. Our results suggest that acetylcholine release is decreased in synaptosomes prepared from old rats although the presynaptic muscarinic regulation of release is functional. Thus, muscarinic receptor-mediated release-modulation is a potential site for pharmacologically altering ACh release.     

Chromatin interactions in differentiating keratinocytes reveal novel atopic dermatitis– and psoriasis-associated genes

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Nuclear organization of centromeric domains is not perturbed by inhibition of histone deacetylases

It is well established that modification of lysines in histone molecules correlates with gene expression and chromatin structure. It is not known whether this operates entirely at a local level, e.g. through the recruitment of specific proteins, or whether histone modifications might impact on more long-range aspects of chromatin organization. There is a distinctive organization of chromatin within the nucleus and the chromatin at the nuclear periphery of mammalian cells appears to be hypoacetylated. Previously it had been suggested that inhibition of histone deacetylases by TSA causes a gross remodeling of nuclear structure, specifically the recruitment of centromeric heterochromatin to the nuclear periphery. Here, we have quantified the nuclear organization of histone modifications and the localization of centromeric domains in human cells before and after TSA treatment. TSA alters the nuclear distribution of histone acetylation, but not that of histone methylation. TSA elevates levels of histone acetylation at the nuclear periphery but we see no alteration in the position of centromeric domains in the nuclei of treated cells. We conclude that the distinctive nuclear localization of centromeric domains is independent of histone acetylation.     

H3K9Ac在卵巢上皮性肿瘤中的表达及其临床意义

目的 研究组蛋白H3赖氨酸残基9位乙酰化（H3KgAc）在卵巢上皮性肿瘤中的表达及其与卵巢癌组织学分级、临床分期之间的关系。方法 采用免疫组织化学ABC法检测20例良性、16例交界性、40例恶性卵巢上皮性肿瘤中H3K9Ac的表达情况，并分析其与临床病理指标间的关系。结果 ①H3K9Ac在良性、交界性、恶性卵巢上皮性肿瘤中的表达逐渐降低，差异有显著统计学意义（P〈0．01）；②与粘液性囊腺癌相比，H3KgAc在浆液性囊腺癌中表达较低，差异有统计学意义（P〈0．05）；③H3KgAc的表达与卵巢上皮性癌的组织分化程度及临床分期有关，在Ⅲ、Ⅳ期卵巢癌中H3KgAc的表达明显低于Ⅰ、Ⅱ期卵巢癌（P〈0．01）；随着组织分化程度降低，H3KgAc表达也逐渐降低，两两比较差异有统计学意义（P〈0．05）。结论 H3KgAc在良性、交界性、恶性卵巢上皮性肿瘤中的表达差异显著，且随着卵巢上皮性癌恶性程度的增高，表达逐渐降低，H3K9Ac可能为卵巢上皮性肿瘤的良恶性及其预后判断提供一个新的生物学指标。