The unusual histone variant macroH2A (mH2A) has been associated with repression of transcription, but the molecular mechanisms by which it exerts this function are unknown. Here we have identified a mechanism by which the different domains of mH2A may be involved in the repression of transcription. Evidence is presented that the presence of mH2A in a positioned nucleosome interferes with the binding of the transcription factor NF-kappaB. The nonhistone region of mH2A was identified to be associated with this interference. Importantly, the presence of macroH2A was found to severely impede SWI/SNF nucleosome remodeling and movement to neighboring DNA segments. This property of mH2A was demonstrated to reside only in its H2A-like domain. A hypothesis explaining the role of histone variants in transcriptional regulation is proposed.

The histone H2ABbd is a novel histone variant of H2A with a totally unknown function. We have investigated the behaviour of the H2ABbd nucleosomes. Nucleosomes were reconstituted with recombinant histone H2ABbd and changes in their conformations at different salt concentrations were studied by analytical centrifugation. The data are in agreement with H2ABbd being less tightly bound compared with conventional H2A in the nucleosome. In addition, stable cell lines expressing either green fluorescent protein (GFP)-H2A or GFP-H2ABbd were established and the mobility of both fusions was measured by fluorescence recovery after photobleaching. We show that GFP-H2ABbd exchanges much more rapidly than GFP-H2A within the nucleosome. The reported data are compatible with a lower stability of the variant H2ABbd nucleosome compared with the conventional H2A particle.

Aurora kinases are serine/threonine protein kinases that are involved in cancer development and are important targets for cancer therapy. By high throughput screening of a chemical library we found that benzo[e]pyridoindole derivatives inhibited Aurora kinases. The most potent compound (compound 1) was found to be an ATP competitive inhibitor, which inhibited in vitro Aurora kinases at the nanomolar range. It prevented, ex vivo, the phosphorylation of Histone H3, induced mitosis exit without chromosome segregation, known phenomena observed upon Aurora B inactivation. This compound was also shown to affect the localization of Aurora B, since in the presence of the inhibitor the enzyme was delocalized on the whole chromosomes and remained associated with the chromatin of newly formed nuclei.In addition, compound 1 inhibited the growth of different cell lines derived from different carcinoma. Its IC 50 for H358 NSCLC (Non-Small Cancer Lung Cells), the most sensitive cell line, was 145 nM. Furthermore compound 1 was found to be efficient towards multicellular tumor spheroid growth. It exhibited minimal toxicity in mice while it had some potency towards aggressive NSCLC tumors. Benzo[e]pyridoindoles represent thus a potential new lead for the development of Aurora kinase inhibitors.

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We have studied the dynamics of Aurora B and Survivin during mitosis in living cells, using C-terminal GFP chimeras of the two proteins. These chimeras showed identical localization and behave as bona fide wild type proteins. The mobility of Aurora B-GFP and Survivin-GFP was analyzed by FRAP. The data show that Survivin-GFP, in contrast to Aurora B-GFP, is highly mobile at prometaphase and metaphase. At telophase and cell cleavage, both chimeras are found to be fully immobile. The ablation of Aurora B by siRNA results in a dramatic decrease of the Survivin-GFP mobility. These results demonstrate that Survivin, but not Aurora B, is weakly associated with the centromeric chromatin at prometaphase and metaphase. The weak association of Survivin with centromeric chromatin is dependent on the presence of Aurora B and is not affected by treatment with either nocodazole or taxol. The rapid and conditional interchange between passenger proteins that we show by live imaging indicates that the high affinity interactions demonstrated with in vitro analysis of passenger protein binding are, in fact, static "snapshots" of highly dynamic and regulated in vivo interactions in mitotic cells.

We studied the enrichment and distribution of the histone variant mH2A1 in the condensed inactive X (Xi) chromosome. By using highly specific antibodies against mH2A1 and stable HEK 293 cell lines expressing either green fluorescent protein (GFP)-mH2A1 or GFP-H2A, we found that the Xi chromosome contains ϳ1.5-fold more mH2A1 than the autosomes. To determine the in vivo distribution of mH2A1 along the X chromosome, we used a native chromatin immunoprecipitation-on-chip technique. DNA isolated from mH2A1-immunoprecipitated nucleosomes from either male or female mouse liver were hybridized to tiling microarrays covering 5 kb around most promoters or the entire X chromosome. The data show that mH2A1 is uniformly distributed across the entire Xi chromosome. Interestingly, a stronger mH2A1 enrichment along the pseudoautosomal X chromosome region was observed in both sexes. Our results indicate a potential role for macroH2A in large-scale chromosome structure and genome stability.

The chromosomal protein passenger complex, a key mitotic regulator, consists of at least four proteins, INCENP, Aurora B, Survivin and Borealin. Survivin, in contrast to the other members of the chromosomal protein passenger complex (CPC), is mobile at metaphase. This protein is also phosphorylated by Aurora B at Threonine 117. In this work we have studied the role of the phosphorylation of Survivin in mitosis by using non phosphorylable T117A and phosphomimic T117E silent resistant Survivin mutants, inducible cell lines expressing these mutants and a combination of siRNA, time-lapse microscopy and FRAP analysis. Time lapse microscopy and FRAP analysis show that Survivin T117A mutant is very stably associated with centromeres and its expression induces a prometaphasic arrest in endogenous survivin depleted cells. In addition, Survivin T117A was unable to rescue the phenotypes of the endogenous survivin depleted cells. Expressed in these cells, the phosphomimic Survivin T117E mutant exhibits a very weak interaction with the centromeres and behaves as a dominant negative mutant inducing severe mitotic defects. Our data suggest that the Aurora B generated phosphorylation/dephosphorylation cycle of Survivin is required for proper proceeding of mitosis.

Summary. The role of adhesive interactions with the extracellular matrix components of the bone marrow (BM) stroma has been widely studied in the differentiation of erythroid and myelomonocytic cells, but not in the megakaryocytic lineage. The development of efficient culture techniques for the production of megakaryocytes (Mks) from CD34 + purified BM cells, enables the study of the expression and function of adhesion receptors for collagen (VLA-2), fibronectin (VLA-4 and VLA-5) and laminin (VLA-6) during the maturation of Mks. We have shown that a significant percentage of CFU-MK (roughly 20%) adhere to fibronectin but not to collagen and laminin. The expression and adhesion of Mks developing in liquid culture from BM-CD34 + cells were tested at days 4, 7 and 10 of incubation. The expression of VLA-2, VLA-5 and VLA-6 on day 10 cultured Mks enabled purification of intermediate and large polyploid Mks by FACS sorting whereas VLA-4 appeared to label only immature Mks and myeloid cells. We observed that only a small proportion of mature Mks was able to adhere to collagen without spreading at day 10 of culture, whereas 30% of Mks adhered to fibronectin as early as day 4 of incubation, 40% of which also attached to laminin. Our data suggest that VLA-4 may be involved in the adhesion of CFU-MK and immature Mks on fibronectin, then replaced by VLA-5 in the final stages of maturation. The expression of VLA-6 and the number of adherent Mks on laminin increased sharply between day 7 and 10 of incubation. A number of mature polyploid Mks found in day 10 of culture exhibited characteristic features of intense spreading on laminin and fibronectin which were not observed on collagen.