Odorants are detected by odorant receptors, which are located on olfactory sensory neurons of the nose. Each olfactory sensory neuron expresses one single odorant receptor gene allele from a large family of odorant receptor genes. To gain insight into the mechanisms underlying this monogenic and monoallelic expression, we examined the 3D nuclear organization of olfactory sensory neurons and determined the positions of homologous odorant receptor gene alleles in relation to different nuclear compartments. Our results show that olfactory neurons exhibit a singular nuclear architecture that is characterized by a large centrally localized constitutive heterochromatin block and by the presence of prominent facultative heterochromatin domains that are localized around this constitutive heterochromatin block. We also found that the two homologous alleles of a given odorant receptor gene are frequently segregated to separate compartments in the nucleus, with one of the alleles localized to the constitutive heterochromatin block and the other one localized to the more plastic facultative heterochromatin, or next to it. Our findings suggest that this nuclear compartmentalization may play a critical role in the expression of odorant receptor genes.H3K27me3 | chromatin | OR gene choice

No abstract

Phosphorylation of histone H1 is intimately related to the cell cycle progression in higher eukaryotes, reaching maximum levels during mitosis. We have previously shown that in the flagellated protozoan Trypanosoma cruzi, which does not condense chromatin during mitosis, histone H1 is phosphorylated at a single cyclin-dependent kinase site. By using an antibody that recognizes specifically the phosphorylated T. cruzi histone H1 site, we have now confirmed that T. cruzi histone H1 is also phosphorylated in a cell cycle-dependent manner. Differently from core histones, the bulk of nonphosphorylated histone H1 in G 1 and S phases of the cell cycle is concentrated in the central regions of the nucleus, which contains the nucleolus and less densely packed chromatin. When cells pass G 2 , histone H1 becomes phosphorylated and starts to diffuse. At the onset of mitosis, histone H1 phosphorylation is maximal and found in the entire nuclear space. As permeabilized parasites preferentially lose phosphorylated histone H1, we conclude that this modification promotes its release from less condensed and nucleolar chromatin after G 2 .The nucleosome, the basic chromatin unit, is assembled by wrapping DNA around an octamer formed by two copies of histone H2A, H2B, H3, and H4 proteins. A fifth histone, called histone H1, packs the chromatin by contacting internucleosomal DNA and the nucleosome particle (50). Histone H1 is formed by a globular domain flanked by a long unstructured C-terminal portion, comprising almost half of the protein, and by a short and also nonstructured N-terminal domain. The C-terminal domain is enriched in basic amino acids that interact with the negative phosphodiester charges of DNA through S/TPKK motifs (26). The globular portion contacts the core histones and the nucleosomal DNA (3), favoring chromatin compaction, which prevents the access of chromatin remodeling factors and therefore restricts transcription and replication (10). Fluorescence recovery after photobleaching experiments have shown that the chromatin residence time of histone H1 is much shorter than that of other histones, suggesting that histone H1 dynamically associates with the nucleosome particles, contributing to several nuclear processes involving chromatin condensation and decondensation (10, 31, 36). However, when histone H1 is absent, chromatin decondenses (20, 47).Histone H1 is phosphorylated at the N-and C-terminal domains in a cell cycle-dependent manner (22). The number of phosphorylated residues is small in G 1 phase and starts to increase when the cell progresses through S and G 2 , reaching a maximal level at mitosis (28). Histone H1 phosphorylation is required for DNA replication (24), and protein kinases that activate replication also promote histone H1 phosphorylation (1). Histone H1 phosphorylation affects the heterochromatin structure (23), DNA repair, chromatin remodeling, apoptosis, and cell aging (29,49). Histone H1 phosphorylation is also involved in chromosome condensation during mitosis. In the absence of histon...

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Acute myeloid leukemia (AML) is a complex hematological disorder characterized by blockage of differentiation and high proliferation rates of myeloid progenitors. Anthracycline and cytarabine-based therapy has remained the standard treatment for AML over the last four decades. Although this treatment strategy has increased survival rates, patients often develop resistance to these drugs. Despite efforts to understand the mechanisms underlying cytarabine resistance, there have been few advances in the field. The present study developed an in vitro AML cell line model resistant to cytarabine (HL-60R), and identified chromosomal aberrations by karyotype evaluation and potential molecular mechanisms underlying chemoresistance. Cytarabine decreased cell viability, as determined by MTT assay, and induced cell death and cell cycle arrest in the parental HL-60 cell line, as revealed by Annexin V/propidium iodide (PI) staining and PI DNA incorporation, respectively, whereas no change was observed in the HL-60R cell line. In addition, the HL-60R cell line exhibited a higher tumorigenic capacity in vivo compared with the parental cell line. Notably, no reduction in tumor volume was detected in mice treated with cytarabine and inoculated with HL-60R cells. In addition, western blotting revealed that the protein expression levels of Bcl-2, X-linked inhibitor of apoptosis protein (XIAP) and c-Myc were upregulated in HL-60R cells compared with those in HL-60 cells, along with predominant nuclear localization of the p50 and p65 subunits of NF-κB in HL-60R cells. Furthermore, the antitumor effect of LQB-118 pterocarpanquinone was investigated; this compound induced apoptosis, a reduction in cell viability and a decrease in XIAP expression in cytarabine-resistant cells. Taken together, these data indicated that acquired cytarabine resistance in AML was a multifactorial process, involving chromosomal aberrations, and differential expression of apoptosis and cell proliferation signaling pathways. Furthermore, LQB-118 could be a potential alternative therapeutic approach to treat cytarabine-resistant leukemia cells.