H1 histones control the epigenetic landscape by local chromatin compaction

H1 linker histones are the most abundant chromatin-binding proteins1. In vitro studies indicate that their association with chromatin determines nucleosome spacing and enables arrays of nucleosomes to fold into more compact chromatin structures. However, the in vivo roles of H1 are poorly understood2. Here we show that the local density of H1 controls the balance of repressive and active chromatin domains by promoting genomic compaction. We generated a conditional triple-H1-knockout mouse strain and depleted H1 in haematopoietic cells. H1 depletion in T cells leads to de-repression of T cell activation genes, a process that mimics normal T cell activation. Comparison of chromatin structure in normal and H1-depleted CD8+ T cells reveals that H1-mediated chromatin compaction occurs primarily in regions of the genome containing higher than average levels of H1: the chromosome conformation capture (Hi-C) B compartment and regions of the Hi-C A compartment marked by PRC2. Reduction of H1 stoichiometry leads to decreased H3K27 methylation, increased H3K36 methylation, B-to-A-compartment shifting and an increase in interaction frequency between compartments. In vitro, H1 promotes PRC2-mediated H3K27 methylation and inhibits NSD2-mediated H3K36 methylation. Mechanistically, H1 mediates these opposite effects by promoting physical compaction of the chromatin substrate. Our results establish H1 as a critical regulator of gene silencing through localized control of chromatin compaction, 3D genome organization and the epigenetic landscape.

DNA damage: a sensible mediator of the differentiation decision in hematopoietic stem cells and in leukemia

In the adult, the source of functionally diverse, mature blood cells are hematopoietic stem cells, a rare population of quiescent cells that reside in the bone marrow niche. Like stem cells in other tissues, hematopoietic stem cells are defined by their ability to self-renew, in order to maintain the stem cell population for the lifetime of the organism, and to differentiate, in order to give rise to the multiple lineages of the hematopoietic system. In recent years, increasing evidence has suggested a role for the accumulation of reactive oxygen species and DNA damage in the decision for hematopoietic stem cells to exit quiescence and to differentiate. In this review, we will examine recent work supporting the idea that detection of cell stressors, such as oxidative and genetic damage, is an important mediator of cell fate decisions in hematopoietic stem cells. We will explore the benefits of such a system in avoiding the development and progression of malignancies, and in avoiding tissue exhaustion and failure. Additionally, we will discuss new work that examines the accumulation of DNA damage and replication stress in aging hematopoietic stem cells and causes us to rethink ideas of genoprotection in the bone marrow niche.

DNA damage response, redox status and hematopoiesis

The ability of hematopoietic stem cells (HSCs) to self-renew and differentiate into progenitors is essential for homeostasis of the hematopoietic system. The longevity of HSCs makes them vulnerable to accumulating DNA damage, which may be leukemogenic or result in senescence and cell death. Additionally, the ability of HSCs to self-renew and differentiate allows DNA damage to spread throughout the hematologic system, leaving the organism vulnerable to disease. In this review we discuss cell fate decisions made in the face of DNA damage and other cellular stresses, and the role of reactive oxygen species in the long-term maintenance of HSCs and their DNA damage response.

Effect of marketing treatments and transportation on feeder pig performance

During four 6-wk trials, 376 university-raised, crossbred feeder pigs were used to determine the effects of fasting (no feed or water) vs providing feed plus water or water alone at the simulated market and a 563-km transport on shrinkage and subsequent health and performance. Providing feed and water or water alone during a 12-h market simulation for Trials 1, 2 and 4 or a 24-h period for Trial 3 did not affect subsequent rate of gain (ADG) or feed efficiency (F/G) compared with pigs fasted during the market stay. However, overall shrinkage (market phase plus transportation) of pigs provided feed and water at the market was less (P less than .05) than for fasted pigs (4.0 vs 8.4% for Trials 1, 2 and 4; 4.5 vs 11.8% for Trial 3). Transporting the fasted or nonfasted feeder pig 563 k did not affect subsequent health or performance. Overall shrinkage of the fasted pigs that were transported 563 km vs those that were not transported during Trial 4 was not different (8.8 vs 9.1%; P greater than .05). In conclusion, one-owner feeder pigs can cope with the stress of fasting and transporting long distances without any extended detrimental effects on subsequent health and performance.

Effect of supplemental potassium in the receiving diet and form of antibiotic on feeder pig performance

In five separate trials 901 feeder pigs (769 purchased and 132 university-raised) were used to determine the effect of level of dietary K (.64 vs 1.00 vs 1.40%), form of neoterramycin (feed-grade vs water-soluble) and a long-lasting oxytetracycline injection on subsequent performance. Purchased pigs fed a 28-d receiving diet with 1.00% K gained faster (P less than .05) than the control pigs fed a .64% K diet (.64 vs .60 kg) during a 1980 summer trial. Feed efficiency was not affected by level of dietary K. Three additional trials conducted during January, July and October of 1981 failed to substantiate this beneficial effect on rate of gain. Feeder pig performance was not different (P greater than .05) when either feed-grade or water-soluble neoterramycin was used as a 14-d receiving treatment at preventative levels. However, in one trial nearly twice as many pigs on the medicated feed diet required additional treatment for sickness compared with pigs receiving water medication (14.5 vs 7.9%; P less than .1). Giving pigs an injection of a long-lasting oxytetracycline (LA-200) either at the market or upon arrival at the finishing facility had no effect on performance; however, the pigs were home-raised rather than purchased; consequently, their health was excellent.