Histone acetylation has been recognized as an important post‐translational modification of core nucleosomal histones that changes access to the chromatin to allow gene transcription, DNA replication, ...and repair. Histone acetyltransferases were initially identified as co‐activators that link DNA‐binding transcription factors to the general transcriptional machinery. Over the years, more chromatin‐binding modes have been discovered suggesting direct interaction of histone acetyltransferases and their protein complex partners with histone proteins. While much progress has been made in characterizing histone acetyltransferase complexes biochemically, cell‐free activity assay results are often at odds with in‐cell histone acetyltransferase activities. In‐cell studies suggest specific histone lysine targets, but broad recruitment modes, apparently not relying on specific DNA sequences, but on chromatin of a specific functional state. Here we review the evidence for general versus specific roles of individual nuclear lysine acetyltransferases in light of in vivo and in vitro data in the mammalian system.
Recruitment of HATs by DNA‐binding transcription factors suggested specificity for subsets of genes (A). Chromatin‐binding domains within HAT protein complexes indicate recruitment to chromatin of a specific functional state, supported by HAT occupancy of many or most active genes (B). Within cells, individual HATs acetylate (Ac) specific histone lysine residues.
Apoptotic cells are commonly observed in a broad range of tissues during mammalian embryonic and fetal development. Specific requirements and functions of programmed cell death were inferred from ...early observations. These inferences did not hold up to functional proof for a requirement of apoptosis for normal tissue development in all cases. In this review, we summarize how the appraisal of the importance of developmental apoptosis has changed over the years, in particular with detailed functional assessment, such as by using gene-targeted mice lacking essential initiators or mediators of apoptosis. In recent years, the essentials of developmental apoptosis have emerged. We hypothesize that apoptosis is predominantly required to balance cell proliferation. The two interdependent processes-cell proliferation and apoptosis-together more powerfully regulate tissue growth than does each process alone. We proposed that this ensures that tissues and cell populations attain the appropriate size that allows fusion in the body midline and retain the size of cavities once formed. In addition, a limited number of tissues, albeit not all previously proposed, rely on apoptosis for remodeling, chiefly aortic arch remodeling, elimination of supernumerary neurons, removal of vaginal septa, and removal of interdigital webs in the formation of hands and feet.
Intrinsic apoptosis, reliant on BAX and BAK, has been postulated to be fundamental for morphogenesis, but its precise contribution to this process has not been fully explored in mammals. Our ...structural analysis of BOK suggests close resemblance to BAX and BAK structures. Notably, Bok−/−Bax−/−Bak−/− animals exhibited more severe defects and died earlier than Bax−/−Bak−/− mice, implying that BOK has overlapping roles with BAX and BAK during developmental cell death. By analyzing Bok−/−Bax−/−Bak−/− triple-knockout mice whose cells are incapable of undergoing intrinsic apoptosis, we identified tissues that formed well without this process. We provide evidence that necroptosis, pyroptosis, or autophagy does not substantially substitute for the loss of apoptosis. Albeit very rare, unexpected attainment of adult Bok−/−Bax−/−Bak−/− mice suggests that morphogenesis can proceed entirely without apoptosis mediated by these proteins and possibly without cell death in general.
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•BOK, BAX, and BAK have similar structures and overlapping roles during development•BOK/BAX/BAK triple-null mice have more severe defects than BAX/BAK double knockouts•Intrinsic apoptosis is critical for midline fusion and aortic arch formation•A small number of BOK/BAX/BAK triple-null mice develop entirely without apoptosis
Although apoptosis has been observed in many organs and tissues during embryonic development, it is not strictly required for the development of most organs.
Hematopoietic stem cells (HSCs) are conventionally thought to be at the apex of a hierarchy that produces all mature cells of the blood. The quintessential property of these cells is their ability to ...reconstitute the entire hematopoietic system of hemoablated recipients. This characteristic has enabled HSCs to be used to replenish the hematopoietic system of patients after chemotherapy or radiotherapy. Here, we use deletion of the monocytic leukemia zinc finger gene (Moz/Kat6a/Myst3) to examine the effects of removing HSCs. Loss of MOZ in adult mice leads to the rapid loss of HSCs as defined by transplantation. This is accompanied by a reduction of the LSK-CD48–CD150+ and LSK-CD34–Flt3– populations in the bone marrow and a reduction in quiescent cells in G0. Surprisingly, the loss of classically defined HSCs did not affect mouse viability, and there was no recovery of the LSK-CD48–CD150+ and LSK-CD34–Flt3– populations 15 to 18 months after Moz deletion. Clonal analysis of myeloid progenitors, which produce short-lived granulocytes, demonstrate that these are derived from cells that had undergone recombination at the Moz locus up to 2 years earlier, suggesting that early progenitors have acquired extended self-renewal. Our results establish that there are essential differences in HSC requirement for steady-state blood cell production compared with the artificial situation of reconstitution after transplantation into a hemoablated host. A better understanding of steady-state hematopoiesis may facilitate the development of novel therapies engaging hematopoietic cell populations with previously unrecognized traits, as well as characterizing potential vulnerability to oncogenic transformation.
•MOZ deletion in adult mice leads to a rapid loss of cells with HSC cell surface immuno-phenotype and transplantation ability.•Absence of classically defined HSCs for an extended period does not substantially affect steady-state hematopoiesis.
The intrinsic apoptosis pathway, regulated by the BCL‐2 protein family, is essential for embryonic development. Using mice lacking all known apoptosis effectors, BAX, BAK and BOK, we have previously ...defined the processes during development that require apoptosis. Rare Bok−/−Bax−/−Bak−/− triple knockout (TKO) mice developed to adulthood and several tissues that were thought to require apoptosis during development appeared normal. This raises the question if all apoptosis had been abolished in the TKO mice or if other BCL‐2 family members could act as effectors of apoptosis. Here, we investigated the role of BID, generally considered to link the extrinsic and intrinsic apoptosis pathways, acting as a BH3‐only protein initiating apoptosis upstream of BAX and BAK. We found that Bok−/−Bax−/− Bak−/−Bid−/− quadruple knockout (QKO) mice have additional developmental anomalies compared to TKO mice, consistent with a role of BID, not only upstream but also in parallel to BAX, BAK and BOK. Mitochondrial experiments identified a small cytochrome c‐releasing activity of full‐length BID. Collectively, these findings suggest a new effector role for BID in the intrinsic apoptosis pathway.
Synopsis
The combined loss of the apoptosis effectors BAX, BAK and BOK abrogates detectable developmental apoptosis, but some tissues known to undergo apoptosis develop normally. Here, loss of BID, previously considered to only act as a BH3‐only protein upstream of BAX and BAK, is found to increase developmental defects in mice also lacking BAX, BAK and BOK, suggesting that it may promote apoptosis in a multi‐BH domain pro‐apoptotic protein‐like manner.
BID deletion in a background of BAX/BAK/BOK knockout (quadruple KO) causes additional birth defects, implying BID action in parallel and not just upstream.
In vitro, full‐length BID can induce some cytochrome c release from mitochondria, suggesting that BID, like BAX and BAK, may form pores in the outer mitochondrial membrane.
The pro‐apoptotic protein BID affects embryonic development independent of its known role as an upstream regulator of Bax, Bak and Bok, likely by directly promoting mitochondrial cytochrome c release.
The histone acetyltransferase HBO1 (MYST2, KAT7) is indispensable for postgastrulation development, histone H3 lysine 14 acetylation (H3K14Ac), and the expression of embryonic patterning genes. In ...this study, we report the role of HBO1 in regulating hematopoietic stem cell function in adult hematopoiesis. We used 2 complementary cre-recombinase transgenes to conditionally delete Hbo1 (Mx1-Cre and Rosa26-CreERT2). Hbo1-null mice became moribund due to hematopoietic failure with pancytopenia in the blood and bone marrow 2 to 6 weeks after Hbo1 deletion. Hbo1-deleted bone marrow cells failed to repopulate hemoablated recipients in competitive transplantation experiments. Hbo1 deletion caused a rapid loss of hematopoietic progenitors. The numbers of lineage-restricted progenitors for the erythroid, myeloid, B-, and T-cell lineages were reduced. Loss of HBO1 resulted in an abnormally high rate of recruitment of quiescent hematopoietic stem cells (HSCs) into the cell cycle. Cycling HSCs produced progenitors at the expense of self-renewal, which led to the exhaustion of the HSC pool. Mechanistically, genes important for HSC functions were downregulated in HSC-enriched cell populations after Hbo1 deletion, including genes essential for HSC quiescence and self-renewal, such as Mpl, Tek(Tie-2), Gfi1b, Egr1, Tal1(Scl), Gata2, Erg, Pbx1, Meis1, and Hox9, as well as genes important for multipotent progenitor cells and lineage-specific progenitor cells, such as Gata1. HBO1 was required for H3K14Ac through the genome and particularly at gene loci required for HSC quiescence and self-renewal. Our data indicate that HBO1 promotes the expression of a transcription factor network essential for HSC maintenance and self-renewal in adult hematopoiesis.
•In the absence of HBO1, H3K14ac is lost, and hematopoietic stem cells differentiate into progenitors at the expense of self-renewal.•HBO1 governs stem cell quiescence and self-renewal by promoting gene transcription, including Gata2, Mpl, Erg, Pbx1, Meis1, and Hoxa9.
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Acetylation of histones is an essential element regulating chromatin structure and transcription. MYST (Moz, Ybf2/Sas3, Sas2, Tip60) proteins form the largest family of histone acetyltransferases and ...are present in all eukaryotes. Surprisingly, until recently this protein family was poorly studied. However, in the last few years there has been a substantial increase in interest in the MYST proteins and a number of key studies have shown that these chromatin modifiers are required for a diverse range of cellular processes, both in health and disease. Translocations affecting MYST histone acetyltransferases can lead to leukemia and solid tumors. Some members of the MYST family are required for the development and self-renewal of stem cell populations; other members are essential for the prevention of inappropriate heterochromatin spreading and for the maintenance of adequate levels of gene expression. In this review we discuss the function of MYST proteins in vivo.
DiGeorge syndrome, caused by a 22q11 microdeletion or mutation of the TBX1 gene, varies in severity greatly, even among monozygotic twins. Epigenetic phenomena have been invoked to explain phenotypic ...differences in individuals of identical genetic composition, although specific chromatin modifications relevant to DiGeorge syndrome are elusive. Here we show that lack of the histone acetyltransferase MOZ (MYST3/KAT6A) phenocopies DiGeorge syndrome, and the MOZ complex occupies the Tbx1 locus, promoting its expression and histone 3 lysine 9 acetylation. Importantly, DiGeorge syndrome-like anomalies are present in mice with homozygous mutation of Moz and in heterozygous Moz mutants when combined with Tbx1 haploinsufficiency or oversupply of retinoic acid. Conversely, a Tbx1 transgene rescues the heart phenotype in Moz mutants. Our data reveal a molecular mechanism for a specific chromatin modification of the Tbx1 locus intersecting with an environmental determinant, modeling variability in DiGeorge syndrome.
► Homozygous mutation of Moz causes DiGeorge syndrome-like anomalies ► MOZ is required for palate, thymus, aortic arch, and cardiac septal development ► MOZ acts through H3K9 acetylation and promotion of transcription at the Tbx1 locus ► Mutation of Moz sensitizes embryonic development to environmental insults
Voss et al. show that loss of the histone acetyltransferase Moz disrupts histone H3K9 acetylation and transcription of Tbx1. A Tbx1 transgene partially rescues DiGeorge syndrome-like phenotypes in Moz mutant mice. Moz mutants offer opportunities to study how environmental insults such as retinoids influence genetic models of disease.
We report that embryos deficient in the histone acetyltransferase Moz (Myst3/Kat6a) show histone H3 lysine 9 (H3K9) hypoacetylation, corresponding H3K9 hypermethylation, and reduced transcription at
...Hox gene loci. Consistent with an observed caudal shift in
Hox gene expression, segment identity is shifted anteriorly, such that Moz-deficient mice show a profound homeotic transformation of the axial skeleton and the nervous system. Intriguingly, histone acetylation defects are relatively specific to H3K9 at
Hox loci, as neither
Hox H3K14 acetylation nor bulk H3K9 acetylation levels throughout the genome are strongly affected; H4K16 acetylation actually increases in the absence of Moz. H3K9 hypoacetylation,
Hox gene repression, and the homeotic transformation caused by lack of Moz are all reversed by treatment with retinoic acid (RA). In conclusion, our data show that Moz regulates H3K9 acetylation at
Hox gene loci and that RA can act independently of Moz to establish specific Hox gene expression boundaries.
Inhibitor of apoptosis (IAP) proteins cIAP1, cIAP2, and XIAP (X‐linked IAP) regulate apoptosis and cytokine receptor signalling, but their overlapping functions make it difficult to distinguish their ...individual roles. To do so, we deleted the genes for IAPs separately and in combination. While lack of any one of the IAPs produced no overt phenotype in mice, deletion of cIap1 with cIap2 or Xiap resulted in mid‐embryonic lethality. In contrast, Xiap−/−cIap2−/− mice were viable. The death of cIap2−/−cIap1−/− double mutants was rescued to birth by deletion of tumour necrosis factor (TNF) receptor 1, but not TNFR2 genes. Remarkably, hemizygosity for receptor‐interacting protein kinase 1 (Ripk1) allowed Xiap−/−cIap1−/− double mutants to survive past birth, and prolonged cIap2−/−cIap1−/− embryonic survival. Similarly, deletion of Ripk3 was able to rescue the mid‐gestation defect of cIap2−/−cIap1−/− embryos, as these embryos survived to E15.5. cIAPs are therefore required during development to limit activity of RIP kinases in the TNF receptor 1 signalling pathway.
The inhibitor of apoptosis proteins cIAP1, cIAP2, and XIAP exert overlapping functions in apoptosis and cytokine signalling. A series of single‐ and double‐knockout mice reveal an essential function of IAP proteins in preventing TNF receptor 1‐induced, RIP kinase 1‐ and 3‐dependent cell death during embryogenesis.
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