Article

Meis Cofactors Control HDAC and CBP Accessibility at Hox-Regulated Promoters during Zebrafish Embryogenesis

Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, 364 Plantation Street, LRB822, Worcester, MA 01605, USA.
Developmental Cell (Impact Factor: 10.37). 10/2009; 17(4):561-7. DOI: 10.1016/j.devcel.2009.08.007
Source: PubMed

ABSTRACT Hox proteins form complexes with Pbx and Meis cofactors to control gene expression, but the role of Meis is unclear. We demonstrate that Hoxb1-regulated promoters are highly acetylated on histone H4 (AcH4) and occupied by Hoxb1, Pbx, and Meis in zebrafish tissues where these promoters are active. Inhibition of Meis blocks gene expression and reduces AcH4 levels at these promoters, suggesting a role for Meis in maintaining AcH4. Within Hox transcription complexes, Meis binds directly to Pbx and we find that this binding displaces histone deacetylases (HDACs) from Hoxb1-regulated promoters in zebrafish embryos. Accordingly, Pbx mutants that cannot bind Meis act as repressors by recruiting HDACs and reducing AcH4 levels, while Pbx mutants that bind neither HDAC nor Meis are constitutively active and recruit CBP to increase AcH4 levels. We conclude that Meis acts, at least in part, by controlling access of HDAC and CBP to Hox-regulated promoters.

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    • "These findings also raise the question as to how changes in subunit composition can alter complex function . In the case of the Hox:PBC complexes discussed here, the functional switch is caused by Meis proteins competing with HDACs for binding to the Pbx N-terminus (Choe et al., 2009). In the absence of Meis cofactors , HDAC is bound to Pbx and the complex is repressive, but in the presence of Meis proteins, Meis associates with Pbx and HDAC is displaced. "
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    ABSTRACT: Hox genes encode transcription factors with important roles during embryogenesis and tissue differentiation. Genetic analyses initially demonstrated that interfering with Hox genes has profound effects on the specification of cell identity, suggesting that Hox proteins regulate very specific sets of target genes. However, subsequent biochemical analyses revealed that Hox proteins bind DNA with relatively low affinity and specificity. Furthermore, it became clear that a given Hox protein could activate or repress transcription depending on the context. A resolution to these paradoxes presented itself with the discovery that Hox proteins do not function in isolation, but interact with other factors in complexes. The first such "cofactors" were members of the Extradenticle/Pbx and Homothorax/Meis/Prep families. However, the list of Hox-interacting proteins has continued to grow, suggesting that Hox complexes contain many more components than initially thought. Additionally, the activities of the various components and the exact mechanisms whereby they modulate the activity of the complex remain puzzling. Here we review the various proteins known to participate in Hox complexes and discuss their likely functions. We also consider that Hox complexes of different compositions may have different activities and discuss mechanisms whereby Hox complexes may be switched between active and inactive states. Developmental Dynamics, 2013. © 2013 Wiley Periodicals, Inc.
    Developmental Dynamics 01/2014; 243(1). DOI:10.1002/dvdy.23997 · 2.67 Impact Factor
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    • "Pbx proteins have been shown to inhibit gene expression through interactions with transcriptional repressors (Saleh et al., 2000; Berghella et al., 2008; Choe et al., 2009), but activating gene expression by repressing a transcriptional repressor may entail a new mechanism for Pbx activity. Pbx proteins could take an active role in inhibiting Prdm1a, while promoting gene expression, by modifying chromatin states at fast muscle gene promoters through Pbx interactions with chromatin factors such as Brg1 or CBP (de la Serna et al., 2005; Choe et al., 2009). Other fast muscle genes, such as srl, are Prdm1a-independent but require Pbx for their activation. "
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    ABSTRACT: The basic helix-loop-helix factor Myod initiates skeletal muscle differentiation by directly and sequentially activating sets of muscle differentiation genes, including those encoding muscle contractile proteins. We hypothesize that Pbx homeodomain proteins direct Myod to a subset of its transcriptional targets, in particular fast-twitch muscle differentiation genes, thereby regulating the competence of muscle precursor cells to differentiate. We have previously shown that Pbx proteins bind with Myod on the promoter of the zebrafish fast muscle gene mylpfa and that Pbx proteins are required for Myod to activate mylpfa expression and the fast-twitch muscle-specific differentiation program in zebrafish embryos. Here we have investigated the interactions of Pbx with another muscle fiber-type regulator, Prdm1a, a SET-domain DNA-binding factor that directly represses mylpfa expression and fast muscle differentiation. The prdm1a mutant phenotype, early and increased fast muscle differentiation, is the opposite of the Pbx-null phenotype, delayed and reduced fast muscle differentiation. To determine whether Pbx and Prdm1a have opposing activities on a common set of genes, we used RNA-seq analysis to globally assess gene expression in zebrafish embryos with single- and double-losses-of-function for Pbx and Prdm1a. We find that the levels of expression of certain fast muscle genes are increased or approximately wild type in pbx2/4-MO;prdm1a-/- embryos, suggesting that Pbx activity normally counters the repressive action of Prdm1a for a subset of the fast muscle program. However, other fast muscle genes require Pbx but are not regulated by Prdm1a. Thus, our findings reveal that subsets of the fast muscle program are differentially regulated by Pbx and Prdm1a. Our findings provide an example of how Pbx homeodomain proteins act in a balance with other transcription factors to regulate subsets of a cellular differentiation program.
    06/2013; 2(6):546-555. DOI:10.1242/bio.20133921
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    • "The 1.0kbp fragment was placed upstream of a 53bp sequence encoding the basal promoter of human β-globin followed by the eGFP gene and used to generate stable transgenic lines (Fig. 1B, see methods section for details). The existence of this line has been reported previously (Choe et al., 2009), but it has not been described in detail. Analysis of the hoxb1a(β-globin):eGFP um8 transgenic line demonstrates detectable eGFP mRNA in the zebrafish hindbrain at 10hpf (Fig. 2D). "
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    ABSTRACT: The zebrafish is well established as a model organism for the study of vertebrate embryogenesis, but transgenic lines enabling restricted gene expression are still lacking for many tissues. We first generated the hoxb1a(β-globin):eGFP(um8) line that expresses eGFP in hindbrain rhombomere 4 (r4), as well as in facial motor neurons migrating caudally from r4. Second, we generated the hoxb1a(β-globin) Gal4VP16(um60) line to express the exogenous Gal4VP16 transcription factor in r4. Lastly, we prepared the UAS(β-actin):hoxa3a(um61) line where the hoxa3a gene, which is normally expressed in r5 and r6, is under control of Gal4-regulated UAS elements. Crossing the hoxb1a(β-globin):Gal4VP16(um60) line to the UAS(β-actin):hoxa3a(um61) line drives robust hoxa3a expression in r4. We find that transgenic expression of hoxa3a in r4 does not affect hoxb1a expression, but has variable effects on migration of facial motorneurons and formation of Mauthner neurons. While cases of somatic transgene silencing have been reported in zebrafish, we have not observed such silencing to date, possibly because of our efforts to minimize repetitive sequences in the transgenic constructs. We have generated three transgenic lines that will be useful for future studies by permitting the labeling of r4-derived cells, as well as by enabling r4-specific expression of various transgenes.
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