Pbx homeodomain proteins direct Myod activity to promote fast-muscle differentiation

Division of Human Biology, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, USA.
Development (Impact Factor: 6.46). 10/2007; 134(18):3371-82. DOI: 10.1242/dev.003905
Source: PubMed


The basic helix-loop-helix (bHLH) transcription factor Myod directly regulates gene expression throughout the program of skeletal muscle differentiation. It is not known how a Myod-driven myogenic program is modulated to achieve muscle fiber-type-specific gene expression. Pbx homeodomain proteins mark promoters of a subset of Myod target genes, including myogenin (Myog); thus, Pbx proteins might modulate the program of myogenesis driven by Myod. By inhibiting Pbx function in zebrafish embryos, we show that Pbx proteins are required in order for Myod to induce the expression of a subset of muscle genes in the somites. In the absence of Pbx function, expression of myog and of fast-muscle genes is inhibited, whereas slow-muscle gene expression appears normal. By knocking down Pbx or Myod function in combination with another bHLH myogenic factor, Myf5, we show that Pbx is required for Myod to regulate fast-muscle, but not slow-muscle, development. Furthermore, we show that Sonic hedgehog requires Myod in order to induce both fast- and slow-muscle markers but requires Pbx only to induce fast-muscle markers. Our results reveal that Pbx proteins modulate Myod activity to drive fast-muscle gene expression, thus showing that homeodomain proteins can direct bHLH proteins to establish a specific cell-type identity.

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    • "Notably, mH2A1.2 mediated chromatin engagement of Pbx1, a homeodomain transcription factor priming MyoD gene targets for activation (Berkes et al., 2004;Maves et al., 2007). In aggregate, these findings assign a role to mH2A1.2 in conferring enhancer marking and activation via regulation of transcription factors' recruitment and H3K27 acetylation. "
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    ABSTRACT: Histone variants complement and integrate histone post-translational modifications in regulating transcription. The histone variant macroH2A1 (mH2A1) is almost three times the size of its canonical H2A counterpart, due to the presence of an ∼25 kDa evolutionarily conserved non-histone macro domain. Strikingly, mH2A1 can mediate both gene repression and activation. However, the molecular determinants conferring these alternative functions remain elusive. Here, we report that mH2A1.2 is required for the activation of the myogenic gene regulatory network and muscle cell differentiation. H3K27 acetylation at prospective enhancers is exquisitely sensitive to mH2A1.2, indicating a role of mH2A1.2 in imparting enhancer activation. Both H3K27 acetylation and recruitment of the transcription factor Pbx1 at prospective enhancers are regulated by mH2A1.2. Overall, our findings indicate a role of mH2A1.2 in marking regulatory regions for activation.
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    • "Asterisks indicate t-test p-values; * p = 2.2268x10 -12 , ** p = 4.49158x10 -18 , *** p = 5.39191x10 -19 . Regulation of a Drosophila Tubular Muscle Troponin Gene 4, 7, 10, 13, 16, 19), or probe plus nuclear extract (lanes 2, 5, 8, 11, 14, 17, 20), or probe plus nuclear extract plus 100-fold excess of unlabeled probe (lanes 3, 6, 9, these two cases, additional regulatory elements are necessary for full enhancer activity [37, 22], much like we observe here for TpnC41C. In other instances, multiple MEF2 sites have been identified within regulatory elements (see for example [38]). "
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    ABSTRACT: Most animals express multiple isoforms of structural muscle proteins to produce tissues with different physiological properties. In Drosophila, the adult muscles include tubular-type muscles and the fibrillar indirect flight muscles. Regulatory processes specifying tubular muscle fate remain incompletely understood, therefore we chose to analyze the transcriptional regulation of TpnC41C, a Troponin C gene expressed in the tubular jump muscles, but not in the fibrillar flight muscles. We identified a 300-bp promoter fragment of TpnC41C sufficient for the fiber-specific reporter expression. Through an analysis of this regulatory element, we identified two sites necessary for the activation of the enhancer. Mutations in each of these sites resulted in 70% reduction of enhancer activity. One site was characterized as a binding site for Myocyte Enhancer Factor-2. In addition, we identified a repressive element that prevents activation of the enhancer in other muscle fiber types. Mutation of this site increased jump muscle-specific expression of the reporter, but more importantly reporter expression expanded into the indirect flight muscles. Our findings demonstrate that expression of the TpnC41C gene in jump muscles requires integration of multiple positive and negative transcriptional inputs. Identification of the transcriptional regulators binding the cis-elements that we identified will reveal the regulatory pathways controlling muscle fiber differentiation.
    Full-text · Article · Dec 2015 · PLoS ONE
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    • "Studies in mice and other model organisms have revealed varied roles for Pbx proteins in promoting and inhibiting cellular differentiation in many developmental contexts [13,50]. Studies of Pbx functions in skeletal muscle have demonstrated the critical role of Pbx proteins in promoting skeletal muscle differentiation, in particular through acting as pioneer factors for Myod14151651525354. Our studies here not only demonstrate that Pbx proteins promote early myocardial differentiation, but also reveal that pbx4 appears to inhibit later myocardial differentiation. "
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    ABSTRACT: Proper control of the temporal onset of cellular differentiation is critical for regulating cell lineage decisions and morphogenesis during development. Pbx homeodomain transcription factors have emerged as important regulators of cellular differentiation. We previously showed, by using antisense morpholino knockdown, that Pbx factors are needed for the timely activation of myocardial differentiation in zebrafish. In order to gain further insight into the roles of Pbx factors in heart development, we show here that zebrafish pbx4 mutant embryos exhibit delayed onset of myocardial differentiation, such as delayed activation of tnnt2a expression in early cardiomyocytes in the anterior lateral plate mesoderm. We also observe delayed myocardial morphogenesis and dysmorphic patterning of the ventricle and atrium, consistent with our previous Pbx knock-down studies. In addition, we find that pbx4 mutant larvae have aberrant outflow tracts and defective expression of the proepicardial marker tbx18. Finally, we present evidence for Pbx expression in cardiomyocyte precursors as well as heterogeneous Pbx expression among the pan-cytokeratin-expressing proepicardial cells near the developing ventricle. In summary, our data show that Pbx4 is required for the proper temporal activation of myocardial differentiation and establish a basis for studying additional roles of Pbx factors in heart development.
    Preview · Article · Nov 2015
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