Histone deacetylase activity is required for the induction of the MyoD muscle cell lineage in Xenopus

Sumoylation of histone deacetylase 1 regulates MyoD signaling during myogenesis

Sumoylation, the conjugation of a small ubiquitin-like modifier (SUMO) protein to a target, has diverse cellular effects. However, the functional roles of the SUMO modification during myogenesis have not been fully elucidated. Here, we report that basal sumoylation of histone deacetylase 1 (HDAC1) enhances the deacetylation of MyoD in undifferentiated myoblasts, whereas further sumoylation of HDAC1 contributes to switching its binding partners from MyoD to Rb to induce myocyte differentiation. Differentiation in C2C12 skeletal myoblasts induced new immunoblot bands above HDAC1 that were gradually enhanced during differentiation. Using SUMO inhibitors and sumoylation assays, we showed that the upper band was caused by sumoylation of HDAC1 during differentiation. Basal deacetylase activity was not altered in the SUMO modification-resistant mutant HDAC1 K444/476R (HDAC1 2R). Either differentiation or transfection of SUMO1 increased HDAC1 activity that was attenuated in HDAC1 2R. Furthermore, HDAC1 2R failed to deacetylate MyoD. Binding of HDAC1 to MyoD was attenuated by K444/476R. Binding of HDAC1 to MyoD was gradually reduced after 2 days of differentiation. Transfection of SUMO1 induced dissociation of HDAC1 from MyoD but potentiated its binding to Rb. SUMO1 transfection further attenuated HDAC1-induced inhibition of muscle creatine kinase luciferase activity that was reversed in HDAC1 2R. HDAC1 2R failed to inhibit myogenesis and muscle gene expression. In conclusion, HDAC1 sumoylation plays a dual role in MyoD signaling: enhancement of HDAC1 deacetylation of MyoD in the basally sumoylated state of undifferentiated myoblasts and dissociation of HDAC1 from MyoD during myogenesis.

Sepsis and glucocorticoids upregulate p300 and downregulate HDAC6 expression and activity in skeletal muscle

Muscle wasting during sepsis is in part regulated by glucocorticoids. In recent studies, treatment of cultured muscle cells in vitro with dexamethasone upregulated expression and activity of p300, a histone acetyl transferase (HAT), and reduced expression and activity of the histone deacetylases-3 (HDAC3) and -6, changes that favor hyperacetylation. Here, we tested the hypothesis that sepsis and glucocorticoids regulate p300 and HDAC3 and -6 in skeletal muscle in vivo. Because sepsis-induced metabolic changes are particularly pronounced in white, fast-twitch skeletal muscle, most experiments were performed in extensor digitorum longus muscles. Sepsis in rats upregulated p300 mRNA and protein levels, stimulated HAT activity, and reduced HDAC6 expression and HDAC activity. The sepsis-induced changes in p300 and HDAC expression were prevented by the glucocorticoid receptor antagonist RU38486. Treatment of rats with dexamethasone increased expression of p300 and HAT activity, reduced expression of HDAC3 and -6, and inhibited HDAC activity. Finally, treatment with the HDAC inhibitor trichostatin A resulted in increased muscle proteolysis and expression of the ubiquitin ligase atrogin-1. Taken together, our results suggest for the first time that sepsis-induced muscle wasting may be regulated by glucocorticoid-dependent hyperacetylation caused by increased p300 and reduced HDAC expression and activity. The recent development of pharmacological HDAC activators may provide a novel avenue to prevent and treat muscle wasting in sepsis and other catabolic conditions.

The reorganisation of constitutive heterochromatin in differentiating muscle requires HDAC activity

Constitutive heterochromatin was once thought to be remarkably stable in composition and transcriptionally inert, but has recently been shown to be surprisingly dynamic. Here, we show that terminal muscle differentiation results in a global reorganisation and spatial clustering of constitutive heterochromatin. This is accompanied by enhanced histone H3K9 and H4K20 tri-methylation across major satellite regions and increased levels of major and minor satellite-encoded transcripts. Histone deacetylase (HDAC) activity is known to be important for initiating muscle differentiation. However, here, we show that low doses of HDAC inhibitors applied after the onset of muscle differentiation prevent the spatial reorganisation of constitutive heterochromatin while allowing terminal differentiation to proceed. Under these conditions, HDAC inhibition interferes with histone methylation and blocks centromere clustering, but does not prevent the temporal expression of muscle regulatory factors or the accumulation of centromere-derived transcripts. The demonstration that HDAC activity is required for spatial relocation of centromeres in differentiating muscle provides a convenient system in which the molecular drivers of differentiation-induced chromosome repositioning can be dissected.

Signaling chromatin to make muscle

Several findings published within the past year have further established key roles for chromatin-modifying enzymes in the control of muscle gene expression, and have thus refined our thinking of how chromatin structure influences muscle differentiation, hypertrophy and fiber type determination. We discuss the interface between chromatin-modifying enzymes and myogenic transcription factors, signaling mechanisms that impinge on these transcriptional complexes, and how these multicomponent regulatory cascades may be exploited in the development of novel therapeutics to more effectively treat myopathies in humans.

Stage-specific modulation of skeletal myogenesis by inhibitors of nuclear deacetylases

Nuclear acetyltransferases promote and deacetylases inhibit skeletal muscle-gene expression, suggesting the potential effectiveness of deacetylase inhibitors (DIs) in modulating skeletal myogenesis. Surprisingly, previous studies have indicated that DIs suppress myogenesis. The recent observations that histone deacetylases associate with the muscle-regulatory proteins MyoD and MEF2C only in undifferentiated myoblasts prompted us to evaluate the effect of DIs at distinct stages of the myogenic program. We found that exposure of established rodent and human muscle cells to distinct DIs has stage-specific effects. Exposure of undifferentiated skeletal myoblasts to DIs, followed by incubation in differentiation medium, enhanced the expression of muscle-specific reporters and increased the levels of endogenous muscle proteins, leading to a dramatic increase in the formation of multinucleated myotubes. By contrast, simultaneous exposure of muscle cells to differentiation medium and DIs inhibited the myogenic program. Likewise, embryos exposed in utero to nonteratogenic doses of DI at the early stages of somitic myogenesis (embryonic day 8.5) exhibited an increased number of somites and augmented expression of a muscle-specific transgene as well as endogenous muscle genes. The functional effects induced by DIs were mirrored by changes in the state of acetylation of histones present at a muscle-gene enhancer and of MyoD itself. These results represent the first evidence that DIs can enhance muscle differentiation and suggest the rationale for their use in manipulating adult and embryonic skeletal myogenesis.

When the embryonic genome flexes its muscles

During the development of multicellular organisms, both transient and stable gene expression patterns have to be established in a precisely orchestrated sequence. Evidence from diverse model organisms indicates that this epigenetic program involves not only transcription factors, but also the local structure, composition, and modification of chromatin, which define and maintain the accessibility and transcriptional competence of the nucleosomal DNA template. A paradigm for the interdependence of development and chromatin is constituted by the mechanisms controlling the specification and differentiation of the skeletal muscle cell lineage in vertebrates, which is the topic of this review.

CBP/p300 and muscle differentiation: no HAT, no muscle

Terminal differentiation of muscle cells follows a precisely orchestrated program of transcriptional regulatory events at the promoters of both muscle-specific and ubiquitous genes. Two distinct families of transcriptional co-activators, GCN5/PCAF and CREB-binding protein (CBP)/p300, are crucial to this process. While both possess histone acetyl-transferase (HAT) activity, previous studies have failed to identify a requirement for CBP/p300 HAT function in myogenic differentiation. We have addressed this issue directly using a chemical inhibitor of CBP/p300 in addition to a negative transdominant mutant. Our results clearly demonstrate that CBP/p300 HAT activity is critical for myogenic terminal differentiation. Furthermore, this requirement is restricted to a subset of events in the differentiation program: cell fusion and specific gene expression. These data help to define the requirements for enzymatic function of distinct coactivators at different stages of the muscle cell differentiation program.