Casein kinase 2-mediated phosphorylation of Brahma-related gene 1 controls myoblast proliferation and contributes to SWI/SNF complex composition

Transcriptional regulation is modulated in part by chromatin-remodeling enzymes that control gene accessibility by altering chromatin compaction or nucleosome positioning. Brahma-related gene 1 (Brg1), a catalytic subunit of the mammalian SWI/SNF chromatin-remodeling enzymes, is required for both myoblast proliferation and differentiation, and the control of Brg1 phosphorylation by calcineurin, PKCβ1, and p38 regulates the transition to differentiation. However, we hypothesized that Brg1 activity might be regulated by additional kinases. Here, we report that Brg1 is also a target of casein kinase 2 (CK2), a serine/threonine kinase, in proliferating myoblasts. We found that CK2 interacts with Brg1, and mutation of putative phosphorylation sites to non-phosphorylatable (Ser to Ala, SA) or phosphomimetic residues (Ser to Glu, SE) reduced Brg1 phosphorylation by CK2. Although BRG1-deleted myoblasts that ectopically express the SA-Brg1 mutant proliferated similarly to the parental cells or cells ectopically expressing wild-type (WT) Brg1, ectopic expression of the SE-Brg1 mutant reduced proliferation and increased cell death, similar to observations from cells lacking Brg1. Moreover, pharmacological inhibition of CK2 increased myoblast proliferation. Furthermore, the Pax7 promoter, which controls expression of a key transcription factor required for myoblast proliferation, was in an inaccessible chromatin state in the SE-Brg1 mutant, suggesting that hyperphosphorylated Brg1 cannot remodel chromatin. WT-, SA-, and SE-Brg1 exhibited distinct differences in interacting with and affecting expression of the SWI/SNF subunits Baf155 and Baf170 and displayed differential sub-nuclear localization. Our results indicate that CK2-mediated phosphorylation of Brg1 regulates myoblast proliferation and provides insight into one mechanism by which composition of the mammalian SWI/SNF enzyme complex is regulated.

Identification of a Chemical Probe for Family VIII Bromodomains through Optimization of a Fragment Hit

The acetyl post-translational modification of chromatin at selected histone lysine residues is interpreted by an acetyl-lysine specific interaction with bromodomain reader modules. Here we report the discovery of the potent, acetyl-lysine-competitive, and cell active inhibitor PFI-3 that binds to certain family VIII bromodomains while displaying significant, broader bromodomain family selectivity. The high specificity of PFI-3 for family VIII was achieved through a novel bromodomain binding mode of a phenolic headgroup that led to the unusual displacement of water molecules that are generally retained by most other bromodomain inhibitors reported to date. The medicinal chemistry program that led to PFI-3 from an initial fragment screening hit is described in detail, and additional analogues with differing family VIII bromodomain selectivity profiles are also reported. We also describe the full pharmacological characterization of PFI-3 as a chemical probe, along with phenotypic data on adipocyte and myoblast cell differentiation assays.

Brg1 Controls the Expression of Pax7 to Promote Viability and Proliferation of Mouse Primary Myoblasts

Brg1 (Brahma-related gene 1) is a catalytic component of the evolutionarily conserved mammalian SWI/SNF ATP-dependent chromatin remodeling enzymes that disrupt histone-DNA contacts on the nucleosome. While the requirement for the SWI/SNF enzymes in cell differentiation has been extensively studied, its role in precursor cell proliferation and survival is not as well defined. Muscle satellite cells constitute the stem cell pool that sustains and regenerates myofibers in adult skeletal muscle. Here, we show that deletion of Brg1 in primary mouse myoblasts derived from muscle satellite cells cultured ex vivo leads to a cell proliferation defect and apoptosis. We determined that Brg1 regulates cell proliferation and survival by controlling chromatin remodeling and activating transcription at the Pax7 promoter, which is expressed during somite development and is required for controlling viability of the satellite cell population. Reintroduction of catalytically active Brg1 or of Pax7 into Brg1-deficient satellite cells rescued the apoptotic phenotype and restored proliferation. These data demonstrate that Brg1 functions as a positive regulator for cellular proliferation and survival of primary myoblasts. Therefore, the regulation of gene expression through Brg1-mediated chromatin remodeling is critical not just for skeletal muscle differentiation but for maintaining the myoblast population as well.

Opposing calcium-dependent signalling pathways control skeletal muscle differentiation by regulating a chromatin remodelling enzyme

Calcium signaling is important for differentiation-dependent gene expression, but is also involved in other cellular functions. Therefore mechanisms must exist to distinguish calcium signaling relevant to differentiation. Calcineurin is a calcium-regulated phosphatase that is required for myogenic gene expression and skeletal muscle differentiation. Here, we demonstrate that inhibition of calcineurin blocks chromatin remodeling and that the Brg1 ATPase of the SWI/SNF chromatin remodeling enzyme, which is required for the activation of myogenic gene expression, is a calcineurin substrate. Furthermore, we identify the calcium-regulated classical protein kinase C beta (PKCβ) as a repressor of myogenesis and as the enzyme that opposes calcineurin function. Replacement of endogenous Brg1 with a phosphomimetic mutant in primary myoblasts inhibits myogenesis, while replacement with a non-phosphorylatable mutant allows myogenesis despite inhibition of calcineurin signaling, demonstrating the functionality of calcineurin/PKC modified residues. Thus the Brg1 chromatin remodeling enzyme integrates two antagonistic calcium-dependent signaling pathways that control myogenic differentiation.

The Scaffold attachment factor b1 (Safb1) regulates myogenic differentiation by facilitating the transition of myogenic gene chromatin from a repressed to an activated state

The regulation of skeletal muscle gene expression during myogenesis is mediated by lineage-specific transcription factors in combination with numerous cofactors, many of which modify chromatin structure. However, the involvement of scaffolding proteins that organize chromatin and chromatin-associated regulatory proteins has not extensively been explored in myogenic differentiation. Here, we report that Scaffold attachment factor b1 (Safb1), primarily associated with transcriptional repression, functions as a positive regulator of myogenic differentiation. Knockdown of Safb1 inhibited skeletal muscle marker gene expression and differentiation in cultured C2C12 myoblasts. In contrast, over-expression resulted in the premature expression of critical muscle structural proteins and formation of enlarged thickened myotubes. Safb1 co-immunoprecipitated with MyoD and was co-localized on myogenic promoters. Upon Safb1 knockdown, the repressive H3K27me3 histone mark and binding of the Polycomb histone methyltransferase Ezh2 persisted at differentiation-dependent gene promoters. In contrast, the appearance of histone marks and regulators associated with myogenic gene activation, such as myogenin and the SWI/SNF chromatin remodelling enzyme ATPase, Brg1, was blocked. These results indicate that the scaffold protein Safb1 contributes to the activation of skeletal muscle gene expression during myogenic differentiation by facilitating the transition of promoter sequences from a repressive chromatin structure to one that is transcriptionally permissive.

Myogenic microRNA expression requires ATP-dependent chromatin remodeling enzyme function

Knockdown of the Brg1 ATPase subunit of SWI/SNF chromatin remodeling enzymes in developing zebrafish caused stunted tail formation and altered sarcomeric actin organization, which phenocopies the loss of the microRNA processing enzyme Dicer, or the knockdown of myogenic microRNAs. Furthermore, myogenic microRNA expression and differentiation was blocked in Brg1 conditional myoblasts differentiated ex vivo. The binding of Brg1 upstream of myogenic microRNA sequences correlated with MyoD binding and accessible chromatin structure in satellite cells and myofibers, and it was required for chromatin accessibility and microRNA expression in a tissue culture model for myogenesis. The results implicate ATP-dependent chromatin remodelers in myogenic microRNA gene regulation.

The genome of the diploid anuran Xenopus tropicalis contains a novel array of sarcoplasmic myosin heavy chain genes expressed in larval muscle and larynx

The sarcomeric myosin heavy chain (MyHC) proteins are a family of molecular motors responsible for the transduction of chemical energy into mechanical work in striated muscle. The vertebrate genome contains multiple copies of the MyHC gene, and expression of different isoforms correlates with differences in the physiological properties of muscle fibers. Most MyHC isoforms are found in two arrays, one containing the "fast-twitch" skeletal muscle isoforms and the other the "slow-twitch" or cardiac isoforms. To extend our understanding of MyHC evolution, we have examined the genome of the anuran Xenopus tropicalis. The X. tropicalis genome includes 15 full-length MyHC genes organized in seven genomic locations. One unique array of MyHC genes is similar to the mammalian fast-skeletal array, but is not found in amniotes. The isoforms in this array are expressed during larval stages and in muscles of the adult larynx. Duplication of the fast-skeletal MyHC array appears to have led to expression divergence of muscle proteins in the larval and adult stages of the anuran life cycle. A striking similarity of gene order between regions flanking X. tropicalis MyHC arrays and human arrays was evident; genomic organization of MyHC isoforms may thus be highly conserved across tetrapods.

Direct action of gonadotropin in brain integrates behavioral and reproductive functions

Essential roles for gonadotropins in gonadal development and reproduction are well established. Over the past decade, however, the expression of luteinizing hormone receptor (LHR) has also been reported in the brain of various mammals and birds. Although suggestive, it has not yet been determined whether this expression pattern supports a novel function for gonadotropins. Here, we demonstrate a CNS-mediated role of gonadotropins in a reproductive behavior: the courtship songs of the South African clawed frog, Xenopus laevis. Male advertisement calling in this species depends on a nongonadal action of gonadotropin. To determine whether this effect is due to action on the CNS, we administered gonadotropin intracerebroventricularly (ICV) or systemically to intact or castrated males with or without concomitant androgen replacement. In intact and androgen-replaced gonadectomized males, gonadotropin significantly increased calling within 1 h after ICV injection. The effective dosage via ICV injections was less than one hundredth of the effective systemic dose. In situ hybridization with a cloned fragment of Xenopus LHR revealed strong expression in ventral forebrain areas important for vocal control. Further, gonadotropin treatment of brain in vitro up-regulates immunoreactivity for the LHR downstream target, egr-1, specifically in these vocal forebrain areas. Up-regulation occurs even when synaptic transmission is suppressed by incubation in Ca2+ free/high magnesium saline. These results demonstrate a neural role for gonadotropin in the control of calling behavior, potentially mediated via LHRs in forebrain vocal nuclei. Gonadotropin may play a novel integrative role in modulating both reproductive physiology and behavior.