Essential and redundant functions of the MyoD distal regulatory region revealed by targeted mutagenesis

3D organization of enhancers in MuSCs

Skeletal muscle stem cells (MuSCs), also known as satellite cells, are essential for muscle growth and injury induced regeneration. In healthy adult muscle, MuSCs remain in a quiescent state located in a specialized niche beneath the basal lamina. Upon injury, these dormant MuSCs can quickly activate to re-enter the cell cycle and differentiate into new myofibers, while a subset undergoes self-renewal and returns to quiescence to restore the stem cell pool. The myogenic lineage progression is intricately controlled by complex intrinsic and extrinsic cues and coupled with dynamic transcriptional programs. In transcriptional regulation, enhancers are key regulatory elements controlling spatiotemporal gene expression through physical contacting promoters of target genes. The three-dimensional (3D) chromatin architecture is known to orchestrate the establishment of proper enhancer-promoter interactions throughout development and aging. However, studies dissecting the 3D organization of enhancers in MuSCs are just emerging. Here, we provide an overview of the general properties of enhancers and newly developed methods for assessing their activity. In particular, we summarize recent discoveries regarding the 3D rewiring of enhancers during MuSC specification, lineage progression as well as aging.

The myogenesis program drives clonal selection and drug resistance in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a pediatric cancer with features of skeletal muscle; patients with unresectable or metastatic RMS fare poorly due to high rates of disease recurrence. Here, we use single-cell and single-nucleus RNA sequencing to show that RMS tumors recapitulate the spectrum of embryonal myogenesis. Using matched patient samples from a clinical trial and orthotopic patient-derived xenografts (O-PDXs), we show that chemotherapy eliminates the most proliferative component with features of myoblasts within embryonal RMS; after treatment, the immature population with features of paraxial mesoderm expands to reconstitute the developmental hierarchy of the original tumor. We discovered that this paraxial mesoderm population is dependent on EGFR signaling and is sensitive to EGFR inhibitors. Taken together, these data serve as a proof of concept that targeting each developmental state in embryonal RMS is an effective strategy for improving outcomes by preventing disease recurrence.

Propionate hampers differentiation and modifies histone propionylation and acetylation in skeletal muscle cells

Protein acylation via metabolic acyl-CoA intermediates provides a link between cellular metabolism and protein functionality. A process in which acetyl-CoA and acetylation are fine-tuned is during myogenic differentiation. However, the roles of other protein acylations remain unknown. Protein propionylation could be functionally relevant because propionyl-CoA can be derived from the catabolism of amino acids and fatty acids and was shown to decrease during muscle differentiation. We aimed to explore the potential role of protein propionylation in muscle differentiation, by mimicking a pathophysiological situation with high extracellular propionate which increases propionyl-CoA and protein propionylation, rendering it a model to study increased protein propionylation. Exposure to extracellular propionate, but not acetate, impaired myogenic differentiation in C2C12 cells and propionate exposure impaired myogenic differentiation in primary human muscle cells. Impaired differentiation was accompanied by an increase in histone propionylation as well as histone acetylation. Furthermore, chromatin immunoprecipitation showed increased histone propionylation at specific regulatory myogenic differentiation sites of the Myod gene. Intramuscular propionylcarnitine levels are higher in old compared to young males and females, possibly indicating increased propionyl-CoA levels with age. The findings suggest a role for propionylation and propionyl-CoA in regulation of muscle cell differentiation and ageing, possibly via alterations in histone acylation.

Development and patterning of rib primordia are dependent on associated musculature

The importance of skeletal muscle for rib development and patterning in the mouse embryo has not been resolved, largely because different experimental approaches have yielded disparate results. In this study, we utilize both gene knockouts and muscle cell ablation approaches to re-visit the extent to which rib growth and patterning are dependent on developing musculature. Consistent with previous studies, we show that rib formation is highly dependent on the MYOD family of myogenic regulatory factors (MRFs), and demonstrate that the extent of rib formation is gene-, allele-, and dosage-dependent. In the absence of Myf5 and MyoD, one allele of Mrf4 is sufficient for extensive rib growth, although patterning is abnormal. Under conditions of limiting MRF dosage, MyoD is identified as a positive regulator of rib patterning, presumably due to improved intercostal muscle development. In contrast to previous muscle ablation studies, we show that diphtheria toxin subunit A (DTA)-mediated ablation of muscle progenitors or differentiated muscle, using MyoDiCre or HSA-Cre drivers, respectively, profoundly disrupts rib development. Further, a comparison of three independently derived Rosa26-based DTA knockin alleles demonstrates that the degree of rib perturbations in MyoDiCre/+/DTA embryos is markedly dependent on the DTA allele used, and may in part explain discrepancies with previous findings. The results support the conclusion that the extent and quality of rib formation is largely dependent on the dosage of Myf5 and Mrf4, and that both early myotome-sclerotome interactions, as well as later muscle-rib interactions, are important for proper rib growth and patterning.

PTH/PTHrP Receptor Signaling Restricts Arterial Fibrosis in Diabetic LDLR-/- Mice by Inhibiting Myocardin-Related Transcription Factor Relays

RATIONALE

The PTH1R (PTH [parathyroid hormone]/PTHrP [PTH-related protein] receptor) is expressed in vascular smooth muscle (VSM) and increased VSM PTH1R signaling mitigates diet-induced arteriosclerosis in LDLR-/- mice.

OBJECTIVE

To study the impact of VSM PTH1R deficiency, we generated mice SM22-Cre:PTH1R(fl/fl);LDLR-/- mice (PTH1R-VKO) and Cre-negative controls.

METHODS AND RESULTS

Immunofluorescence and Western blot confirmed PTH1R expression in arterial VSM that was reduced by Cre-mediated knockout. PTH1R-VKO cohorts exhibited increased aortic collagen accumulation in vivo, and VSM cultures from PTH1R-VKO mice elaborated more collagen (2.5-fold; P=0.01) with elevated Col3a1 and Col1a1 expression. To better understand these profibrotic responses, we performed mass spectrometry on nuclear proteins extracted from Cre-negative controls and PTH1R-VKO VSM. PTH1R deficiency reduced Gata6 but upregulated the MADS (MCM1, Agamous, Deficiens, and Srf DNA-binding domain)-box transcriptional co-regulator, Mkl-1 (megakaryoblastic leukemia [translocation] 1). Co-transfection assays (Col3a1 promoter-luciferase reporter) confirmed PTH1R-mediated inhibition and Mkl-1-mediated activation of Col3a1 transcription. Regulation mapped to a conserved hybrid CT(A/T)6GG MADS-box cognate in the Col3a1 promoter. Mutations of C/G in this motif markedly reduced Col3a1 transcriptional regulation by PTH1R and Mkl-1. Upregulation of Col3a1 and Col1a1 in PTH1R-VKO VSM was inhibited by small interfering RNA targeting Mkl1 and by treatment with the Mkl-1 antagonist CCG1423 or the Rock (Rho-associated coiled-coil containing protein kinase)-2 inhibitor KD025. Chromatin precipitation demonstrated that VSM PTH1R deficiency increased Mkl-1 binding to Col3a1 and Col1a1, but not TNF, promoters. Proteomic studies of plasma extracellular vesicles and VSM from PTH1R-VKO mice identified C1r (complement component 1, r) and C1s (complement component 1, s), complement proteins involved in vascular collagen metabolism, as potential biomarkers. VSM C1r protein and C1r message were increased with PTH1R deficiency, mediated by Mkl-1-dependent transcription and inhibited by CCG1423 or KD025.

CONCLUSIONS

PTH1R signaling restricts collagen production in the VSM lineage, in part, via Mkl-1 regulatory circuits that control collagen gene transcription. Strategies that maintain homeostatic VSM PTH1R signaling, as reflected in extracellular vesicle biomarkers of VSM PTH1R/Mkl-1 action, may help mitigate arteriosclerosis and vascular fibrosis.

Master control: transcriptional regulation of mammalian Myod

MYOD is a master regulator of the skeletal myogenic program. But what regulates expression of Myod? More than 20 years ago, studies established that Myod expression is largely controlled by just two enhancer regions located within a region 24 kb upstream of the transcription start site in mammals, which regulate Myod expression in the embryo, fetus and adult. Despite this apparently simple arrangement, Myod regulation is complex, with different combinations of transcription factors acting on these enhancers in different muscle progenitor cells and phases of differentiation. A range of epigenetic modifications in the Myod upstream region also play a part in activating and repressing Myod expression during development and regeneration. Here the evidence for this binding at Myod control regions is summarized, giving an overview of our current understanding of Myod expression regulation in mammals.

MUNC, an Enhancer RNA Upstream from the MYOD Gene, Induces a Subgroup of Myogenic Transcripts in trans Independently of MyoD

MyoD upstream noncoding RNA (MUNC) initiates in the distal regulatory region (DRR) enhancer of MYOD and is formally classified as an enhancer RNA (DRR^eRNA). MUNC is required for optimal myogenic differentiation, induces specific myogenic transcripts in trans (MYOD, MYOGENIN, and MYH3), and has a functional human homolog. The vast majority of eRNAs are believed to act in cis primarily on their neighboring genes (1, 2), making it likely that MUNC action is dependent on the induction of MYOD RNA. Surprisingly, MUNC overexpression in MYOD ^-/- C2C12 cells induces many myogenic transcripts in the complete absence of MyoD protein. Genomewide analysis showed that, while many genes are regulated by MUNC in a MyoD-dependent manner, there is a set of genes that are regulated by MUNC, both upward and downward, independently of MyoD. MUNC and MyoD even appear to act antagonistically on certain transcripts. Deletion mutagenesis showed that there are at least two independent functional sites on the MUNC long noncoding RNA (lncRNA), with exon 1 more active than exon 2 and with very little activity from the intron. Thus, although MUNC is an eRNA of MYOD, it is also a trans-acting lncRNA whose sequence, structure, and cooperating factors, which include but are not limited to MyoD, determine the regulation of many myogenic genes.

Non-coding Transcripts from Enhancers: New Insights into Enhancer Activity and Gene Expression Regulation

Long non-coding RNAs (lncRNAs) have gained widespread interest in the past decade owing to their enormous amount and surprising functions implicated in a variety of biological processes. Some lncRNAs exert function as enhancers, i.e., activating gene transcription by serving as the cis-regulatory molecules. Furthermore, recent studies have demonstrated that many enhancer elements can be transcribed and produce RNA molecules, which are termed as enhancer RNAs (eRNAs). The eRNAs are not merely the by-product of the enhancer transcription. In fact, many of them directly exert or regulate enhancer activity in gene activation through diverse mechanisms. Here, we provide an overview of enhancer activity, transcription of enhancer itself, characteristics of eRNAs, as well as their roles in regulating enhancer activity and gene expression.

Muscle cell identity requires Pax7-mediated lineage-specific DNA demethylation

Background

Skeletal muscle stem cells enable the formation, growth, maintenance, and regeneration of skeletal muscle throughout life. The regeneration process is compromised in several pathological conditions, and muscle progenitors derived from pluripotent stem cells have been suggested as a potential therapeutic source for tissue replacement. DNA methylation is an important epigenetic mechanism in the setting and maintenance of cellular identity, but its role in stem cell determination towards the myogenic lineage is unknown. Here we addressed the DNA methylation dynamics of the major genes orchestrating the myogenic determination and differentiation programs in embryonic stem (ES) cells, their Pax7-induced myogenic derivatives, and muscle stem cells in proliferating and differentiating conditions.

Results

Our data showed a common muscle-specific DNA demethylation signature required to acquire and maintain the muscle-cell identity. This specific-DNA demethylation is Pax7-mediated, and it is a prime event in muscle stem cells gene activation. Notably, downregulation of the demethylation-related enzyme Apobec2 in ES-derived myogenic precursors reduced myogenin-associated DNA demethylation and dramatically impaired the expression of differentiation markers and, ultimately, muscle differentiation.

Conclusions

Our results underscore DNA demethylation as a key mechanism driving myogenesis and identify specific Pax7-mediated DNA demethylation signatures to acquire and maintain the muscle-cell identity. Additionally, we provide a panel of epigenetic markers for the efficient and safe generation of ES- and induced pluripotent stem cell (iPS)-derived myogenic progenitors for therapeutic applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0250-9) contains supplementary material, which is available to authorized users.

Lineage-affiliated transcription factors bind the Gata3 Tce1 enhancer to mediate lineage-specific programs

The transcription factor GATA3 is essential for the genesis and maturation of the T cell lineage, and GATA3 dysregulation has pathological consequences. Previous studies have shown that GATA3 function in T cell development is regulated by multiple signaling pathways and that the Notch nuclear effector, RBP-J, binds specifically to the Gata3 promoter. We previously identified a T cell-specific Gata3 enhancer (Tce1) lying 280 kb downstream from the structural gene and demonstrated in transgenic mice that Tce1 promoted T lymphocyte-specific transcription of reporter genes throughout T cell development; however, it was not clear if Tce1 is required for Gata3 transcription in vivo. Here, we determined that the canonical Gata3 promoter is insufficient for Gata3 transcriptional activation in T cells in vivo, precluding the possibility that promoter binding by a host of previously implicated transcription factors alone is responsible for Gata3 expression in T cells. Instead, we demonstrated that multiple lineage-affiliated transcription factors bind to Tce1 and that this enhancer confers T lymphocyte-specific Gata3 activation in vivo, as targeted deletion of Tce1 in a mouse model abrogated critical functions of this T cell-regulatory element. Together, our data show that Tce1 is both necessary and sufficient for critical aspects of Gata3 T cell-specific transcriptional activity.

Regulation of Skeletal Muscle Stem Cell Quiescence by Suv4-20h1-Dependent Facultative Heterochromatin Formation

Skeletal muscle stem cells (MuSCs) are required for regeneration of adult muscle following injury, a response that demands activation of mainly quiescent MuSCs. Despite the need for dynamic regulation of MuSC quiescence, relatively little is known about the determinants of this property. Here, we show that Suv4-20h1, an H4K20 dimethyltransferase, controls MuSC quiescence by promoting formation of facultative heterochromatin (fHC). Deletion of Suv4-20h1 reduces fHC and induces transcriptional activation and repositioning of the MyoD locus away from the heterochromatic nuclear periphery. These effects promote MuSC activation, resulting in stem cell depletion and impaired long-term muscle regeneration. Genetic reduction of MyoD expression rescues fHC formation and lost MuSC quiescence, restoring muscle regeneration capacity in Suv4-20h1 mutants. Together, these findings reveal that Suv4-20h1 actively regulates MuSC quiescence via fHC formation and control of the MyoD locus, thereby guarding and preserving the stem cell pool over a lifetime.

Tissue-specific epigenetics in gene neighborhoods: myogenic transcription factor genes

Myogenic regulatory factor (MRF) genes, MYOD1, MYOG, MYF6 and MYF5, are critical for the skeletal muscle lineage. Here, we used various epigenome profiles from human myoblasts (Mb), myotubes (Mt), muscle and diverse non-muscle samples to elucidate the involvement of multigene neighborhoods in the regulation of MRF genes. We found more far-distal enhancer chromatin associated with MRF genes in Mb and Mt than previously reported from studies in mice. For the MYF5/MYF6 gene-pair, regions of Mb-associated enhancer chromatin were located throughout the adjacent 236-kb PTPRQ gene even though Mb expressed negligible amounts of PTPRQ mRNA. Some enhancer chromatin regions inside PTPRQ in Mb were also seen in PTPRQ mRNA-expressing non-myogenic cells. This suggests dual-purpose PTPRQ enhancers that upregulate expression of PTPRQ in non-myogenic cells and MYF5/MYF6 in myogenic cells. In contrast, the myogenic enhancer chromatin regions distal to MYOD1 were intergenic and up to 19 kb long. Two of them contain small, known MYOD1 enhancers, and one displayed an unusually high level of 5-hydroxymethylcytosine in a quantitative DNA hydroxymethylation assay. Unexpectedly, three regions of MYOD1-distal enhancer chromatin in Mb and Mt overlapped enhancer chromatin in umbilical vein endothelial cells, which might upregulate a distant gene (PIK3C2A). Lastly, genes surrounding MYOG were preferentially transcribed in Mt, like MYOG itself, and exhibited nearby myogenic enhancer chromatin. These neighboring chromatin regions may be enhancers acting in concert to regulate myogenic expression of multiple adjacent genes. Our findings reveal the very different and complex organization of gene neighborhoods containing closely related transcription factor genes.

Expression of myogenic regulatory factors in chicken embryos during somite and limb development

The expression of the myogenic regulatory factors (MRFs), Myf5, MyoD, myogenin (Mgn) and MRF4 have been analysed during the development of chicken embryo somites and limbs. In somites, Myf5 is expressed first in somites and paraxial mesoderm at HH stage 9 followed by MyoD at HH stage 12, and Mgn and MRF4 at HH stage 14. In older somites, Myf5 and MyoD are also expressed in the ventrally extending myotome prior to Mgn and MRF4 expression. In limb muscles a similar temporal sequence is observed with Myf5 expression detected first in forelimbs at HH stage 22, MyoD at HH stage 23, Mgn at HH stage 24 and MRF4 at HH stage 30. This report describes the precise time of onset of expression of each MRF in somites and limbs during chicken embryo development, and provides a detailed comparative timeline of MRF expression in different embryonic muscle groups.

Expression of Non-acetylatable H2A.Z in Myoblast Cells Blocks Myoblast Differentiation through Disruption of MyoD Expression

H2A.Z is a histone H2A variant that is essential for viability in Tetrahymena and Drosophila and also during embryonic development of mice. Although implicated in diverse cellular processes, including transcriptional regulation, chromosome segregation, and heterochromatin formation, its essential function in cells remains unknown. Cellular differentiation is part of the developmental process of multicellular organisms. To elucidate the roles of H2A.Z and H2A.Z acetylation in cellular differentiation, we examined the effects of expressing wild type (WT) or a non-acetylatable form of H2A.Z in the growth and differentiation of the myoblast C2C12 cell line. Ectopic expression of wild type or mutant H2A.Z resulted in distinct phenotypes in the differentiation of the C2C12 cells and the formation of myotubes. Most strikingly, expression of the H2A.Z non-acetylatable mutant (H2A.Z-Ac-mut) resulted in a complete block of myoblast differentiation. We determined that this phenotype is caused by a loss of MyoD expression in the Ac-mut-expressing cells prior to and after induction of differentiation. Moreover, chromatin accessibility assays showed that the promoter region of MyoD is less accessible in the differentiation-defective cells. Altogether, these new findings show that expression of the Ac-mut form of H2A.Z resulted in a dominant phenotype that blocked differentiation due to chromatin changes at the MyoD promoter.

MUNC, a long noncoding RNA that facilitates the function of MyoD in skeletal myogenesis

An in silico screen for myogenic long noncoding RNAs (lncRNAs) revealed nine lncRNAs that are upregulated more than 10-fold in myotubes versus levels in myoblasts. One of these lncRNAs, MyoD upstream noncoding (MUNC, also known as DRR(eRNA)), is encoded 5 kb upstream of the transcription start site of MyoD, a myogenic transcription factor gene. MUNC is specifically expressed in skeletal muscle and exists as in unspliced and spliced isoforms, and its 5' end overlaps with the cis-acting distal regulatory region (DRR) of MyoD. Small interfering RNA (siRNA) of MUNC reduced myoblast differentiation and specifically reduced the association of MyoD to the DRR enhancer and myogenin promoter but not to another MyoD-dependent enhancer. Stable overexpression of MUNC from a heterologous promoter increased endogenous MyoD, Myogenin, and Myh3 (myosin heavy chain, [MHC] gene) mRNAs but not the cognate proteins, suggesting that MUNC can act in trans to promote gene expression but that this activity does not require an induction of MyoD protein. MUNC also stimulates the transcription of other genes that are not recognized as MyoD-inducible genes. Knockdown of MUNC in vivo impaired murine muscle regeneration, implicating MUNC in primary satellite cell differentiation in the animal. We also discovered a human MUNC that is induced during differentiation of myoblasts and whose knockdown decreases differentiation, suggesting an evolutionarily conserved role of MUNC lncRNA in myogenesis. Although MUNC overlaps with the DRR enhancer, our results suggest that MUNC is not a classic cis-acting enhancer RNA (e-RNA) acting exclusively by stimulating the neighboring MyoD gene but more like a promyogenic lncRNA that acts directly or indirectly on multiple promoters to increase myogenic gene expression.

MyoD-expressing progenitors are essential for skeletal myogenesis and satellite cell development

Skeletal myogenesis in the embryo is regulated by the coordinated expression of the MyoD family of muscle regulatory factors (MRFs). MyoD and Myf-5, which are the primary muscle lineage-determining factors, function in a partially redundant manner to establish muscle progenitor cell identity. Previous diphtheria toxin (DTA)-mediated ablation studies showed that MyoD+ progenitors rescue myogenesis in embryos in which Myf-5-expressing cells were targeted for ablation, raising the possibility that the regulative behavior of distinct, MRF-expressing populations explains the functional compensatory activities of these MRFs. Using MyoD(iCre) mice, we show that DTA-mediated ablation of MyoD-expressing cells results in the cessation of myogenesis by embryonic day 12.5 (E12.5), as assayed by myosin heavy chain (MyHC) and Myogenin staining. Importantly, MyoD(iCre/+);R26(DTA/+) embryos exhibited a concomitant loss of Myf-5+ progenitors, indicating that the vast majority of Myf-5+ progenitors express MyoD, a conclusion consistent with immunofluorescence analysis of Myf-5 protein expression in MyoD(iCre) lineage-labeled embryos. Surprisingly, staining for the paired box transcription factor, Pax7, which functions genetically upstream of MyoD in the trunk and is a marker for fetal myoblasts and satellite cell progenitors, was also lost by E12.5. Specific ablation of differentiating skeletal muscle in ACTA1Cre;R26(DTA/+) embryos resulted in comparatively minor effects on MyoD+, Myf-5+ and Pax7+ progenitors, indicating that cell non-autonomous effects are unlikely to explain the rapid loss of myogenic progenitors in MyoD(iCre/+);R26(DTA/+) embryos. We conclude that the vast majority of myogenic cells transit through a MyoD+ state, and that MyoD+ progenitors are essential for myogenesis and stem cell development.

eRNAs promote transcription by establishing chromatin accessibility at defined genomic loci

Transcription factors and DNA regulatory binding motifs are fundamental components of the gene regulatory network. Here, by using genome-wide binding profiling, we show extensive occupancy of transcription factors of myogenesis (MyoD and Myogenin) at extragenic enhancer regions coinciding with RNA synthesis (i.e., eRNA). In particular, multiple regions were transcribed to eRNA within the regulatory region of MYOD1, including previously characterized distal regulatory regions (DRR) and core enhancer (CE). While (CE)RNA enhanced RNA polymerase II (Pol II) occupancy and transcription at MYOD1, (DRR)RNA acted to activate the downstream myogenic genes. The deployment of transcriptional machinery to appropriate loci is contingent on chromatin accessibility, a rate-limiting step preceding Pol II assembly. By nuclease sensitivity assay, we found that eRNAs regulate genomic access of the transcriptional complex to defined regulatory regions. In conclusion, our data suggest that eRNAs contribute to establishing a cell-type-specific transcriptional circuitry by directing chromatin-remodeling events.

Six1 regulates MyoD expression in adult muscle progenitor cells

Quiescent satellite cells are myogenic progenitors that enable regeneration of skeletal muscle. One of the early events of satellite cell activation following myotrauma is the induction of the myogenic regulatory factor MyoD, which eventually induces terminal differentiation and muscle function gene expression. The purpose of this study was to elucidate the mechanism by which MyoD is induced during activation of satellite cells in mouse muscle undergoing regeneration. We show that Six1, a transcription factor essential for embryonic myogenesis, also regulates MyoD expression in muscle progenitor cells. Six1 knock-down by RNA interference leads to decreased expression of MyoD in myoblasts. Chromatin immunoprecipitation assays reveal that Six1 binds the Core Enhancer Region of MyoD. Further, transcriptional reporter assays demonstrate that Core Enhancer Region reporter gene activity in myoblasts and in regenerating muscle depends on the expression of Six1 and on Six1 binding sites. Finally, we provide evidence indicating that Six1 is required for the proper chromatin structure at the Core Enhancer Region, as well as for MyoD binding at its own enhancer. Together, our results reveal that MyoD expression in satellite cells depends on Six1, supporting the idea that Six1 plays an important role in adult myogenesis, in addition to its role in embryonic muscle formation.

Six homeoproteins directly activate Myod expression in the gene regulatory networks that control early myogenesis

In mammals, several genetic pathways have been characterized that govern engagement of multipotent embryonic progenitors into the myogenic program through the control of the key myogenic regulatory gene Myod. Here we demonstrate the involvement of Six homeoproteins. We first targeted into a Pax3 allele a sequence encoding a negative form of Six4 that binds DNA but cannot interact with essential Eya co-factors. The resulting embryos present hypoplasic skeletal muscles and impaired Myod activation in the trunk in the absence of Myf5/Mrf4. At the axial level, we further show that Myod is still expressed in compound Six1/Six4:Pax3 but not in Six1/Six4:Myf5 triple mutant embryos, demonstrating that Six1/4 participates in the Pax3-Myod genetic pathway. Myod expression and head myogenesis is preserved in Six1/Six4:Myf5 triple mutant embryos, illustrating that upstream regulators of Myod in different embryonic territories are distinct. We show that Myod regulatory regions are directly controlled by Six proteins and that, in the absence of Six1 and Six4, Six2 can compensate.

The onset of skeletal muscle formation is controlled by complex gene regulatory networks. By manipulation of these genetic pathways in the mouse embryo, we have examined the interplay between genes encoding the transcriptional regulator Pax3; the major myogenic determination proteins Myf5, Mrf4, and Myod; as well as genes encoding homeodomain proteins Six1 and Six4. In the absence of Myf5 and Six1/4, Myod expression is compromised. We demonstrate that key regulatory elements of the Myod gene are directly targeted by Six factors, including Six2, which is unexpectedly upregulated in the absence of Six1 and Six4. This work therefore reveals new aspects of the gene regulatory networks that control myogenesis.

Hedgehog signaling regulates MyoD expression and activity

The inhibition of MyoD expression is important for obtaining muscle progenitors that can replenish the satellite cell niche during muscle repair. Progenitors could be derived from either embryonic stem cells or satellite cells. Hedgehog (Hh) signaling is important for MyoD expression during embryogenesis and adult muscle regeneration. To date, the mechanistic understanding of MyoD regulation by Hh signaling is unclear. Here, we demonstrate that the Hh effector, Gli2, regulates MyoD expression and associates with MyoD gene elements. Gain- and loss-of-function experiments in pluripotent P19 cells show that Gli2 activity is sufficient and required for efficient MyoD expression during skeletal myogenesis. Inhibition of Hh signaling reduces MyoD expression during satellite cell activation in vitro. In addition to regulating MyoD expression, Hh signaling regulates MyoD transcriptional activity, and MyoD activates Hh signaling in myogenic conversion assays. Finally, Gli2, MyoD, and MEF2C form a protein complex, which enhances MyoD activity on skeletal muscle-related promoters. We therefore link Hh signaling to the function and expression of MyoD protein during myogenesis in stem cells.

Targeted deletion of the antisilencer/enhancer (ASE) element from intron 1 of the myelin proteolipid protein gene (Plp1) in mouse reveals that the element is dispensable for Plp1 expression in brain during development and remyelination

Myelin proteolipid protein gene (Plp1) expression is temporally regulated in brain, which peaks during the active myelination period of CNS development. Previous studies with Plp1-lacZ transgenic mice demonstrated that (mouse) Plp1 intron 1 DNA is required for high levels of expression in oligodendrocytes. Deletion-transfection analysis revealed the intron contains a single positive regulatory element operative in the N20.1 oligodendroglial cell line, which was named ASE (antisilencer/enhancer) based on its functional properties in these cells. To investigate the role of the ASE in vivo, the element was deleted from the native gene in mouse using a Cre/lox strategy. Although removal of the ASE from Plp1-lacZ constructs profoundly decreased expression in transfected oligodendroglial cell lines (N20.1 and Oli-neu), the element was dispensable to achieve normal levels of Plp1 gene expression in mouse during development (except perhaps at postnatal day 15) and throughout the remyelination period following cuprizone-induced (acute) demyelination. Thus, it is possible that the ASE is non-functional in vivo, or that loss of the ASE from the native gene in mouse can be compensated for by the presence of other regulatory elements within the Plp1 gene.

Musculin and TCF21 coordinate the maintenance of myogenic regulatory factor expression levels during mouse craniofacial development

The specification of the skeletal muscle lineage during craniofacial development is dependent on the activity of MYF5 and MYOD, two members of the myogenic regulatory factor family. In the absence of MYF5 or MYOD there is not an overt muscle phenotype, whereas in the double Myf5;MyoD knockout branchiomeric myogenic precursors fail to be specified and skeletal muscle is not formed. The transcriptional regulation of Myf5 is controlled by a multitude of regulatory elements acting at different times and anatomical locations, with at least five operating in the branchial arches. By contrast, only two enhancers have been implicated in the regulation of MyoD. In this work, we characterize an enhancer element that drives Myf5 expression in the branchial arches from 9.5 days post-coitum and show that its activity in the context of the entire locus is dependent on two highly conserved E-boxes. These binding sites are required in a subset of Myf5-expressing cells including both progenitors and those which have entered the myogenic pathway. The correct levels of expression of Myf5 and MyoD result from activation by musculin and TCF21 through direct binding to specific enhancers. Consistent with this, we show that in the absence of musculin the timing of activation of Myf5 and MyoD is not affected but the expression levels are significantly reduced. Importantly, normal levels of Myf5 expression are restored at later stages, which might explain the absence of particular muscles in the Msc;Tcf21 double-knockout mice.

Sim2 prevents entry into the myogenic program by repressing MyoD transcription during limb embryonic myogenesis

The basic helix-loop-helix transcription factor MyoD is a central actor that triggers the skeletal myogenic program. Cell-autonomous and non-cell-autonomous regulatory pathways must tightly control MyoD expression to ensure correct initiation of the muscle program at different places in the embryo and at different developmental times. In the present study, we have addressed the involvement of Sim2 (single-minded 2) in limb embryonic myogenesis. Sim2 is a bHLH-PAS transcription factor that inhibits transcription by active repression and displays enhanced expression in ventral limb muscle masses during chick and mouse embryonic myogenesis. We have demonstrated that Sim2 is expressed in muscle progenitors that have not entered the myogenic program, in different experimental conditions. MyoD expression is transiently upregulated in limb muscle masses of Sim2(-/-) mice. Conversely, Sim2 gain-of-function experiments in chick and Xenopus embryos showed that Sim2 represses MyoD expression. In addition, we show that Sim2 represses the activity of the mouse MyoD promoter in primary myoblasts and is recruited to the MyoD core enhancer in embryonic mouse limbs. Sim2 expression is non-autonomously and negatively regulated by the dorsalising factor Lmx1b. We propose that Sim2 represses MyoD transcription in limb muscle masses, through Sim2 recruitment to the MyoD core enhancer, in order to prevent premature entry into the myogenic program. This MyoD repression is predominant in ventral limb regions and is likely to contribute to the differential increase of the global mass of ventral muscles versus dorsal muscles.

MASTR directs MyoD-dependent satellite cell differentiation during skeletal muscle regeneration

In response to skeletal muscle injury, satellite cells, which function as a myogenic stem cell population, become activated, expand through proliferation, and ultimately fuse with each other and with damaged myofibers to promote muscle regeneration. Here, we show that members of the Myocardin family of transcriptional coactivators, MASTR and MRTF-A, are up-regulated in satellite cells in response to skeletal muscle injury and muscular dystrophy. Global and satellite cell-specific deletion of MASTR in mice impairs skeletal muscle regeneration. This impairment is substantially greater when MRTF-A is also deleted and is due to aberrant differentiation and excessive proliferation of satellite cells. These abnormalities mimic those associated with genetic deletion of MyoD, a master regulator of myogenesis, which is down-regulated in the absence of MASTR and MRTF-A. Consistent with an essential role of MASTR in transcriptional regulation of MyoD expression, MASTR activates a muscle-specific postnatal MyoD enhancer through associations with MEF2 and members of the Myocardin family. Our results provide new insights into the genetic circuitry of muscle regeneration and identify MASTR as a central regulator of this process.

Shadow enhancers: frequently asked questions about distributed cis-regulatory information and enhancer redundancy

This paper, in the form of a frequently asked questions page (FAQ), addresses outstanding questions about "shadow enhancers", quasi-redundant cis-regulatory elements, and their proposed roles in transcriptional control. Questions include: What exactly are shadow enhancers? How many genes have shadow/redundant/distributed enhancers? How redundant are these elements? What is the function of distributed enhancers? How modular are enhancers? Is it useful to study a single enhancer in isolation? In addition, a revised definition of "shadow enhancers" is proposed, and possible mechanisms of shadow enhancer function and evolution are discussed.

A non-canonical E-box within the MyoD core enhancer is necessary for circadian expression in skeletal muscle

The myogenic differentiation 1 (MyoD) gene is a master regulator of myogenesis. We previously reported that the expression of MyoD mRNA oscillates over 24 h in skeletal muscle and that the circadian clock transcription factors, BMAL1 (brain and muscle ARNT-like 1) and CLOCK (circadian locomotor output cycles kaput), were bound to the core enhancer (CE) of the MyoD gene in vivo. In this study, we provide in vivo and in vitro evidence that the CE is necessary for circadian expression of MyoD in adult muscle. Gel shift assays identified a conserved non-canonical E-box within the CE that is bound by CLOCK and BMAL1. Functional analysis revealed that this E-box was required for full activation by BMAL1/CLOCK and for in vitro circadian oscillation. Expression profiling of muscle of CE(loxP/loxP) mice found approximately 1300 genes mis-expressed relative to wild-type. Based on the informatics results, we analyzed the respiratory function of mitochondria isolated from wild-type and CE(loxP/loxP) mice. These assays determined that State 5 respiration was significantly reduced in CE(loxP/loxP) muscle. The results of this work identify a novel element in the MyoD enhancer that confers circadian regulation to MyoD in skeletal muscle and suggest that loss of circadian regulation leads to changes in myogenic expression and downstream mitochondrial function.

Stress-induced C/EBP homology protein (CHOP) represses MyoD transcription to delay myoblast differentiation

When mouse myoblasts or satellite cells differentiate in culture, the expression of myogenic regulatory factor, MyoD, is downregulated in a subset of cells that do not differentiate. The mechanism involved in the repression of MyoD expression remains largely unknown. Here we report that a stress-response pathway repressing MyoD transcription is transiently activated in mouse-derived C2C12 myoblasts growing under differentiation-promoting conditions. We show that phosphorylation of the α subunit of the translation initiation factor 2 (eIF2α) is followed by expression of C/EBP homology protein (CHOP) in some myoblasts. ShRNA-driven knockdown of CHOP expression caused earlier and more robust differentiation, whereas its constitutive expression delayed differentiation relative to wild type myoblasts. Cells expressing CHOP did not express the myogenic regulatory factors MyoD and myogenin. These results indicated that CHOP directly repressed the transcription of the MyoD gene. In support of this view, CHOP associated with upstream regulatory region of the MyoD gene and its activity reduced histone acetylation at the enhancer region of MyoD. CHOP interacted with histone deacetylase 1 (HDAC1) in cells. This protein complex may reduce histone acetylation when bound to MyoD regulatory regions. Overall, our results suggest that the activation of a stress pathway in myoblasts transiently downregulate the myogenic program.

MyoD gene suppression by Oct4 is required for reprogramming in myoblasts to produce induced pluripotent stem cells

Expression of the four transcription factors, that is, Oct4, Sox2, cMyc, and Klf4 has been shown to generate induced pluripotent stem cells (iPSCs) from many types of specialized differentiated somatic cells. It remains unclear, however, whether fully committed skeletal muscle progenitor cells (myoblasts) have the potency to undergo reprogramming to develop iPSCs in line with previously reported cases. To test this, we have isolated genetically marked myoblasts derived from satellite cell of adult mouse muscles using the Cre-loxP system (Pax7-CreER:R26R and Myf5-Cre:R26R). On infection with retroviral vectors expressing the four factors, these myoblasts gave rise to myogenic lineage tracer lacZ-positive embryonic stem cell (ESC)-like colonies. These cells expressed ESC-specific genes and were competent to differentiate into all three germ layers and germ cells, indicating the successful generation of myoblast-derived iPSCs. Continuous expression of the MyoD gene, a master transcription factor for skeletal muscle specification, inhibited this reprogramming process in myoblasts. In contrast, reprogramming myoblasts isolated from mice lacking the MyoD gene led to an increase in reprogramming efficiency. Our data also indicated that Oct4 acts as a transcriptional suppressor of MyoD gene expression through its interaction with the upstream enhancer region. Taken together, these results indicate that suppression of MyoD gene expression by Oct4 is required for the initial reprogramming step in the development of iPSCs from myoblasts. This data suggests that the skeletal muscle system provides a well-defined differentiation model to further elaborate on the effects of iPSC reprogramming in somatic cells.

CLOCK and BMAL1 regulate MyoD and are necessary for maintenance of skeletal muscle phenotype and function

MyoD, a master regulator of myogenesis, exhibits a circadian rhythm in its mRNA and protein levels, suggesting a possible role in the daily maintenance of muscle phenotype and function. We report that MyoD is a direct target of the circadian transcriptional activators CLOCK and BMAL1, which bind in a rhythmic manner to the core enhancer of the MyoD promoter. Skeletal muscle of Clock(Δ19) and Bmal1(-/-) mutant mice exhibited ∼30% reductions in normalized maximal force. A similar reduction in force was observed at the single-fiber level. Electron microscopy (EM) showed that the myofilament architecture was disrupted in skeletal muscle of Clock(Δ19), Bmal1(-/-), and MyoD(-/-) mice. The alteration in myofilament organization was associated with decreased expression of actin, myosins, titin, and several MyoD target genes. EM analysis also demonstrated that muscle from both Clock(Δ19) and Bmal1(-/-) mice had a 40% reduction in mitochondrial volume. The remaining mitochondria in these mutant mice displayed aberrant morphology and increased uncoupling of respiration. This mitochondrial pathology was not seen in muscle of MyoD(-/-) mice. We suggest that altered expression of both Pgc-1α and Pgc-1β in Clock(Δ19) and Bmal1(-/-) mice may underlie this pathology. Taken together, our results demonstrate that disruption of CLOCK or BMAL1 leads to structural and functional alterations at the cellular level in skeletal muscle. The identification of MyoD as a clock-controlled gene provides a mechanism by which the circadian clock may generate a muscle-specific circadian transcriptome in an adaptive role for the daily maintenance of adult skeletal muscle.

Pitx2 defines alternate pathways acting through MyoD during limb and somitic myogenesis

The MyoD gene is part of the core regulatory network that governs skeletal myogenesis and acts as an essential determinant of the myogenic cell fate. Although generic regulatory networks converging on this gene have been described, the specific mechanisms leading to MyoD expression in muscles of different ontology remain misunderstood. We now show that the homeobox gene Pitx2 is required for initial activation of the MyoD gene in limb muscle precursors through direct binding of Pitx2 to the MyoD core enhancer. Whereas Myf5 and Mrf4 are dispensable for limb muscle progenitor fate, inactivation of Myf5 and Mrf4 in Pitx2 mutants results in a drastic decrease of limb MyoD expression. Thus, Pitx2 and Myf5 define parallel genetic pathways for limb myogenesis. We show a similar dependence on Pitx2 and Myf5(Mrf4) in myotome, where MyoD expression is initially activated by Myf5 and Mrf4. In their absence, MyoD expression is eventually rescued by a Pax3-dependent mechanism. We now provide evidence that Pitx2 contributes to the rescue of MyoD expression and that it acts downstream of Pax3. We thus propose that myogenic differentiation of somite-derived muscle cells relies on two parallel genetic pathways, with the Pitx2 pathway being of primary importance for limb myogenesis but the Myf5 and Mrf4 pathway predominating in myotome. Muscle-specific wiring of regulatory networks composed of similar transcription factors thus underlies development of distinct skeletal muscles.

Regulation of gene expression in vertebrate skeletal muscle

During embryonic development the integration of numerous synergistic signalling pathways turns a single cell into a multicellular organism with specialized cell types and highly structured, organized tissues. To achieve this, cells must grow, proliferate, differentiate and die according to their spatiotemporal position. Unravelling the mechanisms by which a cell adopts the correct fate in response to its local environment remains one of the fundamental goals of biological research. In vertebrates skeletal myogenesis is coordinated by the activation of the myogenic regulatory factors (MRFs) in response to signals that are interpreted by their associated regulatory elements in different precursor cells during development. The MRFs trigger a cascade of transcription factors and downstream structural genes, ultimately resulting in the generation of one of the fundamental histotypes. In this review we discuss the regulation of the different MRFs in relation to their position in the myogenic cascade, the changes in the general transcriptional machinery during muscle differentiation and the emerging importance of miRNA regulation in skeletal myogenesis.

Distinct enhancers at the Pax3 locus can function redundantly to regulate neural tube and neural crest expressions

Pax3 is a transcription factor expressed in somitic mesoderm, dorsal neural tube and pre-migratory neural crest during embryonic development. We have previously identified cis-acting enhancer elements within the proximal upstream genomic region of Pax3 that are sufficient to direct functional expression of Pax3 in neural crest. These elements direct expression of a reporter gene to pre-migratory neural crest in transgenic mice, and transgenic expression of a Pax3 cDNA using these elements is sufficient to rescue neural crest development in mice otherwise lacking endogenous Pax3. We show here that deletion of these enhancer sequences by homologous recombination is insufficient to abrogate neural crest expression of Pax3 and results in viable mice. We identify a distinct enhancer in the fourth intron that is also capable of mediating neural crest expression in transgenic mice and zebrafish. Our analysis suggests the existence of functionally redundant neural crest enhancer modules for Pax3.

Non-canonical role for the TRAIL receptor DR5/FADD/caspase pathway in the regulation of MyoD expression and skeletal myoblast differentiation

We report herein that the TRAIL receptor DR5/FADD/caspase pathway plays a role in skeletal myoblast differentiation through modulation of the expression of the muscle regulatory transcription factor MyoD. Specifically, treatment with the selective caspase 3 inhibitor DEVD-fmk or the selective caspase 8 inhibitor IETD-fmk in growth media (GM), prior to culture in differentiation media (DM), inhibited differentiation. Further, this treatment resulted in decreased levels of MyoD message and protein. We next explored a role for the TRAIL receptor DR5/FADD pathway. We found that expression of either dominant negative (dn) FADD or dominant negative (dn) DR5 also resulted in decreased levels of MyoD mRNA and protein and blocked differentiation. This decreased level of MyoD mRNA was not a consequence of altered stability. Treatment with TSA, an inhibitor of histone deacetylases (HDACs), allowed MyoD expression in myoblasts expressing dnDR5. Finally, acetylation of histones associated with the distal regulatory region (DRR) enhancer of MyoD was decreased in myoblasts expressing dnDR5. Thus, our data suggests a non-canonical role for the TRAIL receptor/FADD pathway in the regulation of MyoD expression and skeletal myoblast differentiation.

Codependent activators direct myoblast-specific MyoD transcription

Although FoxO and Pax proteins represent two important families of transcription factors in determining cell fate, they had not been functionally or physically linked together in mediating regulation of a common target gene during normal cellular transcription programs. Here, we identify MyoD, a key regulator of myogenesis, as a direct target of FoxO3 and Pax3/7 in myoblasts. Our cell-based assays and in vitro studies reveal a tight codependent partnership between FoxO3 and Pax3/7 to coordinately recruit RNA polymerase II and form a preinitiation complex (PIC) to activate MyoD transcription in myoblasts. The role of FoxO3 in regulating muscle differentiation is confirmed in vivo by observed defects in muscle regeneration caused by MyoD downregulation in FoxO3 null mice. These data establish a mutual interdependence and functional link between two families of transcription activators serving as potential signaling sensors and regulators of cell fate commitment in directing tissue specific MyoD transcription.

MyoD- and nerve-dependent maintenance of MyoD expression in mature muscle fibres acts through the DRR/PRR element

BACKGROUND

MyoD is a transcription factor implicated in the regulation of adult muscle gene expression. Distinguishing the expression of MyoD in satellite myoblasts and muscle fibres has proved difficult in vivo leading to controversy over the significance of MyoD expression within adult innervated muscle fibres. Here we employ the MD6.0-lacZ transgenic mouse, in which the 6 kb proximal enhancer/promoter (DRR/PRR) of MyoD drives lacZ, to show that MyoD is present and transcriptionally active in many adult muscle fibres.

RESULTS

In culture, MD6.0-lacZ expresses in myotubes but not myogenic cells, unlike endogenous MyoD. Reporter expression in vivo is in muscle fibre nuclei and is reduced in MyoD null mice. The MD6.0-lacZ reporter is down-regulated both in adult muscle fibres by denervation or muscle disuse and in cultured myotubes by inhibition of activity. Activity induces and represses MyoD through the DRR and PRR, respectively. During the postnatal period, accumulation of beta-galactosidase correlates with maturation of innervation. Strikingly, endogenous MyoD expression is up-regulated in fibres by complete denervation, arguing for a separate activity-dependent suppression of MyoD requiring regulatory elements outside the DRR/PRR.

CONCLUSION

The data show that MyoD regulation is more complex than previously supposed. Two factors, MyoD protein itself and fibre activity are required for essentially all expression of the 6 kb proximal enhancer/promoter (DRR/PRR) of MyoD in adult fibres. We propose that modulation of MyoD positive feedback by electrical activity determines the set point of MyoD expression in innervated fibres through the DRR/PRR element.

The RhoA effector mDiaphanous regulates MyoD expression and cell cycle progression via SRF-dependent and SRF-independent pathways

Expression of the key muscle transcription factor MyoD is regulated by RhoA GTPase, which is an important regulator of adhesion-dependent signaling. We show that mDiaphanous (mDia)--an adaptor protein that mediates the effects of RhoA on cell motility and the cytoskeleton--is an upstream regulator of MyoD in C2C12 mouse myoblasts. Knockdown of mDia1 reduced MyoD expression and proliferation via a serum-response factor (SRF)-dependent pathway. Surprisingly, overexpression of a Rho-independent form of mDia1 (mDiaDeltaN3), despite activating SRF, also suppressed MyoD and the cell cycle, suggesting the presence of a second pathway downstream of mDia1. We present evidence that the alternative pathway by which mDia1 regulates MyoD involves T-cell factor (TCF)/lymphoid enhancer factor (LEF) and its co-activator, beta-catenin. TCF activity was suppressed by mDiaDeltaN3 and induced by silencing mDia. mDiaDeltaN3 disrupted the signal-dependent nuclear localization of beta-catenin and suppressed MyoD expression. Co-expression of a degradation-resistant form of beta-catenin with mDiaDeltaN3 restored MyoD expression, suggesting a mechanistic link between the two signaling proteins. We also implicate a region encompassing the FH1 domain of mDia1 in beta-catenin-TCF regulation. Taken together, our results suggest that a balance between two pathways downstream of mDia regulates MyoD expression and cell cycle progression.

Identification of a new hybrid serum response factor and myocyte enhancer factor 2-binding element in MyoD enhancer required for MyoD expression during myogenesis

MyoD is a critical myogenic factor induced rapidly upon activation of quiescent satellite cells, and required for their differentiation during muscle regeneration. One of the two enhancers of MyoD, the distal regulatory region, is essential for MyoD expression in postnatal muscle. This enhancer contains a functional divergent serum response factor (SRF)-binding CArG element required for MyoD expression during myoblast growth and muscle regeneration in vivo. Electrophoretic mobility shift assay, chromatin immunoprecipitation, and microinjection analyses show this element is a hybrid SRF- and MEF2 Binding (SMB) sequence where myocyte enhancer factor 2 (MEF2) complexes can compete out binding of SRF at the onset of differentiation. As cells differentiate into postmitotic myotubes, MyoD expression no longer requires SRF but instead MEF2 binding to this dual-specificity element. As such, the MyoD enhancer SMB element is the site for a molecular relay where MyoD expression is first initiated in activated satellite cells in an SRF-dependent manner and then increased and maintained by MEF2 binding in differentiated myotubes. Therefore, SMB is a DNA element with dual and stage-specific binding activity, which modulates the effects of regulatory proteins critical in controlling the balance between proliferation and differentiation.

Signaling to the chromatin during skeletal myogenesis: Novel targets for pharmacological modulation of gene expression

Cellular differentiation entails an extensive reprogramming of the genome toward the expression of discrete subsets of genes, which establish the tissue-specific phenotype. This program is achieved by epigenetic marks of the chromatin at particular loci, and is regulated by environmental cues, such as soluble factors and cell-to-cell interactions. How the intracellular cascades convert the myriad of external stimuli into the nuclear information necessary to reprogram the genome toward specific responses is a question of biological and medical interest. The elucidation of the signaling converting cues from outside the cells into chromatin modifications at individual promoters holds the promise to unveil the targets for selective pharmacological interventions to modulate gene expression for therapeutic purposes. Enhancing muscle regeneration and preventing muscle breakdown are important goals in the therapy of muscular diseases, cancer-associated cachexia and aging-associated sarcopenia. We will summarize the recent progress of our knowledge of the regulation of gene expression by intracellular cascades elicited by external cues during skeletal myogenesis. And will illustrate the potential importance of targeting the chromatin signaling in regenerative medicine--e.g. to boost muscle regeneration.

MyoD-cre transgenic mice: a model for conditional mutagenesis and lineage tracing of skeletal muscle

The Cre-loxP recombination system has been used to great advantage in vivo for conditional gene targeting, lineage tracing, and other applications. To express cre in skeletal myoblasts and muscle fibers, we utilized the well-characterized transcriptional regulatory regions of the muscle determination gene, MyoD. Transgenic mouse lines were produced (F3/-2.5cre) in which the cre gene is driven by the MyoD promoter and core enhancer, which directs the early activation of MyoD. Specificity of cre expression and efficiency of recombination was determined by monitoring reporter gene expression after crossing to the Cre-dependent reporter lines, R26R and Z/AP. Efficient labeling of embryonic and fetal myoblasts and muscle fibers was observed, with timing that was similar (branchial arches and limb buds) or slightly delayed (myotomes) relative to the endogenous MyoD gene. In satellite cell cultures, a strict concordance between MyoD protein and reporter gene expression was observed, demonstrating the muscle specificity and efficiency of Cre-mediated recombination. Nascent muscle fibers were labeled following injury of adult muscle, indicating recombination in satellite cells or their daughters in vivo.

The scl +18/19 stem cell enhancer is not required for hematopoiesis: identification of a 5' bifunctional hematopoietic-endothelial enhancer bound by Fli-1 and Elf-1

Analysis of cis-regulatory elements is central to understanding the genomic program for development. The scl/tal-1 transcription factor is essential for lineage commitment to blood cell formation and previous studies identified an scl enhancer (the +18/19 element) which was sufficient to target the vast majority of hematopoietic stem cells, together with hematopoietic progenitors and endothelium. Moreover, expression of scl under control of the +18/19 enhancer rescued blood progenitor formation in scl(-/-) embryos. However, here we demonstrate by using a knockout approach that, within the endogenous scl locus, the +18/19 enhancer is not necessary for the initiation of scl transcription or for the formation of hematopoietic cells. These results led to the identification of a bifunctional 5' enhancer (-3.8 element), which targets expression to hematopoietic progenitors and endothelium, contains conserved critical Ets sites, and is bound by Ets family transcription factors, including Fli-1 and Elf-1. These data demonstrate that two geographically distinct but functionally related enhancers regulate scl transcription in hematopoietic progenitors and endothelial cells and suggest that enhancers with dual hematopoietic-endothelial activity may represent a general strategy for regulating blood and endothelial development.

The core enhancer is essential for proper timing of MyoD activation in limb buds and branchial arches

Transgenic analyses have defined two transcriptional enhancers that regulate MyoD expression in mammals, the core enhancer and distal regulatory region; these enhancers exhibit complementary activities and together are sufficient to recapitulate MyoD expression in developing and mature skeletal muscle. The core enhancer is activated in presumptive muscle cells and determined myoblasts, suggesting an important role in initiating MyoD expression. Here, targeted mutagenesis in the mouse is used to identify necessary and redundant core enhancer functions. The core enhancer is essential for the timely initiation of MyoD expression in limb buds and branchial arches, as enhancer deletion delayed MyoD activation by 1 to 2 days in these muscle lineages. Functionally, this delay in MyoD transcription delayed the onset of muscle differentiation, as assayed by expression of the gene encoding for the early differentiation marker, Myogenin. In addition to these lineage-specific defects, a generalized, modest reduction in MyoD expression was observed in all muscle lineages and at all embryonic stages examined. Interestingly, however, a specific defect was not observed in the nascent myocytes at the medial and lateral aspects of the myotome, suggesting the existence of at least one other enhancer with this specificity. The core enhancer was also dispensable for Myf-5- and Pax-3-dependent regulation of MyoD transcription. These data demonstrate a differential requirement for core enhancer activity in muscle lineages derived from migratory precursors and suggest redundancy in cis regulatory mechanisms controlling myotomal MyoD expression.

Myogenic regulatory factors and the specification of muscle progenitors in vertebrate embryos

Embryological and genetic studies of mouse, bird, zebrafish, and frog embryos are providing new insights into the regulatory functions of the myogenic regulatory factors, MyoD, Myf5, Myogenin, and MRF4, and the transcriptional and signaling mechanisms that control their expression during the specification and differentiation of muscle progenitors. Myf5 and MyoD genes have genetically redundant, but developmentally distinct regulatory functions in the specification and the differentiation of somite and head muscle progenitor lineages. Myogenin and MRF4 have later functions in muscle differentiation, and Pax and Hox genes coordinate the migration and specification of somite progenitors at sites of hypaxial and limb muscle formation in the embryo body. Transcription enhancers that control Myf5 and MyoD activation in muscle progenitors and maintain their expression during muscle differentiation have been identified by transgenic analysis. In epaxial, hypaxial, limb, and head muscle progenitors, Myf5 is controlled by lineage-specific transcription enhancers, providing evidence that multiple mechanisms control progenitor specification at different sites of myogenesis in the embryo. Developmental signaling ligands and their signal transduction effectors function both interactively and independently to control Myf5 and MyoD activation in muscle progenitor lineages, likely through direct regulation of their transcription enhancers. Future investigations of the signaling and transcriptional mechanisms that control Myf5 and MyoD in the muscle progenitor lineages of different vertebrate embryos can be expected to provide a detailed understanding of the developmental and evolutionary mechanisms for anatomical muscles formation in vertebrates. This knowledge will be a foundation for development of stem cell therapies to repair diseased and damaged muscles.

MyoD distal regulatory region contains an SRF binding CArG element required for MyoD expression in skeletal myoblasts and during muscle regeneration

We show here that the distal regulatory region (DRR) of the mouse and human MyoD gene contains a conserved SRF binding CArG-like element. In electrophoretic mobility shift assays with myoblast nuclear extracts, this CArG sequence, although slightly divergent, bound two complexes containing, respectively, the transcription factor YY1 and SRF associated with the acetyltransferase CBP and members of C/EBP family. A single nucleotide mutation in the MyoD-CArG element suppressed binding of both SRF and YY1 complexes and abolished DRR enhancer activity in stably transfected myoblasts. This MyoD-CArG sequence is active in modulating endogeneous MyoD gene expression because microinjection of oligonucleotides corresponding to the MyoD-CArG sequence specifically and rapidly suppressed MyoD expression in myoblasts. In vivo, the expression of a transgenic construct comprising a minimal MyoD promoter fused to the DRR and beta-galactosidase was induced with the same kinetics as MyoD during mouse muscle regeneration. In contrast induction of this reporter was no longer seen in regenerating muscle from transgenic mice carrying a mutated DRR-CArG. These results show that an SRF binding CArG element present in MyoD gene DRR is involved in the control of MyoD gene expression in skeletal myoblasts and in mature muscle satellite cell activation during muscle regeneration.

Divergence in species and regulatory role of beta -myosin heavy chain proximal promoter muscle-CAT elements

We examined the functional role of distinct muscle-CAT (MCAT) elements during non-weight-bearing (NWB) regulation of a wild-type 293-base pair beta-myosin heavy chain (beta MyHC) transgene. Electrophoretic mobility shift assays (EMSA) revealed decreased NTEF-1, poly(ADP-ribose) polymerase, and Max binding at the human distal MCAT element when using NWB soleus vs. control soleus nuclear extract. Compared with the wild-type transgene, expression assays revealed that distal MCAT element mutation decreased basal transgene expression, which was decreased further in response to NWB. EMSA analysis of the human proximal MCAT (pMCAT) element revealed low levels of NTEF-1 binding that did not differ between control and NWB extract, whereas the rat pMCAT element displayed robust NTEF-1 binding that decreased when using NWB soleus extracts. Differences in binding between human and rat pMCAT elements were consistent whether using rat or mouse nuclear extract or in vitro synthesized human TEF-1 proteins. Our results provide the first evidence that 1) different binding properties and likely regulatory functions are served by the human and rat pMCAT elements, and 2) previously unrecognized beta MyHC proximal promoter elements contribute to NWB regulation.