Expression of MRF4, a myogenic helix-loop-helix protein, produces multiple changes in the myogenic program of BC3H-1 cells

Mechanisms of Binding Specificity among bHLH Transcription Factors

The transcriptome of every cell is orchestrated by the complex network of interaction between transcription factors (TFs) and their binding sites on DNA. Disruption of this network can result in many forms of organism malfunction but also can be the substrate of positive natural selection. However, understanding the specific determinants of each of these individual TF-DNA interactions is a challenging task as it requires integrating the multiple possible mechanisms by which a given TF ends up interacting with a specific genomic region. These mechanisms include DNA motif preferences, which can be determined by nucleotide sequence but also by DNA’s shape; post-translational modifications of the TF, such as phosphorylation; and dimerization partners and co-factors, which can mediate multiple forms of direct or indirect cooperative binding. Binding can also be affected by epigenetic modifications of putative target regions, including DNA methylation and nucleosome occupancy. In this review, we describe how all these mechanisms have a role and crosstalk in one specific family of TFs, the basic helix-loop-helix (bHLH), with a very conserved DNA binding domain and a similar DNA preferred motif, the E-box. Here, we compile and discuss a rich catalog of strategies used by bHLH to acquire TF-specific genome-wide landscapes of binding sites.

Rewiring of an ancestral Tbx1/10-Ebf-Mrf network for pharyngeal muscle specification in distinct embryonic lineages

Skeletal muscles arise from diverse embryonic origins in vertebrates, yet converge on extensively shared regulatory programs that require muscle regulatory factor (MRF)-family genes. Myogenesis in the tail of the simple chordate Ciona exhibits a similar reliance on its single MRF-family gene, and diverse mechanisms activate Ci-Mrf Here, we show that myogenesis in the atrial siphon muscles (ASMs) and oral siphon muscles (OSMs), which control the exhalant and inhalant siphons, respectively, also requires Mrf We characterize the ontogeny of OSM progenitors and compare the molecular basis of Mrf activation in OSM versus ASM. In both muscle types, Ebf and Tbx1/10 are expressed and function upstream of Mrf However, we demonstrate that regulatory relationships between Tbx1/10, Ebf and Mrf differ between the OSM and ASM lineages. We propose that Tbx1, Ebf and Mrf homologs form an ancient conserved regulatory state for pharyngeal muscle specification, whereas their regulatory relationships might be more evolutionarily variable.

Ku70 regulates Bax-mediated pathogenesis in laminin-alpha2-deficient human muscle cells and mouse models of congenital muscular dystrophy

The severely debilitating disease Congenital Muscular Dystrophy Type 1A (MDC1A) is caused by mutations in the gene encoding laminin-alpha2. Bax-mediated muscle cell death is a significant contributor to the severe neuromuscular pathology seen in the Lama2-null mouse model of MDC1A. To extend our understanding of pathogenesis due to laminin-alpha2-deficiency, we have now analyzed molecular mechanisms of Bax regulation in normal and laminin-alpha2-deficient muscles and cells, including myogenic cells obtained from patients with a clinical diagnosis of MDC1A. In mouse myogenic cells, we found that, as in non-muscle cells, Bax co-immunoprecipitated with the multifunctional protein Ku70. In addition, cell permeable pentapeptides designed from Ku70, termed Bax-inhibiting peptides (BIPs), inhibited staurosporine-induced Bax translocation and cell death in mouse myogenic cells. We also found that acetylation of Ku70, which can inhibit binding to Bax and can be an indicator of increased susceptibility to cell death, was more abundant in Lama2-null than in normal mouse muscles. Furthermore, myotubes formed in culture from human laminin-alpha2-deficient patient myoblasts produced high levels of activated caspase-3 when grown on poly-L-lysine, but not when grown on a laminin-alpha2-containing substrate or when treated with BIPs. Finally, cytoplasmic Ku70 in human laminin-alpha2-deficient myotubes was both reduced in amount and more highly acetylated than in normal myotubes. Increased susceptibility to cell death thus appears to be an intrinsic property of human laminin-alpha2-deficient myotubes. These results identify Ku70 as a regulator of Bax-mediated pathogenesis and a therapeutic target in laminin-alpha2-deficiency.

Regeneration of transgenic skeletal muscles with altered timing of expression of the basic helix-loop-helix muscle regulatory factor MRF4

In regenerating muscle cells, muscle regulatory factor (MRF) 4 is normally the last of the four MRFs to be expressed. To analyze how the timing of MRF4 expression affects muscle regeneration, we compared regeneration after local freeze injury of muscles from wild-type mice with muscles from transgenic mice in which MRF4 expression was under control of an approximately 1.6-kb fragment of the myogenin promoter. Three days after injury, masseter and tibialis anterior (TA) muscles in wild-type mice expressed little or no MRF4 mRNA; whereas these muscles in transgenic mice expressed abundant MRF4 mRNA from both the transgene and the endogenous gene. Thus, MRF4 up-regulation was accelerated in transgenic compared to wild-type regenerating muscles, and expression of the transgene appeared to activate, perhaps indirectly, expression of the endogenous MRF4 gene. At 11 days after injury, regeneration, as measured by cross-sectional area and density of regenerated fibers, was significantly impaired in transgenic TA compared to wild-type TA, whereas at 19 days after injury both transgenic and TA muscle fibers had fully recovered to preinjury values. Regeneration of masseter muscles, which normally regenerate much less completely than TA muscles, was unaffected by the transgene. Thus, the timing of MRF4 up-regulation, as well as additional muscle-specific factors, can determine the progress of muscle regeneration.

Further characterization of BC3H1 myogenic cells reveals lack of p53 activity and underexpression of several p53 regulated and extracellular matrix-associated gene products

To catalog factors that may contribute to the completion of myogenesis, we have been looking for molecular differences between BC3H1 and C2C12 cells. Cells of the BC3H1 tumor line, though myogenic, are nonfusing, and withdraw from the cell cycle only reversibly, whereas cells of the C2C12 line fuse, differentiate terminally, and express several muscle-specific gene products that BC3H1 cells do not. Relative to C2C12 cells, BC3H1 cells underaccumulated cyclin-dependent kinase inhibitor p21 and underaccumulated transcripts for p21, GADD45, CDO, decorin, osteopontin, H19, fibronectin, and thrombospondin-1 (tsp-1). Levels of accumulation of H19, tsp-1, and larger isoforms of fibronectin messenger ribonucleic acid (mRNA) were found to increase in response to expression of myogenic regulatory factors as shown by their accumulation in differentiated myogenically converted 10T1/2 cells but not in 10T1/2 fibroblasts. BC3H1s accumulated a temperature-insensitive, geldanamycin-sensitive, misfolded form of p53 incapable of transactivating a p53 responsive reporter, consistent with underexpression of p21, GADD45, and tsp-1. BC3H1 and C2C12 cells were similar with respect to upregulation of p27 protein, downregulation of mitogen-activated protein kinase phosphatase-1 (MKP-1) protein, upregulation of retinoblastoma (Rb) mRNA, and nuclear localization of hypophosphorylated Rb. Cells of both lines expressed the muscle-specific 1b isoform of MEF2D. Although nonfusing in the short term, after more than 18 d in differentiation medium, some cultures of BC3H1 cells formed viable multinucleated cells in which the nuclei did not reinitiate synthesis of DNA in response to serum. Our findings suggest participation of tsp-1 and specific isoforms of fibronectin in myogenesis and suggest additional avenues of research in myogenesis and oncogenesis.

Bc3h1 myogenic cells produce an infectious ecotropic murine leukemia virus

cDNAs representing an endogenous C-type ecotropic murine leukemia virus were isolated from a cDNA library constructed to represent mRNAs present in BC3H1 myogenic cells but not in C2C12 myogenic cells. RNA blot hybridization analysis using the cDNA inserts as probes revealed that BC3H1 cells produce MuLV-related transcirpts of at least three different size classes. A polymerase chain reaction enhanced assay for reverse transcriptase activity revealed the presence of reverse transcriptase in a viral pellet from medium conditioned by BC3H1 cells. A fungizone enhanced assay for syncitium formation provided further evidence of ecotropic retroviral particle production. Exposure of 3T3 cells to medium conditioned by BC3H1 cells, using conditions that facilitate infection, resulted in infection of the 3T3 cells, as confirmed by the syncitium formation assay. We conclude that BC3H1 cells produce an infectious ecotropic murine leukemia virus. Whether or not this feature of BC3H1 cells contributes to their inability to express some muscle-specific genes or to carry out myotube formation is unknown. Investigators will want to take into account that BC3H1 cells are virus producers when planning experiments that involve coculture of BC3H1 with other cell types, BC3H1 conditioned medium, retrovirally mediated transfection into BC3H1 cells, or study of the mCAT-1 amino acid transporter (the viral receptor) in BC3H1 cells. BC3H1 cells and the virus they produce may be of interest to those studying retroviral genomes and products and their effects.

Molecular regulation of tongue and craniofacial muscle differentiation

The molecular regulation of muscle development is tightly controlled at three distinct stages of the process: determination, differentiation, and maturation. Developmentally, specific populations of myoblasts exhibit distinct molecular phenotypes that begin to limit the ultimate characteristics of the muscle fibers. The expression of the myogenic regulatory factor family of the transcription process plays a key role in muscle development and, ultimately, in the subset of contractile genes expressed in a specific muscle. Craniofacial muscles have distinct functional requirements and associated molecular phenotypes that distinguish them from other skeletal muscles. The general principles of muscle molecular differentiation with specific reference to craniofacial muscles, such as the tongue, are discussed in this review.

Vascular smooth muscle cells spontaneously adopt a skeletal muscle phenotype: a unique Myf5(-)/MyoD(+) myogenic program

Smooth and skeletal muscle tissues are composed of distinct cell types that express related but distinct isoforms of the structural genes used for contraction. These two muscle cell types are also believed to have distinct embryological origins. Nevertheless, the phenomenon of a phenotypic switch from smooth to skeletal muscle has been demonstrated in several in vivo studies. This switch has been minimally analyzed at the cellular level, and the mechanism driving it is unknown. We used immunofluorescence and RT-PCR to demonstrate the expression of the skeletal muscle-specific regulatory genes MyoD and myogenin, and of several skeletal muscle-specific structural genes in cultures of the established rat smooth muscle cell lines PAC1, A10, and A7r5. The skeletal muscle regulatory gene Myf5 was not detected in these three cell lines. We further isolated clonal sublines from PAC1 cultures that homogeneously express smooth muscle characteristics at low density and undergo a coordinated increase in skeletal muscle-specific gene expression at high density. In some of these PAC1 sublines, this process culminates in the high-frequency formation of myotubes. As in the PAC1 parental line, Myf5 was not expressed in the PAC1 sublines. We show that the PAC1 sublines that undergo a more robust transition into the skeletal muscle phenotype also express significantly higher levels of the insulin-like growth factor (IGF1 and IGF2) genes and of FGF receptor 4 (FGFR4) gene. Our results suggest that MyoD expression in itself is not a sufficient condition to promote a coordinated program of skeletal myogenesis in the smooth muscle cells. Insulin administered at a high concentration to PAC1 cell populations with a poor capacity to undergo skeletal muscle differentiation enhances the number of cells displaying the skeletal muscle differentiated phenotype. The findings raise the possibility that the IGF signaling system is involved in the phenotypic switch from smooth to skeletal muscle. The gene expression program described here can now be used to investigate the mechanisms that may underlie the propensity of certain smooth muscle cells to adopt a skeletal muscle identity.(J Histochem Cytochem 48:1173-1193, 2000)

Isolation of a spontaneously fusing BC3H1 muscle cell line: fusion alters the response to serum stimulation

Differentiation of skeletal muscle cells involves two distinct events: exit from the cell cycle and expression of muscle-specific contractile genes and formation of multinucleated myocytes. Although many studies have shown that growth factors regulate the initial step of differentiation, little is known about regulation of fusion. BC3H1 cells are a skeletal muscle cell line characterized by a nonfusing phenotype and an ability to dedifferentiate. When subjected to serum or growth factors, differentiated BC3H1 cells lose muscle-specific gene expression and re-enter the cell cycle. In this study, we describe a spontaneously fusing clone of BC3H1 cells. We demonstrate that this fusion capability is not due to altered muscle regulatory factor or adhesion molecule expression. Furthermore, we show that fusion inhibits dedifferentiation. Multinucleated BC3H1 cells do not lose myosin expression, nor do they re-enter the cell cycle. Fused BC3HI cells react to serum stimulation with a hypertrophic response. Our results suggest that the state of differentiation, mono- or multi-nucleated, is essential to how myocytes react to growth stimulation and may provide a mechanism for how differentiation, fusion, and hypertrophy are regulated in vivo.

Myogenic conversion of NIH3T3 cells by exogenous MyoD family members: dissociation of terminal differentiation from myotube formation

Myogenic regulatory factors (MRF) of the MyoD family regulate the skeletal muscle differentiation program. Non-muscle cells transfected with exogenous MRF either are converted to the myogenic lineage or fail to express the muscle phenotype, depending on the cell type analysed. We report here that MRF-induced myogenic conversion of NIH3T3 cells results in an incomplete reprogramming of these cells. Transfected cells withdrew from the cell cycle and underwent biochemical differentiation but, surprisingly, terminally differentiated myocytes absolutely failed to fuse into multinucleated myotubes. Analysis of muscle regulatory and structural gene expression failed to provide an explanation for the fusion defectiveness. However, myogenic derivatives of NIH3T3 cells were shown to be unable to accumulate the transcripts encoding muscle-specific isoforms of the integrin subunit beta1D and the transcription factor MEF2D1b2, that depend on muscle-specific alternative splicing. Our results suggest that the fusion into myotubes is under a distinct genetic control that might depend, at least partially, on differential splicing.

MRF4 can substitute for myogenin during early stages of myogenesis

MRF4, myogenin, MyoD, and Myf-5 are the four members of the basic helix-loop-helix family of muscle-specific regulatory factors (MRFs). We examined whether MRF4 could substitute for myogenin in vivo by determining if the myofiber- and MRF4-deficient phenotype of myogenin (-/-) mice could be rescued by a myogenin promoter-MRF4 transgene. When the transgene was expressed at a physiological level in myogenin-deficient fetuses, we found that expression of the endogenous MRF4 gene was restored to normal levels, whereas MyoD levels were unchanged. Thus, MRF4 can participate in a positive autoregulatory loop and can substitute for myogenin to activate its own promoter. Myogenin-deficient fetuses that expressed the transgene also had more myosin, more and larger myofibers, and a more normal ribcage morphology than myogenin-deficient littermates without the transgene. The transgene failed, however, to restore normal numbers of myofibers or viability to myogenin-deficient mice, because the approximately 1.6 kb myogenin promoter fragment was not expressed in most late-forming myofibers. These results demonstrate that MRF4 is able to substitute for myogenin to activate MRF4 expression and promote myofiber formation during the early stages of myogenesis.

Acceleration of somitic myogenesis in embryos of myogenin promoter-MRF4 transgenic mice

The four muscle regulatory factors (MRFs) of the MyoD family are expressed in distinct temporal and spatial patterns in developing somites. To examine MRF function and regulation in somites, we generated myogenin promoter-MRF4 transgenic mice in which MRF4 was expressed in rostral somites about a half day earlier than normal. We found that the transgene, which was expressed at about the same level as endogenous MRFs, did not noticeably alter developing or adult mice, whereas the rostral somites of transgenic embryos showed accelerated myocyte formation, as well as precocious expression of the endogenous MRF4 gene. In an individual transgenic somite, MRF4 was expressed in both presumptive myotomal (mesenchymal) and dermatomal (epithelial) cells. Transgenic dermatomal cells also contained myogenin, which is expressed early in myogenesis, but did not contain myosin, which is expressed late in myogenesis. In transgenic myotomal cells, in contrast, precocious expression of MRF4 accelerated late events in myogenesis, including myosin expression and striated myofibril formation. MRF function, therefore, appears to be differentially regulated in dermatomal and myotomal cells.

POU homeodomain genes and myogenesis

We show that members of the POU homeodomain family are among the transcription factors expressed in developing mouse skeletal muscle. From a cDNA library prepared from fetal muscle mRNA, we cloned a cDNA identical to that of Brn-4, a POU class II gene previously cloned from neural tissues. In limb muscle, we found that Brn-4 mRNA expression was highest at embryonic days 15-18, declined-after birth, and was undetectable in adults. The mRNAs of two additional POU genes, Emb (POU class VI) and Oct-1 (POU class II), were also expressed in developing muscle and, unlike Brn-4, continued to be expressed in postnatal and adult muscles. In skeletal muscle, expression of Brn-4 is myogenin-dependent, because muscles from myogenin-deficient fetuses contained much less Brn-4 mRNA than muscles from myogenin-expressing littermates. In contrast, expression of Emb was the same in the presence or absence of myogenin. The distinct pattern of Brn-4 mRNA expression and its dependence on a myogenic regulatory factor suggest that Brn-4 is part of the network of interacting transcription factors that control muscle-specific gene expression during mammalian myogenesis.

Use of p112-deficient myoblasts to determine the temporal order of the in vitro expression of myogenic components

The present investigation examines the function and site(s) of involvement of an ecto-protein kinase and its substrate protein (a cell surface 112 kDa protein) in the in vitro myogenic pathway. The phosphorylated 112 kDa protein (p112) has recently been shown to be involved in myogenesis. Not much information is currently available on the role of the ecto-protein kinase and the 112 kDa protein in modulating the expression of the myogenic factors and various muscle-specific proteins. Five different p112-deficient rat myoblasts were used to examine the temporal order of the in vitro expression of the myogenic components; namely, L6 myoblasts treated with BrdUrd or phloretin, a conditional p112-defective mutant (clone D1), an ecto-protein kinase-deficient mutant (clone F72), and a mutant defective in the 112 kDa protein (clone D1/S4). All these p112-deficient myoblasts were also impaired in myogenesis. The absence of p112, ecto-protein kinase, and/or the 112 kDa protein was found to have no effect on the Myf-5 mRNA level. However, the expected increase in NCAM and Myf-4 mRNAs was not observed in any of the p112-deficient myoblasts examined. This suggests that the p112 site of action is probably located upstream of the Myf-4 and NCAM sites in the myogenic pathway. While 7-28 fold increases in the MLC, MHC, and TnT transcripts were observed during myogenesis, such increases were not observed in the p112-deficient myoblasts. However, when mutant D1/S4 was transfected with the myf-4 cDNA, expression of Myf-4 in the transfectant resulted in increased level of the MLC, MHC, and TnT mRNAs, and in myotube formation, even though the Myf-5 and NCAM mRNA levels and p112 were not altered. This suggests that p112 may function by activating transcription of Myf-4, which will subsequently promote the expression of muscle-specific proteins and myotube formation. In the absence of p112, Myf-5 cannot activate the expression of Myf-4, NCAM, MLC, MHC, TnT, and myotube formation. If all these components are involved in the same myogenic pathway, then p112 may be acting downstream from Myf-5, and upstream from NCAM and Myf-4.

Cellular and molecular diversity in skeletal muscle development: news from in vitro and in vivo

Skeletal muscle formation is studied in vitro with myogenic cell lines and primary muscle cell cultures, and in vivo with embryos of several species. We review several of the notable advances obtained from studies of cultured cells, including the recognition of myoblast diversity, isolation of the MyoD family of muscle regulatory factors, and identification of promoter elements required for muscle-specific gene expression. These studies have led to the ideas that myoblast diversity underlies the formation of the multiple types of fast and slow muscle fibers, and that myogenesis is controlled by a combination of ubiquitous and muscle-specific transcriptional regulators that may be different for each gene. We further review some unexpected results that have been obtained when ideas from work in culture have been tested in developing animals. The studies in vivo point to additional molecular and cellular mechanisms that regulate muscle formation in the animal.

A unique pattern of expression of the four muscle regulatory factor proteins distinguishes somitic from embryonic, fetal and newborn mouse myogenic cells

A unique pattern of expression of the four muscle regulatory factor (MRF) proteins was found to distinguish early somitic from embryonic, fetal and newborn limb myogenic cells in vitro. Expression of the myosin heavy chain (MHC), MyoD, myogenin, Myf-5, and MRF4 proteins was examined by immunocytochemistry in cultures of four distinct types of mouse myogenic cells: somitic (E8.5), embryonic (E11.5), fetal (E16.5) and newborn limb. In embryonic, fetal and newborn cultures, the MRF proteins were expressed in generally similar patterns: MyoD was the first MRF expressed; MyoD and myogenin were expressed by more cells than Myf-5 or MRF4; and each of the four MRFs was found both in cells that expressed MHC and in cells that did not express MHC. In cultures of somitic cells, in contrast, Myf-5 was expressed first and by more cells than MyoD or myogenin; MRF4 was not detected; and the MRFs were never found to be coexpressed with MHC in the same cell. Thus, some somitic cells had the unexpected ability to maintain MHC expression in the absence of detectable MRF protein expression. The different myogenic programs of embryonic, fetal and newborn myogenic cells are not, therefore, a simple result of qualitatively different MRF expression patterns, whereas myogenesis by somitic cells does include a unique pattern of MRF expression.

Inactivation of MyoD in mice leads to up-regulation of the myogenic HLH gene Myf-5 and results in apparently normal muscle development

The myogenic basic HLH transcription factor family of genes, composed of MyoD, myogenin, Myf-5, and Myf-6, are thought to regulate skeletal muscle differentiation. To understand the role of MyoD in myogenesis, we have introduced a null mutation of MyoD into the germline of mice. Surprisingly, mice lacking MyoD are viable and fertile. Histological examination of skeletal muscle failed to reveal any morphological abnormalities in these mice. Furthermore, Northern analysis revealed normal levels of skeletal muscle-specific mRNAs. Significantly, Myf-5 mRNA levels are elevated in postnatal mutant mice. Normally, Myf-5 expression becomes markedly reduced at day 12 of gestation when MyoD mRNA first appears. This suggests that Myf-5 expression is repressed by MyoD. Our results indicate that MyoD is dispensable for skeletal muscle development in mice, revealing some degree of functional redundancy in the control of the skeletal myogenic developmental program.