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Korb A, Tajbakhsh S, Comai GE. Functional specialisation and coordination of myonuclei. Biol Rev Camb Philos Soc 2024; 99:1164-1195. [PMID: 38477382 DOI: 10.1111/brv.13063] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2023] [Revised: 01/30/2024] [Accepted: 02/02/2024] [Indexed: 03/14/2024]
Abstract
Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.
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Affiliation(s)
- Amaury Korb
- Institut Pasteur, Université Paris Cité, CNRS UMR 3738, Stem Cells & Development Unit, 25 rue du Dr. Roux, Institut Pasteur, Paris, F-75015, France
| | - Shahragim Tajbakhsh
- Institut Pasteur, Université Paris Cité, CNRS UMR 3738, Stem Cells & Development Unit, 25 rue du Dr. Roux, Institut Pasteur, Paris, F-75015, France
| | - Glenda E Comai
- Institut Pasteur, Université Paris Cité, CNRS UMR 3738, Stem Cells & Development Unit, 25 rue du Dr. Roux, Institut Pasteur, Paris, F-75015, France
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2
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Abstract
Isoform diversity in striated muscle is largely controlled at the level of transcription. In this review we will concentrate on studies concerning transcriptional regulation of the alkali myosin light chain 1F/3F gene. Uncoupled activity of the MLC1F and 3F promoters, together with complex patterns of transcription in developing skeletal and cardiac muscle, combine to make analysis of this gene particularly intriguing. In vitro and transgenic studies of MLC1F/3F regulatory elements have revealed an array of cis-acting modules that each drive a subset of the expression pattern of the two promoters. These cis-acting regulatory modules, including the MLC1F and 3F promoter regions and two skeletal muscle enhancers, control tissue-specificity, cell or fibre-type specificity, and the spatiotemporal regulation of gene expression, including positional information. How each of these regulatory modules acts and how their individual activites are integrated to coordinate transcription at this locus are discussed.
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Affiliation(s)
- R G Kelly
- CNRS URA 1947, Département de Biologie Moléculaire, Institut Pasteur, 75724 Paris Cedex 15, France
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3
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Abstract
Experiments with somatic cell hybrids and stable heterokaryons have demonstrated that differentiated cells exhibit a remarkable capacity to change. Heterokaryons have been particularly useful in determining the extent to which the differentiated state of a cell is plastic. Cell fate can be altered by a change in the balance of positive and negative trans-acting regulators. Although a single regulator may be sufficient in certain environments to trigger a change in cell fate, that regulator may be ineffective in other cell contexts where it encounters a different composition of regulators.
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Affiliation(s)
- H M Blau
- Department of Molecular Pharmacology, Stanford University School of Medicine, Stanford, CA 94305, USA
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Musarò A, Rosenthal N. Maturation of the myogenic program is induced by postmitotic expression of insulin-like growth factor I. Mol Cell Biol 1999; 19:3115-24. [PMID: 10082578 PMCID: PMC84105 DOI: 10.1128/mcb.19.4.3115] [Citation(s) in RCA: 127] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The molecular mechanisms underlying myogenic induction by insulin-like growth factor I (IGF-I) are distinct from its proliferative effects on myoblasts. To determine the postmitotic role of IGF-I on muscle cell differentiation, we derived L6E9 muscle cell lines carrying a stably transfected rat IGF-I gene under the control of a myosin light chain (MLC) promoter-enhancer cassette. Expression of MLC-IGF-I exclusively in differentiated L6E9 myotubes, which express the embryonic form of myosin heavy chain (MyHC) and no endogenous IGF-I, resulted in pronounced myotube hypertrophy, accompanied by activation of the neonatal MyHC isoform. The hypertrophic myotubes dramatically increased expression of myogenin, muscle creatine kinase, beta-enolase, and IGF binding protein 5 and activated the myocyte enhancer factor 2C gene which is normally silent in this cell line. MLC-IGF-I induction in differentiated L6E9 cells also increased the expression of a transiently transfected LacZ reporter driven by the myogenin promoter, demonstrating activation of the differentiation program at the transcriptional level. Nuclear reorganization, accumulation of skeletal actin protein, and an increased expression of beta1D integrin were also observed. Inhibition of the phosphatidyl inositol (PI) 3-kinase intermediate in IGF-I-mediated signal transduction confirmed that the PI 3-kinase pathway is required only at early stages for IGF-I-mediated hypertrophy and neonatal MyHC induction in these cells. Expression of IGF-I in postmitotic muscle may therefore play an important role in the maturation of the myogenic program.
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Affiliation(s)
- A Musarò
- Cardiovascular Research Center, Massachusetts General Hospital-East, Charlestown, Massachusetts 02129, USA
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Kelly RG, Zammit PS, Schneider A, Alonso S, Biben C, Buckingham ME. Embryonic and fetal myogenic programs act through separate enhancers at the MLC1F/3F locus. Dev Biol 1997; 187:183-99. [PMID: 9242416 DOI: 10.1006/dbio.1997.8577] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Embryonic and fetal stages of skeletal muscle development are characterized by the differential expression of a number of muscle-specific genes. These include the products of independent promoters at the fast myosin light chain 1F/3F locus. In the mouse embryo MLC1F transcripts accumulate in embryonic skeletal muscle from E9, 4-5 days before high-level accumulation of MLC3F transcripts. A 3' enhancer can activate MLC1F and MLC3F promoters in differentiated muscle cells in vitro and in transgenic mice; both promoters, however, are activated at the time of MLC1F transcript accumulation. We now demonstrate the presence of a second muscle-specific enhancer at this locus, located in the intron separating the MLC1F and MLC3F promoters. Transgenic mice containing the intronic, but lacking the 3' enhancer, express high levels of an nlacZ reporter gene from the MLC3F promoter in adult fast skeletal muscle fibers. In contrast to the 3' enhancer, the intronic element is inactive both in embryonic muscle cells in vivo and in embryonic myocyte cultures. The intronic enhancer is activated at the onset of fetal development in both primary and secondary muscle fibers, at the time of endogenous MLC3F transcript accumulation. Late-activated MLC3F transgenes thus provide a novel in toto marker of fetal myogenesis. These results suggest that temporal regulation of transcription at the MLC1F/3F locus is controlled by separate enhancers which are differentially activated during embryonic and fetal development.
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Affiliation(s)
- R G Kelly
- CNRS URA 1947, Département de Biologie Moléculaire, Institut Pasteur,Paris, France
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6
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Engert JC, Berglund EB, Rosenthal N. Proliferation precedes differentiation in IGF-I-stimulated myogenesis. J Cell Biol 1996; 135:431-40. [PMID: 8896599 PMCID: PMC2121039 DOI: 10.1083/jcb.135.2.431] [Citation(s) in RCA: 223] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
The insulin-like growth factors (IGFs) have dramatic and complex effects on the growth of many tissues and have been implicated in both the proliferation and differentiation of skeletal muscle cells. A detailed analysis of gene expression was performed in L6E9 myoblast cultures treated with IGF-I to dissect the early events leading to the stimulation of myogenic differentiation by this growth factor. A time course of transcript accumulation in confluent L6E9 myoblasts treated with defined media containing IGF-I revealed an initial transient decrease in myogenic factors, accompanied by an increase in cell cycle markers and cell proliferation. This pattern was reversed at later time points, when the subsequent activation of myogenic factors resulted in a net increase in structural gene expression and larger myotubes. The data presented here support the hypothesis that IGF-I activates proliferation first, and subsequently stimulates events leading to the expression of muscle-specific genes in myogenic cell cultures.
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Affiliation(s)
- J C Engert
- Cardiovascular Research Center, Massachusetts General Hospital-East, Charlestown 02129, USA
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7
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Rao MV, Donoghue MJ, Merlie JP, Sanes JR. Distinct regulatory elements control muscle-specific, fiber-type-selective, and axially graded expression of a myosin light-chain gene in transgenic mice. Mol Cell Biol 1996; 16:3909-22. [PMID: 8668209 PMCID: PMC231388 DOI: 10.1128/mcb.16.7.3909] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
The fast alkali myosin light chain 1f/3f (MLC1f/3f) gene is developmentally regulated, muscle specific, and preferentially expressed in fast-twitch fibers. A transgene containing an MLC1f promoter plus a downstream enhancer replicates this pattern of expression in transgenic mice. Unexpectedly, this transgene is also expressed in a striking (approximately 100-fold) rostrocaudal gradient in axial muscles (reviewed by J. R. Sanes, M. J. Donoghue, M. C. Wallace, and J. P. Merlie, Cold Spring Harbor Symp. Quant. Biol. 57:451-460, 1992). Here, we analyzed the expression of mutated transgenes to map sites necessary for muscle-specific, fiber-type-selective, and axially graded expression. We show that two E boxes (myogenic factor binding sites), a homeodomain (hox) protein binding site, and an MEF2 site, which are clustered in an approximately 170-bp core enhancer, are all necessary for maximal transgene activity in muscle but not for fiber-type- or position-dependent expression. A distinct region within the core enhancer promotes selective expression of the transgene in fast-twitch muscles. Sequences that flank the core enhancer are also necessary for high-level activity in transgenic mice but have little influence on activity in transfected cells, suggesting the presence of regions resembling matrix attachment sites. Truncations of the MLC1f promoter affected position-dependent expression of the transgene, revealing distinct regions that repress transgene activity in neck muscles and promote differential expression among intercostal muscles. Thus, the whole-body gradient of expression displayed by the complete transgene may reflect the integrated activities of discrete elements that regulate expression in subsets of muscles. Finally, we show that transgene activity is not significantly affected by deletion or overexpression of the myoD gene, suggesting that intermuscular differences in myogenic factor levels do not affect patterns of transgene expression. Together, our results provide evidence for at least nine distinct sites that exert major effects on the levels and patterns of MLC1f expression in adult muscles.
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MESH Headings
- Animals
- Base Sequence
- Chloramphenicol O-Acetyltransferase/biosynthesis
- Crosses, Genetic
- DNA Footprinting
- DNA Primers
- Deoxyribonuclease I
- Embryo, Mammalian/cytology
- Embryo, Mammalian/physiology
- Embryonic and Fetal Development
- Enhancer Elements, Genetic
- Female
- Gene Expression Regulation, Developmental
- Male
- Mice
- Mice, Inbred C57BL
- Mice, Inbred CBA
- Mice, Transgenic
- Molecular Sequence Data
- Muscle Fibers, Skeletal/metabolism
- Muscle, Skeletal/embryology
- Muscle, Skeletal/metabolism
- Mutagenesis, Site-Directed
- Myosin Light Chains/biosynthesis
- Myosin Light Chains/genetics
- Organ Specificity
- Polymerase Chain Reaction
- Promoter Regions, Genetic
- Recombinant Proteins/biosynthesis
- Regulatory Sequences, Nucleic Acid
- Transfection
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Affiliation(s)
- M V Rao
- Department of Molecular Biology, Washington University School of Medicine, St. Louis, Missouri 63110, USA
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Smith SB, Davis SK, Wilson JJ, Stone RT, Wu FY, Garcia DK, Lunt DK, Schiavetta AM. Bovine fast-twitch myosin light chain 1: cloning and mRNA amount in muscle of cattle treated with clenbuterol. THE AMERICAN JOURNAL OF PHYSIOLOGY 1995; 268:E858-65. [PMID: 7762638 DOI: 10.1152/ajpendo.1995.268.5.e858] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The cDNA clone encoding the fast-twitch isoform of myosin light chain 1 (MLC-1f) was isolated from bovine longissimus dorsi muscle and sequenced in M13 and pUC8. An 0.8-kb subclone, produced by digestion of the cDNA with EcoRI, contained the portion of the molecule common to MLC-1f and MLC-3f. The cDNA in pUC8 contained an additional 81 bp upstream of the EcoR I digestion site, which was unique to MLC-1f. The cDNA clone was used to measure MLC-1f mRNA in longissimus dorsi muscle of cattle chronically administered the beta-adrenergic agonist clenbuterol. Treatment with clenbuterol for 50 days increased succinic dehydrogenase negative (type IIB) and positive (types I and IIA) myofiber cross-sectional areas by 25%. After the 50-day treatment period, the amount of MLC-1f mRNA was 90% greater in longissimus dorsi muscle of treated animals than in the initial group. This effect was lost when clenbuterol treatment was withdrawn for a 78-day period, during which time muscle growth in the treated animals stopped completely. We conclude that we have cloned the bovine cDNA for MLC-1f, which has provided additional evidence that beta-adrenergic agonists increase myofibrillar gene expression.
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Affiliation(s)
- S B Smith
- Department of Animal Science, Texas A&M University, College Station 77843, USA
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9
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Kelly R, Alonso S, Tajbakhsh S, Cossu G, Buckingham M. Myosin light chain 3F regulatory sequences confer regionalized cardiac and skeletal muscle expression in transgenic mice. J Cell Biol 1995; 129:383-96. [PMID: 7721942 PMCID: PMC2199907 DOI: 10.1083/jcb.129.2.383] [Citation(s) in RCA: 212] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
The myosin light chain IF/3F locus contains two independent promoters, MLC1F and MLC3F, which are differentially activated during skeletal muscle development. Transcription at this locus is regulated by a 3' skeletal muscle enhancer element, which directs correct temporal and tissue-specific expression from the MLC1F promoter in transgenic mice. To investigate the role of this enhancer in regulation of the MLC3F promoter in vivo, we have analyzed reporter gene expression in transgenic mice containing lacZ under transcriptional control of the mouse MLC3F promoter and 3' enhancer element. Our results show that these regulatory elements direct strong expression of lacZ in skeletal muscle; the transgene, however, is activated 4-5 d before the endogenous MLC3F promoter, at the time of initiation of MLC1F transcription. In adult mice, transgene activity is downregulated in muscles that have reduced contributions of type IIB fibers (soleus and diaphragm). The rostrocaudal positional gradient of transgene expression documented for MLC1F transgenic mice (Donoghue, M., J. P. Merlie, N. Rosenthal, and J. R. Sanes. 1991. Proc. Natl. Acad. Sci. USA. 88:5847-5851) is not seen in MLC3F transgenic mice. Although MLC3F was previously thought to be restricted to skeletal striated muscle, the MLC3F-lacZ transgene is expressed in cardiac muscle from 7.5 d of development in a spatially restricted manner in the atria and left ventricular compartments, suggesting that transcriptional differences exist between cardiomyocytes in left and right compartments of the heart. We show here that transgene-directed expression of the MLC3F promoter reflects low level expression of endogenous MLC3F transcripts in the mouse heart.
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MESH Headings
- Animals
- Base Sequence
- Down-Regulation
- Embryonic and Fetal Development
- Enhancer Elements, Genetic/genetics
- Female
- Fetal Heart/physiology
- Gene Expression Regulation, Developmental/physiology
- Genes, Reporter/genetics
- Heart/embryology
- Heart/growth & development
- Heart/physiology
- Male
- Mice
- Mice, Inbred C57BL
- Mice, Transgenic
- Molecular Sequence Data
- Muscle Development
- Muscle, Skeletal/embryology
- Muscle, Skeletal/growth & development
- Muscle, Skeletal/physiology
- Myocardium/cytology
- Myosins/genetics
- Promoter Regions, Genetic/genetics
- RNA, Messenger/analysis
- Transcriptional Activation
- beta-Galactosidase/genetics
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Affiliation(s)
- R Kelly
- Centre National de la Recherche Scientifique Unité de Recherche Associée 1947, Department of Molecular Biology, Pasteur Institute, Paris, France
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10
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Induction of endogenous myosin light chain 1 and cardiac alpha-actin expression in L6E9 cells by MyoD1. J Biol Chem 1992. [DOI: 10.1016/s0021-9258(19)37022-x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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