Lineage restriction of the myogenic conversion factor myf-5 in the brain

Brown fat organogenesis and maintenance requires AKT1 and AKT2

Objective

Understanding the signaling mechanisms that control brown adipose tissue (BAT) development is relevant to understanding energy homeostasis and obesity. The AKT kinases are insulin effectors with critical in vivo functions in adipocytes; however, their role in adipocyte development remains poorly understood. The goal of this study was to investigate AKT function in BAT development.

Methods

We conditionally deleted Akt1 and Akt2 either individually or together with Myf5-Cre, which targets early mesenchymal precursors that give rise to brown adipocytes. Because Myf5-Cre also targets skeletal muscle and some white adipocyte lineages, comparisons were made between AKT function in BAT versus white adipose tissue (WAT) and muscle development. We also deleted both Akt1 and Akt2 in mature brown adipocytes with Ucp1-Cre or Ucp1-CreER to investigate AKT1/2 signaling in BAT maintenance.

Results

AKT1 and AKT2 are individually dispensable in Myf5-Cre lineages in vivo for establishing brown and white adipocyte precursor cell pools and for their ability to differentiate (i.e. induce PPARγ). AKT1 and AKT2 are also dispensable for skeletal muscle development, and AKT3 does not compensate in either the adipocyte or muscle lineages. In contrast, AKT2 is required for adipocyte lipid filling and efficient downstream AKT substrate phosphorylation. Mice in which both Akt1 and Akt2 are deleted with Myf5-Cre lack BAT but have normal muscle mass, and doubly deleting Akt1 and Akt2 in mature brown adipocytes, either congenitally (with Ucp1-Cre), or inducibly in older mice (with Ucp1-CreER), also ablates BAT. Mechanistically, AKT signaling promotes adipogenesis in part by stimulating ChREBP activity.

Conclusions

AKT signaling is required in vivo for BAT development but dispensable for skeletal muscle development. AKT1 and AKT2 have both overlapping and distinct functions in BAT development with AKT2 being the most critical individual isoform. AKT1 and AKT2 also have distinct and complementary functions in BAT maintenance.

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AKT1 is dispensable for the differentiation of Myf5-lineage adipocytes.

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AKT2 regulates adipocyte cell size and body fat distribution.

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AKT1 and AKT2 exhibit some compensatory functions in BAT development and maintenance.

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AKT1 and AKT2 are dispensable in the Myf5-lineage for muscle development.

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ChREBP may function downstream of Akt1/Akt2 in brown adipocyte differentiation.

Reporter-Based Isolation of Developmental Myogenic Progenitors

The formation and activity of mammalian tissues entail finely regulated processes, involving the concerted organization and interaction of multiple cell types. In recent years the prospective isolation of distinct progenitor and stem cell populations has become a powerful tool in the hands of developmental biologists and has rendered the investigation of their intrinsic properties possible. In this protocol, we describe how to purify progenitors with different lineage history and degree of differentiation from embryonic and fetal skeletal muscle by fluorescence-activated cell sorting (FACS). The approach takes advantage of a panel of murine strains expressing fluorescent reporter genes specifically in the myogenic progenitors. We provide a detailed description of the dissection procedures and of the enzymatic dissociation required to maximize the yield of mononucleated cells for subsequent FACS-based purification. The procedure takes ~6–7 h to complete and allows for the isolation and the subsequent molecular and phenotypic characterization of developmental myogenic progenitors.

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.

Skeletal myogenesis and Myf5 activation

Myogenic regulatory factors (MRFs) are the master regulators of skeletal myogenesis. Among the MRFs, Myf5 is the earliest to be expressed and is regulated by a complex set of enhancers. The expression of Myf5 defines different muscle populations in the somite. Furthermore, Myf5 expression is also found in non-muscle tissues, such as preadipocytes and neurons. Here, we present a current view on the regulation of skeletal myogenesis by transcription factors and cellular signals, with an emphasis on the complexity of transcriptional activation of Myf5. We also discuss Myf5 expression in different populations of myoblasts, preadipocytes and neuronal tissue.

Prdm16 is required for the maintenance of brown adipocyte identity and function in adult mice

Prdm16 is a transcription factor that regulates the thermogenic gene program in brown and beige adipocytes. However, whether Prdm16 is required for the development or physiological function of brown adipose tissue (BAT) in vivo has been unclear. By analyzing mice that selectively lacked Prdm16 in the brown adipose lineage, we found that Prdm16 was dispensable for embryonic BAT development. However, Prdm16 was required in young mice to suppress the expression of white-fat-selective genes in BAT through recruitment of the histone methyltransferase Ehmt1. Additionally, Prdm16 deficiency caused a severe adult-onset decline in the thermogenic character of interscapular BAT. This resulted in BAT dysfunction and cold sensitivity but did not predispose the animals to obesity. Interestingly, the loss of brown fat identity due to ablation of Prdm16 was accelerated by concurrent deletion of the closely related Prdm3 gene. Together, these results show that Prdm16 and Prdm3 control postnatal BAT identity and function.

Fetal growth restriction caused by MIMT1 deletion alters brain transcriptome in cattle

We examined levels of gene expression in the brains of bovine fetuses carrying a truncated MIMT1 allele, MIMT1(Del), shown to cause late abortion and stillbirth as a result of fetal growth restriction. MIMT1 is a non-protein coding gene that forms part of the imprinted PEG3 (paternally expressed gene 3) domain. Microarray analysis of brain cortex samples from mid-gestation MIMT1(Del/WT) bovine fetuses and wild-type siblings was performed to study the effect of fetal growth restriction on brain gene expression. Statistical analysis revealed 134 genes with increased mRNA levels and 22 with reduced levels in MIMT1(Del/WT) fetuses. Gene set enrichment analysis identified a relatively small number of significant functional clusters representing three major biological processes: response to oxidative stress, angiogenesis, and epithelial cell proliferation. Gene expression microarray analyses identified increased expression of VIPR2, HTRA1, S100A4 and MYH8 in fetuses carrying the deletion and decreased expression of DRD2, ADAM18, miR345, ZNF585A. ADAM18, DRD2 and S100A4 are known to be involved in prenatal brain development. ZNF585A, miR-345, VIPR2, HTRA1, and MYH8 are known to be involved in cell growth and differentiation, but any role in neural developmental has yet to be elucidated.

Ancestral Myf5 gene activity in periocular connective tissue identifies a subset of fibro/adipogenic progenitors but does not connote a myogenic origin

Extraocular muscles (EOM) represent a unique muscle group that controls eye movements and originates from head mesoderm, while the more typically studied body and limb muscles are somite-derived. Aiming to investigate myogenic progenitors (satellite cells) in EOM versus limb and diaphragm of adult mice, we have been using flow cytometry in combination with myogenic-specific Cre-loxP lineage marking for cell isolation. While analyzing cells from the EOM of mice that harbor Myf5(Cre)-driven GFP expression, we identified in addition to the expected GFP(+) myogenic cells (presumably satellite cells), a second dominant GFP(+) population distinguished as being Sca1(+), non-myogenic, and exhibiting a fibro/adipogenic potential. This unexpected population was not only unique to EOM compared to the other muscles but also specific to the Myf5(Cre)-driven reporter when compared to the MyoD(Cre) driver. Histological studies of periocular tissue preparations demonstrated the presence of Myf5(Cre)-driven GFP(+) cells in connective tissue locations adjacent to the muscle masses, including cells in the vasculature wall. These vasculature-associated GFP(+) cells were further identified as mural cells based on the presence of the specific XLacZ4 transgene. Unlike the EOM satellite cells that originate from a Pax3-negative lineage, these non-myogenic Myf5(Cre)-driven GFP(+) cells appear to be related to cells of a Pax3-expressing origin, presumably derived from the neural crest. In all, our lineage tracing based on multiple reporter lines has demonstrated that regardless of common ancestral expression of Myf5, there is a clear distinction between periocular myogenic and non-myogenic cell lineages according to their mutually exclusive antecedence of MyoD and Pax3 gene activity.

Distinct populations of adipogenic and myogenic Myf5-lineage progenitors in white adipose tissues

Brown adipose tissues (BAT) are derived from a myogenic factor 5 (Myf5)-expressing cell lineage and white adipose tissues (WAT) predominantly arise from non-Myf5 lineages, although a subpopulation of adipocytes in some WAT depots can be derived from the Myf5 lineage. However, the functional implication of the Myf5- and non-Myf5-lineage cells in WAT is unclear. We found that the Myf5-lineage constitution in subcutaneous WAT depots is negatively correlated to the expression of classical BAT and newly defined beige/brite adipocyte-specific genes. Consistently, fluorescent-activated cell sorting (FACS)-purified Myf5-lineage adipo-progenitors give rise to adipocytes expressing lower levels of BAT-specific Ucp1, Prdm16, Cidea, and Ppargc1a genes and beige adipocyte-specific CD137, Tmem26, and Tbx1 genes compared with the non-Myf5-lineage adipocytes from the same depots. Ablation of the Myf5-lineage progenitors in WAT stromal vascular cell (SVC) cultures leads to increased expression of BAT and beige cell signature genes. Strikingly, the Myf5-lineage cells in WAT are heterogeneous and contain distinct adipogenic [stem cell antigen 1(Sca1)-positive] and myogenic (Sca1-negative) progenitors. The latter differentiate robustly into myofibers in vitro and in vivo, and they restore dystrophin expression after transplantation into mdx mouse, a model for Duchenne muscular dystrophy. These results demonstrate the heterogeneity and functional differences of the Myf5- and non-Myf5-lineage cells in the white adipose tissue.

Are human and mouse satellite cells really the same?

Satellite cells are quiescent cells located under the basal lamina of skeletal muscle fibers that contribute to muscle growth, maintenance, repair, and regeneration. Mouse satellite cells have been shown to be muscle stem cells that are able to regenerate muscle fibers and self-renew. As human skeletal muscle is also able to regenerate following injury, we assume that the human satellite cell is, like its murine equivalent, a muscle stem cell. In this review, we compare human and mouse satellite cells and highlight their similarities and differences. We discuss gaps in our knowledge of human satellite cells, compared with that of mouse satellite cells, and suggest ways in which we may advance studies on human satellite cells, particularly by finding new markers and attempting to re-create the human satellite cell niche in vitro.

Cells that express MyoD mRNA in the epiblast are stably committed to the skeletal muscle lineage

The epiblast of the chick embryo contains cells that express MyoD mRNA but not MyoD protein. We investigated whether MyoD-positive (MyoD^pos) epiblast cells are stably committed to the skeletal muscle lineage or whether their fate can be altered in different environments. A small number of MyoD^pos epiblast cells were tracked into the heart and nervous system. In these locations, they expressed MyoD mRNA and some synthesized MyoD protein. No MyoD^pos epiblast cells differentiated into cardiac muscle or neurons. Similar results were obtained when MyoD^pos cells were isolated from the epiblast and microinjected into the precardiac mesoderm or neural plate. In contrast, epiblast cells lacking MyoD differentiated according to their environment. These results demonstrate that the epiblast contains both multipotent cells and a subpopulation of cells that are stably committed to the skeletal muscle lineage before the onset of gastrulation. Stable programming in the epiblast may ensure that MyoD^pos cells express similar signaling molecules in a variety of environments.

MyoD-positive epiblast cells regulate skeletal muscle differentiation in the embryo

MyoD mRNA is expressed in a subpopulation of cells within the embryonic epiblast. Most of these cells are incorporated into somites and synthesize Noggin. Ablation of MyoD-positive cells in the epiblast subsequently results in the herniation of organs through the ventral body wall, a decrease in the expression of Noggin, MyoD, Myf5, and myosin in the somites and limbs, and an increase in Pax-3-positive myogenic precursors. The addition of Noggin lateral to the somites compensates for the loss of MyoD-positive epiblast cells. Skeletal muscle stem cells that arise in the epiblast are utilized in the somites to promote muscle differentiation by serving as a source of Noggin.

Mrf4 determines skeletal muscle identity in Myf5:Myod double-mutant mice

In vertebrates, skeletal muscle is a model for the acquisition of cell fate from stem cells. Two determination factors of the basic helix-loop-helix myogenic regulatory factor (MRF) family, Myf5 and Myod, are thought to direct this transition because double-mutant mice totally lack skeletal muscle fibres and myoblasts. In the absence of these factors, progenitor cells remain multipotent and can change their fate. Gene targeting studies have revealed hierarchical relationships between these and the other MRF genes, Mrf4 and myogenin, where the latter are regarded as differentiation genes. Here we show, using an allelic series of three Myf5 mutants that differentially affect the expression of the genetically linked Mrf4 gene, that skeletal muscle is present in the new Myf5:Myod double-null mice only when Mrf4 expression is not compromised. This finding contradicts the widely held view that myogenic identity is conferred solely by Myf5 and Myod, and identifies Mrf4 as a determination gene. We revise the epistatic relationship of the MRFs, in which both Myf5 and Mrf4 act upstream of Myod to direct embryonic multipotent cells into the myogenic lineage.

Skeletal myosin heavy chain function in cultured lung myofibroblasts

Myofibroblasts are unique contractile cells with both muscle and nonmuscle properties. Typically myofibroblasts are identified by the expression of alpha smooth muscle actin (ASMA); however some myofibroblasts also express sarcomeric proteins. In this study, we show that pulmonary myofibroblasts express three of the eight known sarcomeric myosin heavy chains (MyHCs) (IIa, IId, and embryonic) and that skeletal muscle myosin enzymatic activity is required for pulmonary myofibroblast contractility. Furthermore, inhibition of skeletal myosin activity and myofibroblast contraction results in a decrease in both ASMA and skeletal MyHC promoter activity and ASMA protein expression, suggesting a potential coupling of skeletal myosin activity and ASMA expression in myofibroblast differentiation. To understand the molecular mechanisms whereby skeletal muscle genes are regulated in myofibroblasts, we have found that members of the myogenic regulatory factor family of transcription factors and Ca(2+) - regulated pathways are involved in skeletal MyHC promoter activity. Interestingly, the regulation of skeletal myosin expression in myofibroblasts is distinct from that observed in muscle cells and suggests that cell context is important in its control.

Cell differentiation and cell fate during urodele tail and limb regeneration

One of the most striking natural examples of adult tissue plasticity in vertebrates is limb and tail regeneration in urodele amphibians. In this setting, amputation triggers the destabilization of cell differentiation and the production of progenitor cells that extensively proliferate and pattern themselves to recreate a perfect replica of the missing part. A precise understanding of which cells dedifferentiate and how plastic they become has recently begun to emerge. Furthermore, information on which developmental gene programs are activated upon injury is becoming better understood. These studies indicate that, upon injury, an unusual cohort of genes are co-expressed. The future challenge will be to link the systems for studying dedifferentiation with activation of gene expression to understand on a molecular level how cells are 'pushed backward' to regenerate a complex structure such as a limb or tail.

Wnt signaling induces the myogenic specification of resident CD45+ adult stem cells during muscle regeneration

The observation that CD45(+) stem cells injected into the circulation participate in muscle regeneration raised the question of whether CD45(+) stem cells resident in muscle play a physiological role during regeneration. We found that CD45(+) cells cultured from uninjured muscle were uniformly nonmyogenic. However, CD45(+) cells purified from regenerating muscle readily gave rise to determined myoblasts. The number of CD45(+) cells in muscle rapidly expanded following injury, and a high proportion entered the cell cycle. Investigation of candidate pathways involved in embryonic myogenesis revealed that Wnt signaling was sufficient to induce the myogenic specification of muscle-derived CD45(+) stem cells. Moreover, injection of the Wnt antagonists sFRP2/3 into regenerating muscle markedly reduced CD45(+) stem cell proliferation and myogenic specification. Our data therefore suggest that mobilization of resident CD45(+) stem cells is an important factor in regeneration after injury and highlight the Wnt pathway as a potential therapeutic target for degenerative neuromuscular disease.

Treatment with Myf5-morpholino results in somite patterning and brain formation defects in zebrafish

Myf-5 is a stage-dependent transcription factor associated with somitogenesis. To study its biological functions in zebrafish, we injected the Myf5-morpholinos ZMF-MO (antisense nucleotides 28 to 52) and ZMF-OTHER (antisense nucleotides 3 to 27) into zebrafish embryos to establish a myf-5 gene knockdown. No phenotypic abnormalities were observed following injection with 0.2 ng of ZMF-MO, but defects were displayed in 2 of 118 (1.7%) surviving embryos injected with 1 ng ZMF-MO. Morphological defects became more severe with increased dosages: 105 of 270 (38.9%) surviving embryos injected with 4.5 ng of ZMF-MO displayed such abnormalities as the absence of eyes or brains in addition to the following low-dosage defects in 24 hpf embryos: longitudinal yolk sacs, incomplete epiboly coverage, abnormal and suspended tail buds, diffused somite boundaries, and head shrinkage. Similar results were observed in the 4.5 ng ZMF-OTHER injection group. However, when fish were co-injected with 4.5 ng ZMF-MO and 4.5 ng myf-5 mRNA, abnormality rates decreased from 49.6% to 5.5%. Our results show that the brain krox20 gene was down-regulated at rhombomere 3; the pax2.1 gene was completely down-regulated; myoD was expressed normally; myogenin was substantially down-regulated in whole somites; and desmin was partly inhibited in newly forming somites. Our conclusion is that zebrafish Myf-5 may play important roles in brain formation and in the convergence and extension of shield epiblasts and tail buds during early embryogenesis, in addition to its well-understood role as a muscle regulatory factor in somites.

The early epaxial enhancer is essential for the initial expression of the skeletal muscle determination geneMyf5but not for subsequent, multiple phases of somitic myogenesis

Vertebrate myogenesis is controlled by four transcription factors known as the myogenic regulatory factors (MRFs): Myf5, Mrf4, myogenin and MyoD. During mouse development Myf5 is the first MRF to be expressed and it acts by integrating multiple developmental signals to initiate myogenesis. Numerous discrete regulatory elements are involved in the activation and maintenance of Myf5 gene expression in the various muscle precursor populations, reflecting the diversity of the signals that control myogenesis. Here we focus on the enhancer that recapitulates the first phase of Myf5 expression in the epaxial domain of the somite, in order to identify the subset of cells that first transcribes the gene and therefore gain insight into molecular, cellular and anatomical facets of early myogenesis. Deletion of this enhancer from a YAC reporter construct that recapitulates the Myf5 expression pattern demonstrates that this regulatory element is necessary for expression in the early epaxial somite but in no other site of myogenesis. Importantly, Myf5 is subsequently expressed in the epaxial myotome under the control of other elements located far upstream of the gene. Our data suggest that the inductive signals that control Myf5 expression switch rapidly from those that impinge on the early epaxial enhancer to those that impinge on the other enhancers that act later in the epaxial somite, indicating that there are significant changes in either the signalling environment or the responsiveness of the cells along the rostrocaudal axis. We propose that the first phase of Myf5 epaxial expression, driven by the early epaxial enhancer in the dermomyotome, is necessary for early myotome formation, while the subsequent phases are associated with cytodifferentiation within the myotome.

Different regulatory elements within the MyoD promoter control its expression in the brain and inhibit its functional consequences in neurogenesis

MyoD is a key basic helix-loop-helix (bHLH) transcription factor capable of converting many cells into skeletal muscle. Together with Myf5 it is essential for initiating skeletal myogenesis. In this report, the restricted domains of MyoD-lacZ expression have been revealed in the embryonic mouse brain by the analysis of transgenic mice with reporter genes driven by MyoD regulatory elements. The MD6.0-lacZ transgene was localized in the basal plate of pons, medulla oblongata (i.e. the medial longitudinal fasciculus) and spinal cord of wild-type and mutant mouse embryos at various stages of development, whereas the 258/-2.5lacZ transgene was not detected in the brain. In addition, MyoD RNA and MyoD protein accumulations were monitored in neurons expressing MD6.0-lacZ transgene. Although MyoD was detected in muscle myotomal cells, it was absent in MD6.0-lacZ-expressing neurons. This would account for the lack of myogenic conversion in brain structures and the absence of a neural phenotype in MyoD-/- embryos and mice. Together, these data indicate that within the promoter of MyoD different regulatory elements control its expression and prevent the functional consequences of MyoD in neurogenesis.

Myf-5 is transiently expressed in nonmuscle mesoderm and exhibits dynamic regional changes within the presegmented mesoderm and somites I-IV

Myf-5 is one of four myogenic regulatory factors that play important roles in skeletal muscle development. This study provides detailed analysis of Myf-5 expression during early chick development using an in situ hybridization technique that has been optimized to detect low level Myf-5 transcripts. This facilitated detection of heretofore unrecognized dynamic changes in Myf-5 expression patterns. Myf-5 expression is first detected at stage 3 in the primitive streak and exhibits transient low-level expression in nonmyogenic mesoderm. Myf-5 is later expressed in the presegmented mesoderm (psm) in a reiterating pattern that is coordinated with somitogenesis and also colocalizes with the Notch ligand C-Delta-1. In somites (S) I-IV, Myf-5 expression exhibits dynamic regional changes, and in somites rostral to S IV, Myf-5 is expressed at higher levels in muscle precursors in the dorsomedial somite. Semiquantitative comparison of Myf-5 mRNA levels in the psm and in myotome-containing somites indicates about a 10-fold difference. The expression pattern of Myf-5 differs from that of MyoD, which we find is expressed only in the dorsomedial somite. These data reveal that Myf-5 is expressed at low levels several stages before muscle differentiation occurs and suggest that only a subset of cells that initially express Myf-5 will upregulate its expression and differentiate as muscle.

Modular long-range regulation of Myf5 reveals unexpected heterogeneity between skeletal muscles in the mouse embryo

The myogenic factor Myf5 plays a key role in muscle cell determination, in response to signalling cascades that lead to the specification of muscle progenitor cells. We have adopted a YAC transgenic approach to identify regulatory sequences that direct the complex spatiotemporal expression of this gene during myogenesis in the mouse embryo. Important regulatory regions with distinct properties are distributed over 96 kb upstream of the Myf5 gene. The proximal 23 kb region directs early expression in the branchial arches, epaxial dermomyotome and in a central part of the myotome, the epaxial intercalated domain. Robust expression at most sites in the embryo where skeletal muscle forms depends on an enhancer-like sequence located between −58 and −48 kb from the Myf5 gene. This element is active in the epaxial and hypaxial myotome, in limb muscles, in the hypoglossal chord and also at the sites of Myf5 transcription in prosomeres p1 and p4 of the brain. However later expression of Myf5 depends on a more distal region between −96 and −63 kb, which does not behave as an enhancer. This element is necessary for expression in head muscles but strikingly only plays a role in a subset of trunk muscles, notably the hypaxially derived ventral body muscles and also those of the diaphragm and tongue. Transgene expression in limb muscle masses is not affected by removal of the −96/-63 region. Epaxially derived muscles and some hypaxial muscles, such as the intercostals and those of the limb girdles, are also unaffected. This region therefore reveals unexpected heterogeneity between muscle masses, which may be related to different facets of myogenesis at these sites. Such regulatory heterogeneity may underlie the observed restriction of myopathies to particular muscle subgroups.

Fate restriction in limb muscle precursor cells precedes high-level expression of MyoD family member genes

The mechanisms by which pluripotent embryonic cells generate unipotent tissue progenitor cells during development are unknown. Molecular/genetic experiments in cultured cells have led to the hypothesis that the product of a single member of the MyoD gene family (MDF) is necessary and sufficient to establish the positive aspects of the determined state of myogenic precursor cells: i.e., the ability to initiate and maintain the differentiated state (Weintraub, H., Davis, R., Tapscott, S., Thayer, M., Krause, M., Benezra, R., Blackwell, T. K., Turner, D., Rupp, R., Hollenberg, S. et al. (1991) Science 251, 761–766). Embryonic cell type determination also involves negative regulation, such as the restriction of developmental potential for alternative cell types, that is not directly addressed by the MDF model. In the experiments reported here, phenotypic restriction in myogenic precursor cells is assayed by an in vivo ‘notochord challenge’ to evaluate their potential to ‘choose’ between two alternative cell fate endpoints: cartilage and muscle (Williams, B. A. and Ordahl, C. P. (1997) Development 124, 4983–4997). Two separate myogenic precursor cell populations were found to be phenotypically restricted while expressing the Pax3 gene and prior to MDF gene activation. Therefore, while MDF family members act positively during myogenic differentiation, phenotypic restriction, the negative aspect of cell specification, requires cellular and molecular events and interactions that precede MDF expression in myogenic precursor cells. The qualities of muscle formed by the determined myogenic precursor cells in these experiments further indicate that their developmental potential is intermediate between that of myoblastic stem cells taken from fetal or adult tissue (which lack mitotic and morphogenetic potential when tested in vivo) and embryonic stem cells (which are multipotent). We hypothesize that such embryonic myogenic progenitor cells represent a distinct class of determined embryonic cell, one that is responsible for both tissue growth and tissue morphogenesis.

DNA dendrimers localize MyoD mRNA in presomitic tissues of the chick embryo

MyoD expression is thought to be induced in somites in response to factors released by surrounding tissues; however, reverse transcription-PCR and cell culture analyses indicate that myogenic cells are present in the embryo before somite formation. Fluorescently labeled DNA dendrimers were used to identify MyoD expressing cells in presomitic tissues in vivo. Subpopulations of MyoD positive cells were found in the segmental plate, epiblast, mesoderm, and hypoblast. Directly after laying, the epiblast of the two layered embryo contained approximately 20 MyoD positive cells. These results demonstrate that dendrimers are precise and sensitive reagents for localizing low levels of mRNA in tissue sections and whole embryos, and that cells with myogenic potential are present in the embryo before the initiation of gastrulation.

Myf5 is a novel early axonal marker in the mouse brain and is subjected to post-transcriptional regulation in neurons

Myf5 is a key basic Helix-Loop-Helix transcription factor capable of converting many non-muscle cells into muscle. Together with MyoD it is essential for initiating the skeletal muscle programme in the embryo. We previously identified unexpected restricted domains of Myf5 transcription in the embryonic mouse brain, first revealed by Myf5-nlacZ(+/)(−) embryos (Tajbakhsh, S. and Buckingham, M. (1995) Development 121, 4077–4083). We have now further characterized these Myf5 expressing neurons. Retrograde labeling with diI, and the use of a transgenic mouse line expressing lacZ under the control of Myf5 regulatory sequences, show that Myf5 transcription provides a novel axonal marker of the medial longitudinal fasciculus (mlf) and the mammillotegmental tract (mtt), the earliest longitudinal tracts to be established in the embryonic mouse brain. Tracts projecting caudally from the developing olfactory system are also labelled. nlacZ and lacZ expression persist in the adult brain, in a few ventral domains such as the mammillary bodies of the hypothalamus and the interpeduncular nucleus, potentially derived from the embryonic structures where the Myf5 gene is transcribed. To investigate the role of Myf5 in the brain, we monitored Myf5 protein accumulation by immunofluorescence and immunoblotting in neurons transcribing the gene. Although Myf5 was detected in muscle myotomal cells, it was absent in neurons. This would account for the lack of myogenic conversion in brain structures and the absence of a neural phenotype in homozygous null mutants. RT-PCR experiments show that the splicing of Myf5 primary transcripts occurs correctly in neurons, suggesting that the lack of Myf5 protein accumulation is due to regulation at the level of mRNA translation or protein stability. In the embryonic neuroepithelium, Myf5 is transcribed in differentiated neurons after the expression of neural basic Helix-Loop-Helix transcription factors. The signalling molecules Wnt1 and Sonic hedgehog, implicated in the activation of Myf5 in myogenic progenitor cells in the somite, are also produced in the viscinity of the Myf5 expression domain in the mesencephalon. We show that cells expressing Wnt1 can activate neuronal Myf5-nlacZ gene expression in dissected head explants isolated from E9.5 embryos. Furthermore, the gene encoding the basic Helix-Loop-Helix transcription factor mSim1 is expressed in adjacent cells in both the somite and the brain, suggesting that signalling molecules necessary for the activation of mSim1 as well as Myf5 are present at these different sites in the embryo. This phenomenon may be widespread and it remains to be seen how many other potentially potent regulatory genes, in addition to Myf5, when activated do not accumulate protein at inappropriate sites in the embryo.

Ventrally emigrating neural tube cells differentiate into vascular smooth muscle cells

A multipotential cell population originating in the ventral part of the hindbrain neural tube, the ventrally emigrating neural tube cells (VENT cells), has recently been shown to migrate into the craniofacial mesenchyme. Because vascular smooth muscle cells develop from this mesenchyme, we sought to determine if the VENT cells contributed to their differentiation. VENT cells were tagged with replication-deficient retroviral vector with LacZ by microinjection into the lumen of the rostral hindbrain of chick embryos on day 2. Embryos were processed for the detection of LacZ positive cells on day 7. LacZ-positive cells were present in the wall of craniofacial arteries and veins. Immunostaining with the smooth muscle alpha-actin confirmed the labeled cells to be smooth muscle cells. It is concluded that some vascular smooth muscle cells differentiate from neural tube cells. the developmental and functional significance of which remains to be established.

Hierarchical differentiation of multipotent progenitor cells

Selection of a pathway of differentiation in multipotent progenitor cells may depend on the amount of histone H1 or H1 zero relative to the core histones. With low levels of these linker histones, it is proposed that an evolutionarily more ancient cell differentiation occurs. Greater repetition of AT-rich regulatory motifs allows more frequent, hence earlier, transcription of genes, accounting for this type of cell differentiation. It is further proposed that a decrease of total cell protein accumulation leads to an increase of histone H1 or H1 zero relative to the core histones. It is suggested that these linker histones preferentially bind the more AT-rich regulatory sequences, thereby restricting the phylogenetically more ancient differentiation potency. This allows differentiation of an evolutionarily younger cell type.

Ventrally emigrating neural tube cells differentiate into heart muscle

A population of ventrally emigrating neural tube cells has been shown to migrate along the vagus nerve and contribute to the development of the gastrointestinal tract. Since the vagus also goes to the heart, we sought to determine if these cells migrated into the heart. Neural tube cells were tagged with replication-deficient retroviral vectors containing the LacZ gene, to permanently label their progeny. The virus was microinjected into the lumen of the caudal hindbrain of chick embryos on day 2. Embryos were later processed for the detection of LacZ positive cells. Labeled cells were initially confined to the neural tube. Later, they migrated in association with the vagus nerve into the heart, where they were located in the myocardium. Labeled cells were identified as cardiac muscle cells of non-neural crest origin, with specific markers. It is concluded that some cardiac muscle cells differentiate from the neural tube cells.

Ventral neural tube cells differentiate into craniofacial skeletal muscles

Craniofacial skeletal muscle cells are believed to develop from mesoderm. A population of ventral neural tube cells has recently been shown to migrate out of the hindbrain and populate the craniofacial mesenchyme in chick embryos. Since skeletal muscle cells develop from this mesenchyme, we sought to determine if the emigrated neural tube cells contributed to their development. Ventral neural tube cells in the hindbrain of chick embryos were labeled on embryonic day 2 with replication-deficient retroviral vectors containing the gene LacZ, which provides a permanent marker for the progeny. On day 7 embryos were processed for the detection of labeled cells. Labeled cells were seen in craniofacial skeletal muscles. By using muscle-specific markers, the labeled cells were confirmed to be skeletal muscle cells. Thus, some muscle cells are derived from the ventral neural tube cells of the hindbrain.

Development of the avian iris and ciliary body: mechanisms of cellular differentiation during the smooth-to-striated muscle transition

The avian iris and ciliary body undergoes a transition from smooth-to-striated muscle during embryonic development. Using antibodies specific for smooth muscle-specific alpha-actin and myosin heavy chain, we confirm that a smooth-to-striated muscle transition occurs between E8 and E17 in both iris and ciliary body of the chick. To study the mechanisms regulating the transition in muscle type, we analyzed the fate of quail clones derived from E7 iris cells. When cells were cloned alone, 45/71 colonies differentiated into smooth muscle and 10/71 became striated muscle. None of the colonies were mixed with respect to muscle phenotype, indicating a lack of pluripotent stem cells. Furthermore, clones giving rise to nonstriated muscle could not be forced to incorporate into myotubes when cocultured with chick myocytes. Clones grown in coculture with chick embryo fibroblasts or E11 iris cells had very high cloning efficiencies (>98%). Significantly more clones differentiated into striated muscle when cocultured with E11 cells (60/156) than when cocultured with fibroblasts (29/108). This was due to an increased recruitment of undifferentiated cells into striated muscle, rather than a change in the percentage of cells differentiating into smooth muscle. In vivo and in vitro, various smooth and striated muscle-specific markers including contractile proteins, acetylcholine receptor subtypes, and transcription factors were colocalized in cells. Although our data argue against a multipotent stem cell for smooth and striated muscle cells, they cannot exclude a role for transdifferentiation. Cumulatively these results suggest that both smooth muscle and migratory myoblasts contribute to the development of myotubes in the avian iris and that this process is regulated in a non-cell-autonomous fashion by locally generated signals.

MyoD and the specification of muscle and non-muscle fates during postembryonic development of the C. elegans mesoderm

Basic-helix-loop helix factors of the myoD/myf5/ myogenin/MRF4 family have been implicated in acquisition and elaboration of muscle cell fates. Here we describe both myogenic and non-myogenic roles for the Caenorhabditis elegans member of this family (CeMyoD) in postembryonic mesodermal patterning. The postembryonic mesodermal lineage in C. elegans provides a paradigm for many of the issues in mesodermal fate specification: a single mesoblast ('M') divides to generate 14 striated muscles, 16 non-striated muscles, and two non-muscle cells. To study CeMyoD function in the M lineage, we needed to circumvent an embryonic requirement for the protein. Two approaches were used: (1) isolation of mutants that decrease CeMyoD levels while retaining viability, and (2) analysis of genetic mosaics that had lost CeMyoD in the M lineage. With either manipulation, we observed a series of cell-fate transformations affecting a subset of both striated muscles and non-muscle cells. In place of these normal fates, the affected lineages produced a number of myoblast-like cells that initially failed to differentiate, instead swelling to acquire a resemblance to sex myoblasts (M-lineage-derived precursors to non-striated uterine and vulval muscles). Like normal sex myoblasts, the ectopic myoblast-like cells were capable of migration and proliferation followed by differentiation of progeny cells into vulval and uterine muscle. Our results demonstrate a cell-intrinsic contribution of CeMyoD to specification of both non-muscle and muscle fates.

Mouse Mesenchyme forkhead 2 (Mf2): expression, DNA binding and induction by sonic hedgehog during somitogenesis

Cloning and sequencing of mouse Mf2 (mesoderm/mesenchyme forkhead 2) cDNAs revealed an open reading frame encoding a putative protein of 492 amino acids which, after in vitro translation, binds to a DNA consensus sequence. Mf2 is expressed at high levels in the ventral region of newly formed somites, in sclerotomal derivatives, in lateral plate and cephalic mesoderm and in the first and second branchial arches. Other regions of mesodermal expression include the developing tongue, meninges, nose, whiskers, kidney, genital tubercule and limb joints. In the nervous system Mf2 is transcribed in restricted regions of the mid- and forebrain. In several tissues, including the early somite, Mf2 is expressed in cell populations adjacent to regions expressing sonic hedgehog (Shh) and in explant cultures of presomitic mesoderm Mf2 is induced by Shh secreted by COS cells. These results suggest that Mf2, like other murine forkhead genes, has multiple roles in embryogenesis, possibly mediating the response of cells to signaling molecules such as SHH.

Faithful expression of the Myf-5 gene during mouse myogenesis requires distant control regions: a transgene approach using yeast artificial chromosomes

Myf-5, a member of the family of four muscle-specific basic helix-loop-helix (bHLH) transcription factors is the first to be expressed in somites, branchial arches, and limb buds during prenatal mouse development. However, little is known about control mechanisms which actually regulate Myf-5 gene activity within these various muscle-forming domains. To identify control regions that contribute to the correct spatiotemporal activity pattern of the Myf-5 gene during mouse embryogenesis, here we report the characterization of yeast artificial chromosomes (YACs) which faithfully direct muscle-specific expression of the gene in chimeric mouse embryos. Forty-five kilobases of sequence 5' to the Myf-5 gene together with 500 kb of 3' flanking DNA drives the correct Myf-5 expression in the mesenchyme of the visceral arches and in somites but not in the hypaxial muscles of limb buds. An additional 50 kb of DNA at the 5' end is required to activate Myf-5 gene expression in developing limbs. These results demonstrate for the first time that unexpectedly distant regions of the Myf-5 gene are necessary to recapitulate its precise developmental expression pattern. We also show that Myf-5 expression in hypaxial muscles and in somites and visceral arches is regulated by separate and distinct far upstream regions. The identification of these remote regulatory elements on YACs carrying the mouse Myf-5 gene constitutes the first important step toward further dissection of the complex mechanisms by which cell-autonomous and external cues control Myf-5 expression during skeletal muscle formation in the mouse embryo.

Establishing myogenic identity during somitogenesis

Over the past year, interest has focused on identifying signalling molecules--including Wnts, Sonic hedgehog, BMP-4, and noggin--that divert somitic mesodermal cells into the muscle lineage, either by induction or derepression. New mouse mutants have also provided insights into somite formation and differentiation, as well as pointing to novel differences between head, trunk, and limb myogenic programmes. In addition, recent genetic, embryological, and molecular studies have shed new light on somite formation and the establishment of muscle progenitor cells.

Muscle determination: another key player in myogenesis?

The steps that commit multipotential somite cells to muscle differentiation are being elucidated. Recent results show that pax3 is an upstream regulator of myoD, one of the key genes in myogenic lineage determination.

Different MRF4 knockout alleles differentially disrupt Myf-5 expression: cis-regulatory interactions at the MRF4/Myf-5 locus

Three different null alleles of the myogenic bHLH gene MRF4/herculin/Myf-6 were created recently. The three alleles were similar in design but were surprisingly different in the intensity of their phenotypes, which ranged from complete viability of homozygotes to complete lethality. One possible explanation for these differences is that each mutation altered expression from the nearby Myf-5 gene to a different extent. This possibility was first raised by the observation that the most severe MRF4 knockout allele expresses no Myf-5 RNA and is a developmental phenocopy of the Myf-5 null mutation. Furthermore, initial studies of the two weaker alleles had shown that their differences in viability correlate with the intensity of rib skeletal defects, and the most extreme version of this rib defect is the hallmark phenotype of Myf-5 null animals. In the present study we tested this hypothesis for the two milder MRF4 alleles. By analyzing compound heterozygous animals carrying either the intermediate or the weakest MRF4 knockout allele on one chromosome 10 and a Myf-5 knockout allele on the other chromosome, we found that both of these MRF4 alleles apparently downregulate Myf-5 expression by a cis-acting mechanism. Compound heterozygotes showed increased mortality of the normally viable MRF4 allele, together with intensified rib defects for both MRF4 alleles and increased deficits in myotomal Myf-5 expression. The allele-specific gradation in phenotypes also suggested that rib morphogenesis is profoundly sensitive to quantitative differences in Myf-5 function if Myf-5 products drop below hemizygous levels. The mechanistic basis for cis interactions at the MRF4/Myf-5 locus was further examined by fusing a DNA segment containing the entire MRF4 structural gene, including all sequences deleted in the three MRF knockout alleles, with a basal promoter and a lacZ reporter. Transgenic embryos showed specific LacZ expression in myotomes in a pattern that closely resembles the expression of Myf-5 RNA. cis-acting interactions between Myf-5 and MRF4 may therefore play a significant role in regulating expression of these genes in the early myotomes of wildtype embryos.

Embryonic and fetal myogenic programs act through separate enhancers at the MLC1F/3F locus

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.

Redefining the genetic hierarchies controlling skeletal myogenesis: Pax-3 and Myf-5 act upstream of MyoD

We analyzed Pax-3 (splotch), Myf-5 (targeted with nlacZ), and splotch/Myf-5 homozygous mutant mice to investigate the roles that these genes play in programming skeletal myogenesis. In splotch and Myf-5 homozygous embryos, myogenic progenitor cell perturbations and early muscle defects are distinct. Remarkably, splotch/Myf-5 double homozygotes have a dramatic phenotype not seen in the individual mutants: body muscles are absent. MyoD does not rescue this double mutant phenotype since activation of this gene proves to be dependent on either Pax-3 or Myf-5. Therefore, Pax-3 and Myf-5 define two distinct myogenic pathways, and MyoD acts genetically downstream of these genes for myogenesis in the body. This genetic hierarchy does not appear to operate for head muscle formation.

Muscle progenitor cells failing to respond to positional cues adopt non-myogenic fates in myf-5 null mice

Mice that have mutations in both myogenic transcription factors Myf-5 and MyoD totally lack skeletal muscle fibres and their precursor myoblasts, whereas with either mutation alone, muscle is present. Skeletal muscle in the vertebrate body is derived from epithelial somites that respond to environmental signals to form the dorsal epithelial dermomyotome (dermis, muscle) and ventral mesenchymal sclerotome (axial skeleton, ribs). The first muscle, the myotome, forms centrally in the somite, when only myf-5 is programming myogenesis. By targeting the nlacZ reporter gene into the myf-5 locus, we demonstrate that beta-galactosidase+ muscle progenitor cells are present in the dermomyotome of myf-5 null embryos, and that they undergo a normal epithelial-mesenchymal transition; however, they migrate aberrantly. Dorsally, they accumulate under the ectoderm and express a non-muscle dermal marker, Dermo-1. Ventrally, beta-galactosidase+ cells also fail to localize correctly, express a cartilage marker scleraxis, and are subsequently found in ribs. Therefore Myf-5 protein is necessary for cells to respond correctly to positional cues in the embryo and to adopt their myogenic fate. In its absence, muscle progenitors, having activated myf-5, remain multipotent and differentiate into other somitic derivatives according to their local environment.

Defining the regulatory networks for muscle development

The formation of skeletal muscle during vertebrate embryogenesis requires commitment of mesodermal precursor cells to the skeletal muscle lineage, withdrawal of myoblasts from the cell cycle, and transcriptional activation of dozens of muscle structural genes. The myogenic basic helix-loop-helix (bHLH) factors - MyoD, myogenin, Myf5, and MRF4 - act at multiple points in the myogenic lineage to establish myoblast identity and to control terminal differentiation. Recent studies have begun to define the inductive mechanisms that regulate myogenic bHLH gene expression and muscle cell determination in the embryo. Myogenic bHLH factors interact with components of the cell cycle machinery to control withdrawal from the cell cycle and act combinatorially with other transcription factors to induce skeletal muscle transcription. Elucidation of these aspects of the myogenic program is leading to a detailed understanding of the regulatory circuits controlling muscle development.

Gene targeting the myf-5 locus with nlacZ reveals expression of this myogenic factor in mature skeletal muscle fibres as well as early embryonic muscle

We have introduced the nlacZ reporter gene into the locus of the myogenic factor gene myf-5 by homologous recombination in embryonic stem (ES) cells. Targeted ES clones were injected into precompaction morula, and the beta-galactosidase expression pattern was monitored. These mice permit the sensitive visualization of myf-5 expression throughout the embryo, and provide a standard for comparing it with that seen with different myf-5/nlacZ transgenes. Thus, in a comparison using ES cells in chimaeric embryos containing the targeted or randomly integrated myf-5/nlacZ construct, we demonstrate that 5.5 kbp of myf-5 upstream flanking sequence including exon1 and most of intron1 directs some skeletal muscle expression, but this is neither qualitatively nor quantitatively equivalent to that of the endogenous gene. Myf-5 is expressed early, before terminal myogenesis takes place in the medial half of the somite, and subsequently it is a major myogenic factor as skeletal muscle forms. All skeletal muscle shows beta-galactosidase activity, even after birth, indicating that myf-5 expression is not confined to primary myotubes, which are derived from embryonic myoblasts, but is also present in muscles containing different adult fibre types. The presence of myf-5 transcripts from the endogenous gene in older muscle was confirmed by in situ hybridization. These results suggest that the myf-5 gene is not activated in only a subset of muscle cells and are consistent with the results on the MyoD knockout mice.

How is myogenesis initiated in the embryo?

Skeletal myoblasts are derived from paraxial mesoderm, but how myoblasts acquire their identity is still a matter of speculation. The characterization of molecular markers and, in some cases, the analysis of mutations in the corresponding genes, has now made it possible to ask specific questions about this process. Specification of somite cell fate depends on epigenetic factors. Adjacent tissues, such as the neural tube, notochord, dorsal ectoderm and lateral mesoderm, act either positively or negatively on the different myogenic precursor populations in the somite. Candidate molecules for this complex signalling activity include sonic hedgehog and the Wnt proteins as positive signals, and BMP4 as a possible inhibitor. Although it is generally assumed that induction is required, some observations suggest that embryonic cells might have a tendency to undergo myogenesis as a 'default' pathway. By analogy with Drosophila, where the neurogenic genes affect myogenesis, the vertebrate homologues of notch and its ligands could be candidate molecules for a repression or derepression mechanism. Similar studies with cultured muscle cells also implicate other HLH factors as potential inhibitors of the MyoD family and, hence, of inappropriate myogenesis.

Activation of different myogenic pathways: myf-5 is induced by the neural tube and MyoD by the dorsal ectoderm in mouse paraxial mesoderm

Newly formed somites or unsegmented paraxial mesoderm (UPM) have been cultured either in isolation or with adjacent structures to investigate the influence of these tissues on myogenic differentiation in mammals. The extent of differentiation was easily and accurately quantified by counting the number of beta-galactosidase-positive cells, since mesodermal tissues had been isolated from transgenic mice that carry the n-lacZ gene under the transcriptional control of a myosin light chain promoter, restricting expression to striated muscle. The results obtained showed that axial structures are necessary to promote differentiation of paraxial mesoderm, in agreement with previous observations. However, it also appeared that the influence of axial structures could be replaced by dorsolateral tissues, adjacent to the paraxial mesoderm. To elucidate which of these tissues exerts this positive effect, we cultured the paraxial mesoderm with a variety of adjacent structures, either adherent to the mesoderm or recombined in vitro. The results of these experiments indicated that the dorsal ectoderm exerts a positive influence on myogenesis but only if left in physical proximity to it. In contrast, lateral mesoderm delays the positive effect of the ectoderm (and has no effect on its own) suggesting that this tissue produces an inhibitory signal. To investigate whether axial structures and dorsal ectoderm induce myogenesis through common or separate pathways, we dissected the medial half of the unsegmented paraxial mesoderm and cultured it with the adjacent neural tube. We also cultured the lateral half of the unsegmented paraxial mesoderm with adjacent ectoderm. The induction of the myogenic regulatory factors myf-5 and MyoD was monitored by double staining of cultured cells with antibodies against MyoD and beta-galactosidase since the tissues were isolated from mouse embryos that carry n-lacZ targeted to the myf-5 gene, so that myf-5 expressing cells could be easily identified by either histochemical or immunocytochemical staining for beta-galactosidase. After 1 day in culture myogenic cells from the medial half expressed myf-5 but not MyoD, while myogenic cells from the lateral half expressed MyoD but not myf-5. By the next day in vitro, however, most myogenic cells expressed both gene products. These data suggest that the neural tube activates myogenesis in the medial half of paraxial mesoderm through a myf-5-dependent pathway, while the dorsal ectoderm activates myogenesis through a MyoD-dependent pathway. The possible developmental significance of these observations is discussed and a model of myogenic determination in mammals is proposed.