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

Cell cycle-regulated expression of the muscle determination factor Myf5 in proliferating myoblasts

Myf5 is the earliest-known muscle-specific factor to be expressed in vivo and its expression is associated with determination of the myoblast lineage. In C2 cells, we show by immunocytolocalization that Myf5 disappears rapidly from cells in which the differentiation program has been initiated. In proliferating myoblasts, the levels of Myf5 and MyoD detected from cell to cell are very heterogeneous. We find that some of the heterogeneity of Myf5 expression arises from a posttranscriptional regulation of Myf5 by the cell cycle. Immunoblotting of extracts from synchronized cultures reveals that Myf5 undergoes periodic fluctuations during the cell cycle and is absent from cells blocked early in mitosis by use of nocodazole. The disappearance of Myf5 from mitotic cells involves proteolytic degradation of a phosphorylated form of Myf5 specific to this phase of the cell cycle. In contrast, MyoD levels are not depleted in mitotic C2 cells. The mitotic destruction of Myf5 is the first example of a transcription factor showing cell cycle-regulated degradation. These results may be significant in view of the possible role of Myf5 in maintaining the determination of proliferating cells and in timing the onset of differentiation.

Levels of MyoD protein expression following injury of mdx and normal limb muscle are modified by thyroid hormone

Thyroid hormone (T3) affects muscle development and muscle regeneration. It also interacts with the muscle regulatory gene MyoD in culture and affects myoblast proliferation. We studied the localization of MyoD protein using a well-characterized polyclonal antibody for immunohistochemistry. Relative numbers of myogenic precursor cells per field were identified by their MyoD expression during muscle regeneration in normal and mdx dystrophic mice, with particular reference to the expression in mononuclear cells and myotubes at various T3 levels. In regeneration by normal muscles, relatively few MyoD+ nuclei per field were present in mononuclear cells of euthyroid and hypothyroid mice. MyoD staining of mononuclear cell nuclei was approximately doubled in fields of regenerating muscles of normal hyperthyroid compared to euthyroid mice, and was observed in precursors that appeared to be aligned before fusion into myotubes. In mdx regenerating muscle, twofold more mononuclear cells positive for MyoD were present in all three treatment groups compared to normal muscles regenerating under the same conditions. Localization was similar to the pattern in normal euthyroid mice. However, in muscles regenerating in hyperthyroid mdx mice, both mononuclear cell nuclei and centrally located nuclei in a subpopulation (about 15%) of new myotubes formed after the crush injury were intensely stained for MyoD protein. The changes observed are consistent with reports on T3-induced alteration of muscle repair, and propose a link between MyoD regulation and the accelerated differentiation during regeneration under high T3 conditions. (J Histochem Cytochem 46:59-67, 1998)

Persistent expression of MNF identifies myogenic stem cells in postnatal muscles

Skeletal muscles contain an undifferentiated myogenic stem cell pool (satellite cells) that can be mobilized to regenerate myofibers in response to injury. We have determined that the winged helix transcription factor MNF is expressed selectively in quiescent satellite cells, which do not express known regulators of the myogenic program. Following muscle injury, MNF is present transiently in proliferating satellite cells and in centralized nuclei of regenerating myofibers, but expression declines as these fibers mature, until only the residual stem cell pool continues to express detectable levels of MNF. MNF also is expressed selectively but transiently at embryonic stages of myogenesis in the developing myotome, limb bud precursors, and heart tube, but by late fetal stages of development, MNF is down-regulated within differentiated cardiac and skeletal myocytes, and persistently high expression is observed only in satellite cells. These data identify MNF as a marker of quiescent satellite cells and suggest that downstream genes controlled by MNF serve to modulate proliferative growth or differentiation in this unique cell population.

Developmental potential of rat L6 myoblasts in vivo following injection into regenerating muscles

To examine the relative importance of myoblast lineage and environmental influences on the development of muscle fiber types in vivo, the phenotype of muscle fibers formed from rat L6 myoblasts was examined following their injection into different regenerating adult muscles. Myoblasts were infected with a retroviral vector carrying a LacZ reporter gene and their fate in vivo was examined using a panel of antibodies against various myosin heavy chain (MyHC) isoforms. Since L6 myoblasts express IIX MyHC following differentiation in vitro, we wanted to determine if they would form IIX muscle fibers in vivo and whether innervation would alter this fate. Following injection, L6 cells either fused with each other to form homotypic fibers or fused with host muscle cells to form heterotypic fibers. Initially, homotypic fibers expressed embryonic MyHC-similar to L6 myotubes in vitro. However, by 4 weeks postinjection IIX MyHC had replaced embryonic MyHC as the predominant isoform. Single fiber analysis using an antibody specific for NCAM indicated that this transition was independent of innervation. Analysis of heterotypic fibers resulting from the incorporation of donor L6 myoblasts into host fast IIA and IIB fibers revealed that L6-derived nuclei express embryonic and IIX MyHCs for up to 8 weeks postinjection, often as nuclear domains surrounding L6 nuclei. These results suggest that MyHC expression in muscle fibers derived from L6 myoblasts is regulated, in part, by intrinsic factors that limit the fiber type potential of these cells in vivo.

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.

Limitations of nls beta-galactosidase as a marker for studying myogenic lineage or the efficacy of myoblast transfer

BACKGROUND

Nuclear localizing beta-galactosidase (nls beta-gal) is used as a marker for studying myoblast cell lineage and for evaluating myoblast survival after myoblast transfer, a procedure with potential use for gene complementation for muscular dystrophy. Usefulness of this construct depends on the establishment of the extent to which nls beta-gal or its mRNA may be translocated from the nucleus that encodes it to other non-coding myonuclei in hybrid myofibers and the ease with which the encoding and non-coding myonuclei can be distinguished. Previous in vitro studies (Ralston and Hall 1989. Science, 244:1066-1068) have suggested limited translocation of the fusion protein. We re-examined the extent to which nls beta-gal is translocated in hybrid myofibers, both in vitro and in vivo, and evaluated the extent to which one can rely on histochemistry to distinguish encoding from non-coding nuclei in these myofibers.

METHODS

Myotubes formed in co-cultures of a myoblast line (MM14 cells), stably transfected with a construct consisting of a nls beta-gal under the control of the myosin light chain 3F promoter and 3' enhancer (3FlacZ10 cells), and [3H]-thymidine-labeled parental MM14 cells (plated at ratios of 1:6 or 1:20, respectively) were reacted with X-gal. After autoradiography, the distance over which nls beta-gal was translocated in hybrid myotubes was determined. In vivo translocation of nls beta-gal was evaluated by injecting [3H]-thymidine-labeled 3FlacZ10 myoblasts into the regenerating extensor digitorum longus muscle of immunosuppressed normal and mdx (dystrophin deficient) mice. Sections stained with X-gal and subjected to autoradiography permitted determination of the extent of nls beta-gal translocation in hybrid myofibers.

RESULTS

In vitro: All nuclei in > 92% of hybrid myotubes showed evidence of nls beta-gal after exposure to X-gal, suggesting extensive translocation. Within hybrid myotubes, MM14-derived myonuclei approximately 350 microns from a 3FlacZ10-derived myonucleus showed evidence of nls beta-gal. In vivo: Similar translocation of nls beta-gal was observed in vivo. One week after myoblast transfer, donor-derived myonuclei were distinguishable from host-derived myonuclei containing nls beta-gal by the greater accumulation of reaction product in donor myonuclei after X-gal staining. However, 2 weeks after injection, host myonuclei often contained a significant amount of nls beta-gal, and accumulation of reaction product could not be used as the criterion for identification of donor myonuclei.

CONCLUSIONS

Translocation of nls beta-gal (or its mRNA) is significantly greater than previously reported (Ralston and Hall 1989), resulting in large numbers of nls beta-gal positive non-coding myonuclei in hybrid myofibers. One week after myoblast transfer, distinguishing between nls beta-gal encoding and non-coding myonuclei in hybrid myofibers after X-gal staining of sectioned muscle is feasible; however, by 2 weeks, nls beta-gal increases in host myonuclei, making identification of donor-derived myonuclei problematic. Translocation of nls beta-gal to non-coding myonuclei in hybrid myofibers must be considered when nls beta-gal is used for studies of myogenic lineage or the efficacy of myoblast transfer therapy, particularly if long-term survival of hybrid myotubes is required.

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.

Expression of fast myosin heavy chain transcripts in developing and dystrophic chicken skeletal muscle

The expression of fast myosin heavy chain (MyHC) genes was examined in vivo during fast skeletal muscle development in the inbred White Leghorn chicken (line 03) and in adult muscles from the genetically related dystrophic White Leghorn chicken (line 433). RNA dotblot and northern hybridization was employed to monitor MyHC transcript levels utilizing specific oligonucleotide probes. The developmental pattern of MyHC gene expression in the pectoralis major (PM) and the gastrocnemius muscles was similar during embryonic development with three embryonic MyHC isoform genes, Cemb1, Cemb2, and Cemb3, sequentially expressed. Following hatching, MyHC expression patterns in each muscle differed. The expression of MyHC genes was also studied in muscle cell cultures derived from 12-day embryonic pectoralis muscles. In vitro, Cvent, Cemb1, and Cemb2 MyHC genes were expressed; however, little if any Cemb3 MyHC gene expression could be detected, even though Cemb3 was the predominant MyHC gene expressed during late embryonic development in vivo. In most adult muscles other than the PM and anterior latissimus dorsi (ALD), the Cemb3 MyHC gene was the major adult MyHC isoform. In addition, two general patterns of expression were identified in fast muscle. The fast muscles of the leg expressed neonatal (Cneo) and Cemb3 MyHC genes, while other fast muscles expressed adult (Cadult) and Cemb3 MyHC genes. MyHC gene expression in adult dystrophic muscles was found to reflect the expression patterns found in corresponding normal muscles during the neonatal or early post-hatch developmental period, providing additional evidence that avian muscular dystrophy inhibits muscle maturation.

Distal regulatory elements control MRF4 gene expression in early and late myogenic cell populations

MRF4 is a muscle-specific transcription factor that belongs to a family of basic helix-loop-helix proteins known as the myogenic regulatory factors (MRFs). In vitro studies have shown that expression of the MRF4 gene is controlled by a proximal promoter element (-336 to +71) that binds the muscle-specific transcription factors MEF2 and myogenin to activate transcription. To examine further the regulatory elements necessary for endogenous MRF4 gene expression during development, transgenic mice were generated that contained either a proximal MRF4 promoter-LacZ reporter gene (-336 MRF4-nLacZ) or a MRF4-LacZ reporter gene containing 8.5 kb of 5' flanking sequence (-8500 MRF4-nLacZ). Characterization of individual transgenic mouse lines throughout development revealed that expression of both transgenes is restricted to skeletal muscle tissue. However, unlike previous in vitro data, the proximal promoter transgene exhibits only limited transcriptional activity at all developmental time points, whereas the -8500 MRF4-nLacZ lines fully recapitulate the later developmental expression patterns and exhibit transcription in myotomal cells during somitic differentiation. Tissue culture analysis of myogenic cells isolated from E12.5, E16.5, and adults confirmed that the -8500 MRF4-nLacZ transgene is expressed in greater than 90% of the myotubes for all myogenic populations. These results indicate that 8.5 kb of MRF4 5' flanking sequence contains all the regulatory elements necessary for late MRF4 expression and that at least some of these elements lie upstream of the -336 proximal promoter. It is also likely that distant upstream regulatory sequences control early somitic MRF4 expression. These findings, coupled with previous in vitro studies, suggest that the early and late developmental expression patterns of the MRF4 gene are controlled by distinct sets of regulatory elements.

Influence of PDGF-BB on proliferation and transition through the MyoD-myogenin-MEF2A expression program during myogenesis in mouse C2 myoblasts

We have previously demonstrated that PDGF-BB enhances proliferation of C2 myoblasts. This has led us to examine whether the mitogenic influence of PDGF-BB in the C2 model correlates with modulation of specific steps associated with myogenic differentiation. C2 myoblasts transiting through these differentiation specific steps were monitored via immunocytochemistry. We show that the influence of PDGF on enhancing cell proliferation correlates with a delay in the emergence of cells positive for sarcomeric myosin. We further monitored the influence of PDGF-BB on differentiation steps preceding the emergence of myosin+ cells. We demonstrate that mononucleated C2 cells first express MyoD (MyoD+/myogenin- cells) and subsequently, myogenin. Cells negative for both MyoD and myogenin (the phenotype preceding the MyoD+ state) were present at all times in culture and comprised the majority, if not all, of the cells which responded mitogenically to PDGF. Additionally, the frequency of the MyoD+/myogenin+ cell phenotype was reduced in cultures receiving PDGF, suggesting that PDGF can modulate the transition of the cells into the myogenin+ state. We determined that many of the myogenin+ cells subsequently become MEF2A+ and this phenomenon is not influenced by PDGF-BB. FGF-2 also enhanced the proliferation of C2 myoblasts and suppressed the appearance of the myogenin+ cells, but did not influence the subsequent transition into the MEF2A+ state. The study raises the possibility that PDGF-BB and FGF-2 might delay the transition of the C2 cells into the MyoD+/myogenin+ state by depressing a paracrine signal that enhances differentiation.

MyoD protein is differentially accumulated in fast and slow skeletal muscle fibres and required for normal fibre type balance in rodents

MyoD is a muscle-specific transcription factor involved in commitment of cells to myogenesis. MyoD mRNA levels differ between fast and slow muscles, suggesting that MyoD may regulate aspects of fibre type. Here we show that detectable MyoD protein becomes restricted during development to the nuclei of the fastest classes of fibres in fast muscles. myoDm1 mice, in which the myoD gene has been disrupted, show subtle shifts in fibre type of fast muscles toward a slower character, suggesting that MyoD is involved in the maintenance of the fast IIB/IIX fibre type. In contrast, slow muscle shifts to a faster phenotype in myoDm1. Moreover, MD6.0-lacZ transgenic mice with the myoD promoter driving lacZ, show highest beta-galactosidase activity in the fastest fibres of fast muscles, but also express low levels in slow fibres of slow, but not fast, muscles, suggesting distinct regulation of gene expression in slow fibres of fast and slow muscles.

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.

Expression of myosin heavy chain and of myogenic regulatory factor genes in fast or slow rabbit muscle satellite cell cultures

We investigated the myogenic properties of rabbit fast or slow muscle satellite cells during their differentiation in culture, with a particular attention to the expression of myosin heavy chain and myogenic regulatory factor genes. Satellite cells were isolated from Semimembranosus proprius (slow-twitch muscle; 100% type I fibres) and Semimembranosus accessorius (fast-twitch muscle; almost 100% type II fibres) muscles of 3-month-old rabbits. Satellite cells in culture possess different behaviours according to their origin. Cells isolated from slow muscle proliferate faster, fuse earlier into more numerous myotubes and mature more rapidly into striated contractile fibres than do cells isolated from fast muscle. This pattern of proliferation and differentiation is also seen in the expression of myogenic regulatory factor genes. Myf5 is detected in both fast or slow 6-day-old cell cultures, when satellite cells are in the exponential stage of proliferation. MyoD and myogenin are subsequently detected in slow satellite cell cultures, but their expression in fast cell cultures is delayed by 2 and 4 days respectively. MRF4 is detected in both types of cultures when they contain striated and contractile myofibres. Muscle-specific myosin heavy chains are expressed earlier in slow satellite cell cultures. No adult myosin heavy chain isoforms are detected in fast cell cultures for 13 days, whereas cultures from slow cells express neonatal, adult slow and adult fast myosin heavy chain isoforms at that time. In both fast and slow satellite cell cultures containing striated contractile fibres, neonatal and adult myosin heavy chain isoforms are coexpressed. However, cultures made from satellite cells derived from slow muscles express the slow myosin heavy chain isoform, in addition to the neonatal and the fast isoforms. These results are further supported by the expression of the mRNA encoding the adult myosin heavy chain isoforms. These data provide further evidence for the existence of satellite cell diversity between two rabbit muscles of different fibre-type composition, and also suggest the existence of differently preprogrammed satellite cells.

Development and postnatal regulation of adult myoblasts

The myogenic precursor cells of postnatal and adult skeletal muscle are situated underneath the basement membrane of the myofibers. It is because of their unique positions that these precursor cells are often referred to as satellite cells. Such defined satellite cells can first be detected following the formation of a distinct basement membrane around the fiber, which takes place in late stages of embryogenesis. Like myoblasts found during development, satellite cells can proliferate, differentiate, and fuse into myofibers. However, in the normal, uninjured adult muscle, satellite cells are mitotically quiescent. In recent years several important questions concerning the biology of satellite cells have been asked. One aspect has been the relationship between satellite cells and myoblasts found in the developing muscle: are these myogenic populations identical or different? Another aspect has been the physiological cues that control the quiescent, proliferative, and differentiative states of these myogenic precursors: what are the growth regulators and how do they function? These issues are discussed, referring to previous work by others and further emphasizing our own studies on avian and rodent satellite cells. Collectively, the studies presented indicate that satellite cells represent a distinct myogenic population that becomes dominant in late stages of embryogenesis. Moreover, although satellite cells are already destined to be myogenic precursors, they do not express any of the four known myogenic regulatory genes unless their activation is induced in the animal or in culture. Furthermore, multiple growth factors are important regulators of satellite cell proliferation and differentiation. Our work on the role of one of these growth factors [platelet-derived growth factor (PDGF)] during proliferation of adult myoblasts is further discussed with greater detail and the possibility that PDGF is involved in the transition from fetal to adult myoblasts in late embryogenesis is brought forward.

Myogenin binds to and represses c-fos promoter

Myogenin (a member of the myogenic basic helix-loop-helix transcription factor family) seems to be the main effector of proliferation repression, a crucial step which precedes muscle cell terminal differentiation during muscle development. Proliferation repression most likely occurs through inhibition of proliferation-associated genes such as the proto-oncogene, c-fos. Here, we demonstrate that myogenin binds to an E-box located in the main element of the c-fos promoter, the serum response element (SRE). Results from co-transfection experiments indicate that myogenin acts as a repressor for the SRE. Our data suggest that myogenin could play a role in c-fos inhibition at the onset of muscle cell terminal differentiation.

Embryonic activation of the myoD gene is regulated by a highly conserved distal control element

MyoD belongs to a small family of basic helix-loop-helix transcription factors implicated in skeletal muscle lineage determination and differentiation. Previously, we identified a transcriptional enhancer that regulates the embryonic expression of the human myoD gene. This enhancer had been localized to a 4 kb fragment located 18 to 22 kb upstream of the myoD transcriptional start site. We now present a molecular characterization of this enhancer. Transgenic and transfection analyses localize the myoD enhancer to a core sequence of 258 bp. In transgenic mice, this enhancer directs expression of a lacZ reporter gene to skeletal muscle compartments in a spatiotemporal pattern indistinguishable from the normal myoD expression domain, and distinct from expression patterns reported for the other myogenic factors. In contrast to the myoD promoter, the myoD enhancer shows striking conservation between humans and mice both in its sequence and its distal position. Furthermore, a myoD enhancer/heterologous promoter construct exhibits muscle-specific expression in transgenic mice, demonstrating that the myoD promoter is dispensable for myoD activation. With the exception of E-boxes, the myoD enhancer has no apparent sequence similarity with regulatory regions of other characterized muscle-specific structural or regulatory genes. Mutation of these E-boxes, however, does not affect the pattern of lacZ transgene expression, suggesting that myoD activation in the embryo is E-box-independent. DNase I protection assays reveal multiple nuclear protein binding sites in the core enhancer, although none are strictly muscle-specific. Interestingly, extracts from myoblasts and 10T1/2 fibroblasts yield identical protection profiles, indicating a similar complement of enhancer-binding factors in muscle and this non-muscle cell type. However, a clear difference exists between myoblasts and 10T1/2 cells (and other non-muscle cell types) in the chromatin structure of the chromosomal myoD core enhancer, suggesting that the myoD enhancer is repressed by epigenetic mechanisms in 10T1/2 cells. These data indicate that myoD activation is regulated at multiple levels by mechanisms that are distinct from those controlling other characterized muscle-specific genes.

The MyoD family of transcription factors and skeletal myogenesis

Gene targeting has allowed the dissection of complex biological processes at the genetic level. Our understanding of the nuances of skeletal muscle development has been greatly increased by the analysis of mice carrying targeted null mutations in the Myf-5, MyoD and myogenin genes, encoding members of the myogenic regulatory factor (MRF) family. These experiments have elucidated the hierarchical relationships existing between the MRFs, and established that functional redundancy is a feature of the MRF regulatory network. Either MyoD or Myf-5 is sufficient for the formation or survival of skeletal myoblasts. Myogenin acts later in development and plays an essential in vivo role in the terminal differentiation of myotubes.

MyoD expression marks the onset of skeletal myogenesis in Myf-5 mutant mice

The expression pattern of myogenic regulatory factors and myotome-specific contractile proteins was studied during embryonic development of Myf-5 mutant mice by in situ hybridization and immunohistochemistry. In contrast to somites in wild-type embryos, no expression of myogenin and Myf-6 (MRF4), or any other myotomal markers was detected in mutant animals at E9.0 and E10.0 indicating that Myf-5 plays a crucial role during this developmental period. Significantly, the onset of MyoD expression in rostral somites of E10.5 embryos was unaffected by the Myf-5 mutation suggesting that the activation of the MyoD gene occurs independently of Myf-5 at the correct developmental time. Immediately after the activation of MyoD myogenin transcripts and protein accumulated within the myotome. The first contractile proteins of the sarcomeric apparatus appeared slightly later. By E11.5 the expression of muscle markers were indistinguishable between wild-type and Myf-5 mutant mice. The migration of muscle precursor cells that leave the somites to form limb musculature was monitored in Myf-5-mutant mice by Pax-3 expression. Pax-3-positive cells were equally found in somites and limbs of E10.0 wild-type and mutant mice indicating that myogenic factor expression at the level of somites is not a prerequisite for determination and subsequent migration of limb precursor cells.

Temporal expression of regulatory and structural muscle proteins during myogenesis of satellite cells on isolated adult rat fibers

Myogenic precursors in adult skeletal muscle (satellite cells) are mitotically quiescent but can proliferate in response to a variety of stresses including muscle injury. To gain further understanding of adult myoblasts, we analyzed myogenesis of satellite cells on intact fibers isolated from adult rat muscle. In this culture model, satellite cells are maintained in their in situ position underneath the fiber basement membrane. In the present study patterns of satellite cell proliferation, expression of myogenic regulatory factor proteins, and expression of differentiation-specific, cytoskeletal proteins were determined, via immunohistochemistry of cultured fibers. The temporal appearance and the numbers of cells positive for proliferating cell nuclear antigen (PCNA) or for MyoD were similar, suggesting that MyoD is present in detectable amounts in proliferating but not quiescent satellite cells. Satellite cells positive for myogenin, alpha-smooth muscle actin (alpha SMactin), or developmental sarcomeric myosin (DEVmyosin) appeared following the decline in PCNA and MyoD expression. However, expression of myogenin and alpha SMactin was transient, while DEV-myosin expression was continuously maintained. Moreover, the number of DEVmyosin + cells was only half of the number of myogenin + or alpha SMactin + cells--indicating, perhaps, that only 50% of the satellite cell descendants entered the phase of terminal differentiation. We further determined that the number of proliferating satellite cells can be modulated by basic FGF but the overall schedule of cell cycle entry, proliferation, differentiation, and temporal expression of regulatory and structural proteins was unaffected. We thus conclude that satellite cells conform to a highly coordinated program when undergoing myogenesis at their native position along the muscle fiber.

Temporal expression of myogenic regulatory genes during activation, proliferation, and differentiation of rat skeletal muscle satellite cells

The satellite cell is responsible for growth and repair of postnatal skeletal muscle. We investigated the expression of the myogenic regulatory gene (MRG) family in these cells in the stages from quiescence to fusion. Using polymerase chain reaction amplification of reverse-transcribed RNA (RT-PCR) isolated from adult rat satellite cells, we demonstrated a temporal sequence of gene activation, which is distinct from that previously observed in embryonic somatic cells. No MRG expression was detected in predominantly quiescent cells. MyoD is activated by 12 h in cell culture, prior to the first evidence of proliferation. MRF4 and myf-5 appear by 48 h and may be associated with the first division cycle. Myogenin is not detectable until 72 h after satellite cell recovery from the muscle fiber, coincidental with the first evidence of differentiation.

Differential expression of muscle regulatory factor genes in normal and denervated adult rat hindlimb muscles

Skeletal muscle represents an excellent model system in which to examine regulatory mechanisms that modulate gene expression in the mature adult organism. Individual muscle fibers can be categorized as fast- or slow-twitch based upon several physiological and molecular criteria, including metabolic enzyme activity and contractile protein isoforms. Each property can be influenced by a variety of factors such as changes in motor neuron activity or alterations in hormone levels, although the molecular pathways by which environmental factors affect gene expression remain largely unknown. As a first step in identifying potential regulators of fiber-type diversity, the expression patterns of four basic/helix-loop-helix muscle regulatory factors (MRFs), referred to as MyoD, myogenin, Myf-5, and MRF4, were examined in normal adult rat muscles which differed in their phenotypic properties. As expected, all four MRFs were expressed at detectable levels in the muscles studied. However, different muscles accumulated different proportions and combinations of MRF transcripts. For example, myogenin expression was maximally detected in slow-twitch muscles whereas MyoD transcripts were found predominantly in muscles exhibiting a fast-twitch phenotype. Induced phenotypic changes in two fast-twitch muscles via denervation lead to a large and rapid increase in transcript levels of all four MRFs as early as 24 hr following denervation, with myogenin transcripts approaching 150-200-fold higher levels than innervated contralateral muscles within 7 days. These results suggest that myogenin, as well as the other three MRFs, may be involved in both the initial establishment as well as maintenance of fiber-type diversity in the developing organism.

Nkx-2.5: a novel murine homeobox gene expressed in early heart progenitor cells and their myogenic descendants

We have isolated two murine homeobox genes, Nkx-2.5 and Nkx-2.6, that are new members of a sp sub-family of homeobox genes related to Drosophila NK2, NK3 and NK4/msh-2. In this paper, we focus on the Nkx-2.5 gene and its expression pattern during post-implantation development. Nkx-2.5 transcripts are first detected at early headfold stages in myocardiogenic progenitor cells. Expression preceeds the onset of myogenic differentiation, and continues in cardiomyocytes of embryonic, foetal and adult hearts. Transcripts are also detected in future pharyngeal endoderm, the tissue believed to produce the heart inducer. Expression in endoderm is only found laterally, where it is in direct apposition to promyocardium, suggesting an interaction between the two tissues. After foregut closure, Nkx-2.5 expression in endoderm is limited to the pharyngeal floor, dorsal to the developing heart tube. The thyroid primordium, a derivative of the pharyngeal floor, continues to express Nkx-2.5 after transcript levels diminish in the rest of the pharynx. Nkx-2.5 transcripts are also detected in lingual muscle, spleen and stomach. The expression data implicate Nkx-2.5 in commitment to and/or differentiation of the myocardial lineage. The data further demonstrate that cardiogenic progenitors can be distinguished at a molecular level by late gastrulation. Nkx-2.5 expression will therefore be a valuable marker in the analysis of mesoderm development and an early entry point for dissection of the molecular basis of myogenesis in the heart.