Differential Telomere Shortening in Blood versus Arteries in an Animal Model of Type 2 Diabetes

Vascular dysfunction is an early feature of diabetic vascular disease, due to increased oxidative stress and reduced nitric oxide (NO) bioavailability. This can lead to endothelial cell senescence and clinical complications such as stroke. Cells can become senescent by shortened telomeres and oxidative stress is known to accelerate telomere attrition. Sirtuin 1 (SIRT1) has been linked to vascular health by upregulating endothelial nitric oxide synthase (eNOS), suppressing oxidative stress, and attenuating telomere shortening. Accelerated leukocyte telomere attrition appears to be a feature of clinical type 2 diabetes (T2D) and therefore the telomere system may be a potential therapeutic target in preventing vascular complications of T2D. However the effect of T2D on vascular telomere length is currently unknown. We hypothesized that T2D gives rise to shortened leukocyte and vascular telomeres alongside reduced vascular SIRT1 expression and increased oxidative stress. Accelerated telomere attrition was observed in circulating leukocytes, but not arteries, in T2D compared to control rats. T2D rats had blunted arterial SIRT1 and eNOS protein expression levels which were associated with reduced antioxidant defense capacity. Our findings suggest that hyperglycemia and a deficit in vascular SIRT1 per se are not sufficient to prematurely shorten vascular telomeres.

In vitro study of antibacterial activity of the alga Sargassum oligocystum from the Persian Gulf

BACKGROUND AND OBJECTIVES

With due attention to the development of drug-resistant bacteria, discovering of new antibacterial compounds is needed. Algae produce numerous bioactive substances which may have pharmacological properties such as antibacterial activity. The objective of this investigation was to in vitro study of antibacterial activity of brown alga Sargassum oligocystum collected along the Bushehr coast of Persian Gulf (south west of Iran).

MATERIALS AND METHODS

Hot water extract, cold water extract, and hot glycerin extract were prepared. The effect of the extracts were investigated on Staphylococcus aureus (ATCC 25923), Staphylococcus epidermidis (ATCC 14990), Pseudomonas aeruginosa (ATCC 27853), and Escherichia coli (ATCC 25922).

RESULTS

Hot water extract exhibited antibacterial activity against Staphylococcus aureus, Staphylococcus epidermidis, and Pseudomonas aeruginosa. Cold water extract and hot glycerin extract did not show antibacterial activity on any of the four test bacteria. The minimum inhibitory concentration (MIC) of hot water extract for both Staphylococcus aureus and Staphylococcus epidermidis was 3.175 mg/ml. However, the MIC of this extract for Pseudomonas aeruginosa was 9.556 mg/ml.

DISCUSSION

In this study gram-positive bacteria were more susceptible to hot water extract than gram-negative bacteria. Extract of Sargassum oligocystum could be a candidate for purification and further in vivo studies.

Rapid detection of Streptococcus pyogenes in throat swab specimens by fluorescent in situ hybridization

BACKGROUND AND OBJECTIVES

Streptococcus pyogenes (S. pyogenes) is an important cause of pharyngitis. Rapid detection of this microorganism in throat specimens is essential to promptly start antibiotic therapy which could be lead to prevent complications and stop transmission of infection to other individuals. In the present study, fluorescent in situ hybridization (FISH) was compared with culture method for the detection of S. pyogenes in throat swab specimens.

MATERIALS AND METHODS

One hundred eleven patients with pharyngitis were included in this study. The throat swab specimens of these patients were investigated by both conventional culturing and FISH.

RESULTS

Based on the results of this investigation, the sensitivity and specificity of FISH were 88.9% and 97.8%, respectively. Strikingly, in the specimen of one patient who had received antibiotic previous to clinical sampling, S. pyogenes was detected by means of FISH, whereas the culture method could not detect this bacterium.

CONCLUSIONS

It seems that FISH is a suitable method for quick identification of S. pyogenes in throat swab specimens. When FISH is positive, culturing is not necessary. But because of the limited sensitivity of FISH for detection of S. pyogenes in throat swab specimens, culturing shoud be performed if FISH was negative.

Anticancer activity of Sargassum oligocystum water extract against human cancer cell lines

BACKGROUND AND OBJECTIVES

Antitumor drug resistance and side effects of antitumor compounds are the most common problems in medicine. Therefore, finding new antitumor agents with low side effects could be interesting. This study was designed to assay antitumor activity of the extract from brown alga Sargassum oligocystum, gathered from Persian Gulf seashore, against K562 and Daudi human cancer cell lines.

MATERIALS AND METHODS

The research was performed as an in vitro study. The effect of the alga extract on proliferation of cell lines were measured by two methods: MTT assay and trypan blue exclusion test.

RESULTS AND CONCLUSION

The most effective antitumor activity has been shown at concentrations 500 microg/ml and 400 microg/ml of the alga extract against Daudi and K562 cell lines, respectively. The results showed that the extracts of brown alga Sargassum oligocystum have remarkable antitumor activity against K562 and Daudi cell lines. It is justified to be suggested for further research such as algal extract fractionation and purification and in vivo studies in order to formulate natural compounds with antitumor activities.

Skeletal muscle stem cells in developmental versus regenerative myogenesis

Tissue and organ regeneration proceed in a coordinated manner to restore proper function after trauma. Vertebrate skeletal muscle has a remarkable ability to regenerate after repeated and complete destruction of the tissue, yet limited information is available on how muscle stem and progenitor cells, and other nonmuscle cells, reestablish homeostasis after the regenerative process. The genetic pathways that regulate the establishment of skeletal muscle in the embryo have been studied extensively, and many of the genes that govern muscle stem cell maintenance and commitment are redeployed during adult homeostasis and regeneration. Therefore, correlates can be made between embryonic muscle development and postnatal regeneration. However, there are some important distinctions between prenatal development and regeneration - in the context of the cells, niche, anatomy and the regulatory genes employed. The similarities and distinctions between these two scenarios are the focus of this review.

Corrosion of Low-Carbon Steels in Sulfuric Acid

The corrosion behavior of some low-carbon steels in different concentrations of inhibitor in 0.5 M sulfuric acid solution were studied by potentiodynamic polarization measurements with a scan rate of 1mVs-1 at various temperatures (293–323 ± 1K). It was found that the inhibitor is adsorbed on the steel surface and the experimental results are in agreement with Temkin isotherm. Negative values of the free energy of adsorption were obtained indicating the spontaneity of adsorption process. The formation of insoluble complexes as a result of interaction between metal cations and inhibitor is also discussed and some corrosion parameters such as cathodic Tafel slope, corrosion potential, corrosion current, passivation potential, passivation current, surface coverage and inhibition efficiency were calculated.

The homeobox gene Arx is a novel positive regulator of embryonic myogenesis

Skeletal muscle fibers form in overlapping, but distinct phases that depend on the generation of temporally different lineages of myogenic cells. During primary myogenesis (E10.5-E12.5 in the mouse), embryonic myoblasts fuse homotypically to generate primary fibers, whereas during later development (E14.5-E17.5), fetal myoblasts differentiate into secondary fibers. How these myogenic waves are regulated remains largely unknown. Studies have been hampered by the lack of markers which would distinguish embryonic from fetal myoblast populations. We show here that the homeobox gene Arx is strongly expressed in differentiating embryonic muscle, downstream of myogenic basic helix-loop-helix (bHLH) genes. Its expression progressively decreases during development. When overexpressed in the C2C12 myogenic cell line, Arx enhances differentiation. Accordingly, it stimulates the transcriptional activity from the Myogenin promoter and from multimerized E-boxes when co-expressed with MyoD and Mef2C in CH310T1/2. Furthermore, Arx co-immunoprecipitates with Mef2C, suggesting that it participates in the transcriptional regulatory network acting in embryonic muscle. Finally, embryonic myoblasts isolated from Arx-deficient embryos show a delayed differentiation in vivo together with an enhanced clonogenic capacity in vitro. We propose here that Arx acts as a novel positive regulator of embryonic myogenesis by synergizing with Mef2C and MyoD and by establishing an activating loop with Myogenin.

Transplanted primary neonatal myoblasts can give rise to functional satellite cells as identified using the Myf5nlacZl+ mouse

Myoblast transplantation is a potential therapeutic approach for the genetic modification of host skeletal muscle tissue. To be considered an effective, long-lived method of delivery, however, it is essential that at least a proportion of the transplanted cells also retain their proliferative potential. We sought to investigate whether transplanted neonatal myoblasts can contribute to the satellite cell compartment of adult skeletal muscle by using the Myf5nlacZ/+ mouse. The Myf5nlacZ/+ mouse has nlacZ targeted to the Myf5 locus resulting in beta-galactosidase activity in quiescent satellite cells. Following transplantation, beta-galactosidase-labelled nuclei were detected in host muscles, showing that donor cells had been incorporated. Significantly, beta-galactosidase-positive, and therefore donor-derived, satellite cells were detected. When placed in culture, beta-galactosidase marked myogenic cells emanated from the parent fibre. These observations demonstrate that cell transplantation not only results in the incorporation of donor nuclei into the host muscle syncytia, but also that the donor cells can become functional satellite cells. The Myf5nlacZ/+ mouse therefore provides a novel and specific marker for determining the contribution of transplanted cells to the satellite cell pool.

Expression of CD34 and Myf5 defines the majority of quiescent adult skeletal muscle satellite cells

Skeletal muscle is one of a several adult post-mitotic tissues that retain the capacity to regenerate. This relies on a population of quiescent precursors, termed satellite cells. Here we describe two novel markers of quiescent satellite cells: CD34, an established marker of hematopoietic stem cells, and Myf5, the earliest marker of myogenic commitment. CD34(+ve) myoblasts can be detected in proliferating C2C12 cultures. In differentiating cultures, CD34(+ve) cells do not fuse into myotubes, nor express MyoD. Using isolated myofibers as a model of synchronous precursor cell activation, we show that quiescent satellite cells express CD34. An early feature of their activation is alternate splicing followed by complete transcriptional shutdown of CD34. This data implicates CD34 in the maintenance of satellite cell quiescence. In heterozygous Myf5(nlacZ/+) mice, all CD34(+ve) satellite cells also express beta-galactosidase, a marker of activation of Myf5, showing that quiescent satellite cells are committed to myogenesis. All such cells are positive for the accepted satellite cell marker, M-cadherin. We also show that satellite cells can be identified on isolated myofibers of the myosin light chain 3F-nlacZ-2E mouse as those that do not express the transgene. The numbers of satellite cells detected in this way are significantly greater than those identified by the other three markers. We conclude that the expression of CD34, Myf5, and M-cadherin defines quiescent, committed precursors and speculate that the CD34(-ve), Myf5(-ve) minority may be involved in maintaining the lineage-committed majority.

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.

Transdifferentiation of esophageal smooth to skeletal muscle is myogenic bHLH factor-dependent

Previously, coexpression of smooth and skeletal differentiation markers, but not myogenic regulatory factors (MRFs), was observed from E16.5 mouse fetuses in a small percentage of diaphragm level esophageal muscle cells, suggesting that MRFs are not involved in the process of initiation of developmentally programmed transdifferentiation in the esophagus. To investigate smooth-to-skeletal esophageal muscle transition, we analyzed Myf5nlacZ knock-in mice, MyoD-lacZ and myogenin-lacZ transgenic embryos with a panel of the antibodies reactive with myogenic regulatory factors (MRFs) and smooth and skeletal muscle markers. We observed that lacZ-expressing myogenic precursors were not detected in the esophagus before E15.5, arguing against the hypothesis that muscle precursor cells populate the esophagus at an earlier stage of development. Rather, the expression of the MRFs initiated in smooth muscle cells in the upper esophagus of E15.5 mouse embryos and was immediately followed by the expression of skeletal muscle markers. Moreover, transdifferentiation was markedly delayed or absent only in the absence of Myf5, suggesting that appropriate initiation and progression of smooth-to-skeletal muscle transdifferentiation is Myf5-dependent. Accordingly, the esophagus of Myf5(−/−):MyoD(−/−)embryos completely failed to undergo skeletal myogenesis and consisted entirely of smooth muscle. Lastly, extensive proliferation of muscularis precursor cells, without programmed cell death, occurred concomitantly with esophageal smooth-to-skeletal muscle transdifferentiation. Taken together, these results indicate that transdifferentiation is the fate of all smooth muscle cells in the upper esophagus and is normally initiated by Myf5.

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.

Transplacental delivery of the Wnt antagonist Frzb1 inhibits development of caudal paraxial mesoderm and skeletal myogenesis in mouse embryos

Axial structures (neural tube/notochord) and surface ectoderm activate myogenesis in the mouse embryo; their action can be reproduced, at least in part, by several molecules such as Sonic hedgehog and Wnts. Recently, soluble Wnt antagonists have been identified. Among those examined only Frzb1 was found to be expressed in the presomitic mesoderm and newly formed somites and thus its possible role in regulating myogenesis was investigated in detail. When presomitic mesoderm or newly formed somites were cultured with axial structures and surface ectoderm on a feeder layer of C3H10T1/2 cells expressing Frzb1, myogenesis was abolished or severely reduced in presomitic mesoderm and the three most recently formed somites. In contrast, no effect was observed on more mature somites. Inhibition of myogenesis did not appear to be associated with increased cell death since the final number of cells in the explants grown in the presence of Frzb1 was only slightly reduced in comparison with controls. In order to examine the possible function of Frzb1 in vivo, we developed a method based on the overexpression of the soluble antagonist by transient transfection of WOP cells with a Frzb1 expression vector and injection of transfected cells into the placenta of pregnant females before the onset of maternofoetal circulation. Frzb1, secreted by WOP cells, accumulated in the embryo and caused a marked reduction in size of caudal structures. Myogenesis was strongly reduced and, in the most severe cases, abolished. This was not due to a generalized toxic effect since only several genes downstream of the Wnt signaling pathway such as En1, Noggin and Myf5 were downregulated; in contrast, Pax3 and Mox1 expression levels were not affected even in embryos exhibiting the most severe phenotypes. Taken together, these results suggest that Wnt signals may act by regulating both myogenic commitment and expansion of committed cells in the mouse mesoderm.

Sonic hedgehog controls epaxial muscle determination through Myf5 activation

Sonic hedgehog (Shh), produced by the notochord and floor plate, is proposed to function as an inductive and trophic signal that controls somite and neural tube patterning and differentiation. To investigate Shh functions during somite myogenesis in the mouse embryo, we have analyzed the expression of the myogenic determination genes, Myf5 and MyoD, and other regulatory genes in somites of Shh null embryos and in explants of presomitic mesoderm from wild-type and Myf5 null embryos. Our findings establish that Shh has an essential inductive function in the early activation of the myogenic determination genes, Myf5 and MyoD, in the epaxial somite cells that give rise to the progenitors of the deep back muscles. Shh is not required for the activation of Myf5 and MyoD at any of the other sites of myogenesis in the mouse embryo, including the hypaxial dermomyotomal cells that give rise to the abdominal and body wall muscles, or the myogenic progenitor cells that form the limb and head muscles. Shh also functions in somites to establish and maintain the medio-lateral boundaries of epaxial and hypaxial gene expression. Myf5, and not MyoD, is the target of Shh signaling in the epaxial dermomyotome, as MyoD activation by recombinant Shh protein in presomitic mesoderm explants is defective in Myf5 null embryos. In further support of the inductive function of Shh in epaxial myogenesis, we show that Shh is not essential for the survival or the proliferation of epaxial myogenic progenitors. However, Shh is required specifically for the survival of sclerotomal cells in the ventral somite as well as for the survival of ventral and dorsal neural tube cells. We conclude, therefore, that Shh has multiple functions in the somite, including inductive functions in the activation of Myf5, leading to the determination of epaxial dermomyotomal cells to myogenesis, as well as trophic functions in the maintenance of cell survival in the sclerotome and adjacent neural tube.

In vivo satellite cell activation via Myf5 and MyoD in regenerating mouse skeletal muscle

Regeneration of adult skeletal muscle is an asynchronous process requiring the activation, proliferation and fusion of satellite cells, to form new muscle fibres. This study was designed to determine the pattern of expression in vivo of the two myogenic regulatory factors, Myf5 and MyoD during this process. Cardiotoxin was used to induce regeneration in the gastrocnemius and soleus muscles of heterozygous Myf5-nlacZ mice, and the muscles were assayed for the presence of (beta)-galactosidase (Myf5) and MyoD. Adult satellite cells identified by M-cadherin labelling, when activated, initially express either MyoD or Myf5 or both myogenic factors. Subsequently all proliferating myoblasts express MyoD and part of the population is (beta)-galactosidase (Myf5) positive. Furthermore, we demonstrate that activated satellite cells, which express either Myf5 or MyoD, do not accumulate selectively on fast or slow muscle fibres.

The homeobox gene Msx1 is expressed in a subset of somites, and in muscle progenitor cells migrating into the forelimb

In myoblast cell cultures, the Msx1 protein is able to repress myogenesis and maintain cells in an undifferentiated and proliferative state. However, there has been no evidence that Msx1 is expressed in muscle or its precursors in vivo. Using mice with the nlacZ gene integrated into the Msx1 locus, we show that the reporter gene is expressed in the lateral dermomyotome of brachial and thoracic somites. Cells from this region will subsequently contribute to forelimb and intercostal muscles. Using Pax3 gene transcripts as a marker of limb muscle progenitor cells as they migrate from the somites, we have defined precisely the somitic origin and timing of cell migration from somites to limb buds in the mouse. Differences in the timing of migration between chick and mouse are discussed. Somites that label for Msx1(nlacZ)transgene expression in the forelimb region partially overlap with those that contribute Pax3-expressing cells to the forelimb. In order to see whether Msx1 is expressed in this migrating population, we have grafted somites from the forelimb level of Msx1(nlacZ)mouse embryos into a chick host embryo. We show that most cells migrating into the wing field express the Msx1(nlacZ)transgene, together with Pax3. In these experiments, Msx1 expression in the somite depends on the axial position of the graft. Wing mesenchyme is capable of inducing Msx1 transcription in somites that normally would not express the gene; chick hindlimb mesenchyme, while permissive for this expression, does not induce it. In the mouse limb bud, the Msx1(nlacZ)transgene is downregulated prior to the activation of the Myf5 gene, an early marker of myogenic differentiation. These observations are consistent with the proposal that Msx1 is involved in the repression of muscle differentiation in the lateral half of the somite and in limb muscle progenitor cells during their migration.

Differential activation of Myf5 and MyoD by different Wnts in explants of mouse paraxial mesoderm and the later activation of myogenesis in the absence of Myf5

Activation of myogenesis in newly formed somites is dependent upon signals derived from neighboring tissues, namely axial structures (neural tube and notochord) and dorsal ectoderm. In explants of paraxial mesoderm from mouse embryos, axial structures preferentially activate myogenesis through a Myf5-dependent pathway and dorsal ectoderm preferentially through a MyoD-dependent pathway. Here we report that cells expressing Wnt1 will preferentially activate Myf5 while cells expressing Wnt7a will preferentially activate MyoD. Wnt1 is expressed in the dorsal neural tube and Wnt7a in dorsal ectoderm in the early embryo, therefore both can potentially act in vivo to activate Myf5 and MyoD, respectively. Wnt4, Wnt5a and Wnt6 exert an intermediate effect activating both Myf5 and MyoD equivalently in paraxial mesoderm. Sonic Hedgehog synergises with both Wnt1 and Wnt7a in explants from E8.5 paraxial mesoderm but not in explants from E9.5 embryos. Signaling through different myogenic pathways may explain the rescue of muscle formation in Myf5 null embryos, which do not form an early myotome but later develop both epaxial and hypaxial musculature. Explants of unsegmented paraxial mesoderm contain myogenic precursors capable of expressing MyoD in response to signaling from a neural tube isolated from E10.5 embryos, the developmental stage when MyoD is present throughout the embryo. Myogenic cells cannot activate MyoD in response to signaling from a less mature neural tube. Together these data suggest that different Wnt molecules can activate myogenesis through different pathways such that commitment of myogenic precursors is precisely regulated in space and time to achieve the correct pattern of skeletal muscle development.

DAN is a secreted glycoprotein related to Xenopus cerberus

We report that DAN, a potential cell cycle regulator and tumour suppressor, is a secreted glycoprotein related to Xenopus cerberus. DAN, cerberus, its mouse relative Cer-1/cer-l/Cerberus-like/Cerr1, and the recently described factor DRM/Gremlin, appear to be members of the cystine knot superfamily, which includes TGFbetas and BMPs. Like cerberus and mCer-1, DAN-induced cement glands as well as markers of anterior neural tissue and endoderm in Xenopus animal cap assays, features of BMP signalling blockade. During mouse embryogenesis, Dan was expressed from E8.5 in cranial mesenchyme and somites, then later in limb and facial mesenchyme. The pattern in somites was highly dynamic, with transcripts initially localized to the caudal half of the nascent epithelial somite, then, after maturation, to sclerotomal cells adjacent to the neural tube. Dan was also expressed in the developing myotome. The expression domains include sites in which BMP inhibition is known to be important for development. Thus, DAN appears to be a secreted factor belonging to the cystine knot superfamily, and one of a growing number of antagonists acting to modulate BMP signalling during development.

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.

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.

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.

Developmental and tissue-specific regulation of the murine cardiac actin gene in vivo depends on distinct skeletal and cardiac muscle-specific enhancer elements in addition to the proximal promoter

Cardiac actin is an early marker of cardiac and skeletal muscle lineages in the mouse. After birth, the gene is down-regulated in skeletal muscle. High-level expression of the murine cardiac actin gene in skeletal myotubes in vitro involves distal (-7.8/-7.0 kb) and proximal (-5.4/-3.5 kb) enhancer sequences as well as the proximal promoter (-0.7/+0.1 kb). Transgenic mice carrying an nlacZ reporter gene under the control of different fragments of the upstream region of the cardiac actin gene were generated. This analysis led to the conclusions that (1) the proximal promoter is a weak but tissue specific element in vivo, (2) consistent high-level expression in skeletal muscle depends on the presence of at least one of the enhancers, (3) expression in adult cardiac muscle requires a cardiac enhancer located in the (-5.4/-0.7 kb) region, and (4) a construct containing these three elements gives a strong specific expression of the transgene in the heart throughout the life of the animal and in embryonic skeletal muscle. All transgenes tested reproduce the down-regulation observed in adult skeletal muscle for the cardiac actin gene. Nonuniform expression of these transgenes in the heart may mark cardiomyocytes derived from different cardiac progenitors.

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

myf-5 is one of four transcription factors belonging to the MyoD family that play key roles in skeletal muscle determination and differentiation. We have shown earlier by gene targeting nlacZ into the murine myf-5 locus that myf-5 expression in the developing mouse embryo is closely associated with the restriction of precursor muscle cells to the myogenic lineage. We now identify unexpected expression of this myogenic factor in subdomains of the brain. myf-5 expression begins to be detected at embryonic day 8 (E8) in the mesencephalon and coincides with the appearance of the first differentiated neurons; expression in the secondary prosencephalon initiates at E10 and is confined to the ventral domain of prosomere p4, later becoming restricted to the posterior hypothalamus. This expression is observed throughout embryogenesis. No other member of the MyoD family is detected in these regions, consistent with the lack of myogenic conversion. Furthermore, embryonic stem cells expressing the myf-5/nlacZ allele yield both skeletal muscle and neuronal cells when differentiated in vitro. These observations raise questions about the role of myf-5 in neurogenesis as well as myogenesis, and introduce a new lineage marker for the developing brain.

Myosin light chain 3F regulatory sequences confer regionalized cardiac and skeletal muscle expression in transgenic mice

The myosin light chain IF/3F locus contains two independent promoters, MLC1F and MLC3F, which are differentially activated during skeletal muscle development. Transcription at this locus is regulated by a 3' skeletal muscle enhancer element, which directs correct temporal and tissue-specific expression from the MLC1F promoter in transgenic mice. To investigate the role of this enhancer in regulation of the MLC3F promoter in vivo, we have analyzed reporter gene expression in transgenic mice containing lacZ under transcriptional control of the mouse MLC3F promoter and 3' enhancer element. Our results show that these regulatory elements direct strong expression of lacZ in skeletal muscle; the transgene, however, is activated 4-5 d before the endogenous MLC3F promoter, at the time of initiation of MLC1F transcription. In adult mice, transgene activity is downregulated in muscles that have reduced contributions of type IIB fibers (soleus and diaphragm). The rostrocaudal positional gradient of transgene expression documented for MLC1F transgenic mice (Donoghue, M., J. P. Merlie, N. Rosenthal, and J. R. Sanes. 1991. Proc. Natl. Acad. Sci. USA. 88:5847-5851) is not seen in MLC3F transgenic mice. Although MLC3F was previously thought to be restricted to skeletal striated muscle, the MLC3F-lacZ transgene is expressed in cardiac muscle from 7.5 d of development in a spatially restricted manner in the atria and left ventricular compartments, suggesting that transcriptional differences exist between cardiomyocytes in left and right compartments of the heart. We show here that transgene-directed expression of the MLC3F promoter reflects low level expression of endogenous MLC3F transcripts in the mouse heart.

A population of myogenic cells derived from the mouse neural tube

Embryonic mouse neural tubes produce a variety of terminally differentiated cells in vitro, mostly neurons and glia. We report here that some of these cells differentiate into skeletal muscle cells. The possibility of mesoderm contamination was ruled out as follows. First, Dil+ muscle cells were present in cultures from a Dil-labeled neuroepithelium. Second, a small fraction of cultured neural tube cells coexpressed muscle myosin and neuronal beta III tubulin within the same cell. Third, embryos generated from embryonic stem cells in which nlacZ was targeted into the myogenic gene myf-5 expressed nlacZ in a localized region of the neural tube. These myf-5+ cells coexpress neuronal and muscle markers in culture. The developmental significance of this phenomenon is discussed in the context of overlapping regulatory networks between myogenesis and neurogenesis.

Plaque assay and replication of Tipula iridescent virus in Spodoptera frugiperda ovarian cells

A plaque assay was developed for the study of Tipula iridescent virus (TIV) replication using a cell line derived from the fall army worm Spodoptera frugiperda (Sf9). Infection and plaque formation were monitored with time by phase contrast microscopy, video and fluorescent light microscopy. Structure of virions, viroplasmic centres and organelles of infected cells were examined by transmission electron microscopy (TEM). After 4 h postinfection, plaques were visibly detected within the cell monolayer by the presence of localized cell damage and production of numerous vesicular-like cytoplasmic structures. Quantitation of virions present per A260 unit of TIV preparation was determined by TEM. The number of visible plaques corresponded to virus concentration and 1 A260 produced approximately 10(5) plaques. DNA hybridization analysis revealed no gross differences in genomic DNA from TIV propagated in either Sf9 cells or wax moth Galleria mellonella larvae. These findings indicate that Sf9 is permissive for replication of TIV and superior by some parameters to other cell lines currently in use for the study of host cell/TIV interactions.

Mouse limb muscle is determined in the absence of the earliest myogenic factor myf-5

myf-5 is the only member of the MyoD family of myogenic regulatory genes to be expressed in the mouse embryo prior to muscle cell differentiation. We have used the developing limb as a model in which to follow the formation of peripheral muscle, to address the question of whether myogenic precursor cells are already present in the limb bud before expression of myf-5. The lacZ reporter gene has been introduced into the myf-5 gene by homologous recombination so that its expression is under the control of the endogenous myf-5 locus. beta-Galactosidase (beta-gal) coloration provides a sensitive assay for myf-5+ cells. Embryos were generated from embryonic stem cells carrying this mutation, and the appearance of beta-gal+ (myf-5+) cells was followed during limb development in vivo. Limb buds, at a stage when they are beta-gal-, were cultured in vitro. After several days, beta-gal+ cells accumulated in the premuscle mass. We conclude that determined muscle precursor cells in the limb bud do not initially express any member of the MyoD family. Furthermore, myogenic precursor cells in the somite, which, according to the avian model, migrate from the ventral/lateral edge of the dermomyotome to form peripheral muscle masses, are also negative for these factors.

Expression of myogenic factors in the mouse: myf-5, the first member of the MyoD gene family to be transcribed during skeletal myogenesis

There are four myogenic factors of the MyoD family present in mammals, each have distinct patterns of expression during skeletal myogenesis in the mouse. This differs between axial (myotome) and peripheral (limb) muscle masses, which are derived from the medial and lateral parts of the somite, respectively. The onset of expression of the muscle structural genes also differs, suggesting differences in their activation threshold in response to different factors. In all cases myf-5 is the first gene to be expressed. In order to examine this more closely, we have introduced an nlsLacZ sequence into the myf-5 gene by homologous recombination. This has permitted us to follow expression of this gene at the cellular level. While the immediate precursors of the myotome are labelled, we have shown that peripheral muscle masses such as those in the limb are founded by muscle precursor cells which are initially myf-5 negative. At later stages of development the gene is transcribed in all skeletal muscles, irrespective of their origin and fibre type. We discuss the unexpected finding which was made with the myf-5/nlsLacZ mice, that myf-5 is also expressed in the central nervous system.

Adenovirus as an expression vector in muscle cells in vivo

Attempting gene transfer in muscle raises difficult problems: the nuclei of mature muscle fibers do not undergo division, thus excluding strategies involving replicative integration of exogenous DNA. As adenovirus has been reported to be an efficient vector for the transfer of an enzyme encoding gene in mice, we decided to explore its potential for muscle cells. Advantages of adenovirus vectors are their independence of host cell replication, broad host range, and potential capacity for large foreign DNA inserts. We constructed a recombinant adenovirus containing the beta-galactosidase reporter gene under the control of muscle-specific regulatory sequences. This recombinant virus was able to direct expression of the beta-galactosidase in myotubes in vitro. We report its in vivo expression in mouse muscles up to 75 days after infection. The efficiency and stability of expression we obtained compare very favorably with other strategies proposed for gene or myoblast transfer in muscle in vivo.

Molecular cloning and characterization of a late Tipula iridescent virus gene

Virions of the cytoplasmic, icosahedral insect virus, Tipula iridescent virus (TIV), contain two major DNA components (L, greater than 176 kb; and S1, 10.8 kb) and 25-30 proteins. We characterized a gene (L96) whose 3.6-kb transcript is expressed late in the course of TIV infection of cultured of Estigmene acrea (salt marsh caterpillar, permissive host) and Aedes albopictus (mosquito, semipermissive host) cells. The L96 gene has an open reading frame of 867 codons, predicting a protein of 96 kDa with a pI of 10.9. The C terminus of the L96 protein is rich in hydrophobic amino acids and contains a small region of homology spanning a proteolytic cleavage site within two mammalian viral (GAG) polyproteins. Additional identity with H5 lysine-rich histones in the same region and with other DNA-binding proteins suggests that this protein may be involved in TIV structure. The lengths of the 5'- and 3'-untranslated regions of the L96 transcript were determined to be 21 nucleotides (nt) and 700 nt, respectively. Comparison of the TIV L96- and capsid-encoding genes, both of which are expressed late in infection, revealed that their 5' and 3' regions are generally rich in A and T residues, and that their 3' ends encode at least one eukaryotic polyadenylation signal (AATAAA).

Semipermissive replication of Tipula iridescent virus in Aedes albopictus C6/36 cells

Comparative studies were carried out using two different insect cell lines, Aedes albopictus and Estigmene acrea, for Tipula iridescent virus (TIV) propagation. Light microscope autoradiography showed viral DNA present in viroplasmic centers (VCs) and an inhibition of nuclear DNA synthesis. These VCs appeared to be morphologically similar in both cell lines when examined by light and electron microscopy. Radiolabeled cDNA was synthesized from RNA samples obtained from infected cells at different times after infection and hybridized to TIV DNA digested with various restriction endonucleases. The results indicated that the pattern of transcription and the kinetics of TIV infection were qualitatively similar in both cell lines. The major TIV DNA components, L (greater than 174 kbp) and S1 (10.8 kbp) that are found in virions in approximately equivalent amounts, were made in both infected cell lines. However, the infected cell lines produced S1 DNA at higher levels relative to L than in virions. The cDNA hybridization studies also revealed that the S1 DNA has sequences that are transcribed and are TIV specific. While VC morphology, levels of L and S1 DNA synthesis, transcription, and capsid protein synthesis were similar in both cell lines, time course electron microscope studies revealed that progeny virions were detected only in the VCs of E. acrea cells and not in the VCs of A. albopictus cells, even by 96 hr p.i. These data suggest that the A. albopictus C6/36 cell line is semipermissive for TIV replication.

Molecular cloning, characterization, and expression of the Tipula iridescent virus capsid gene

The capsid protein is the major structural component of the icosahedral Tipula iridescent virus (TIV) that replicates in cytoplasmic inclusion bodies of insect cells. TIV capsid protein purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was digested with trypsin and fractionated by reverse-phase high-pressure liquid chromatography. A mixed oligonucleotide constructed from the amino acid sequence of a capsid tryptic peptide was used for the identification and cloning of the corresponding gene. The single-copy capsid gene, located on a 2.47-kilobase-pair HindIII TIV genomic fragment, codes for a 464-amino-acid protein (50,831 daltons) with a predicted pI of 6.34. Analysis of total RNA from infected Estigmene acrea cells indicated that the 1.8-kilobase capsid transcript was maximally produced between 14 and 24 h after infection. Transcript mapping by primer extension indicated that the RNA start site was in the A+T-rich TGCTACTAAT sequence, 19 nucleotides upstream from the first ATG codon of the capsid open reading frame. Expression of the TIV capsid protein in infected E. acrea cells was demonstrated by in vivo labeling of total proteins with [35S]methionine, using anti-capsid antiserum as the probe. Capsid protein was also expressed in Escherichia coli cells by using a pUC19 plasmid containing a lacZ-capsid gene fusion.

DNA components of Tipula iridescent virus

Purified Tipula iridescent virus (TIV) from infected Galleria mellonella larvae was shown to consist of four DNA components: L (> 150 kilobase pairs (kbp)), S1 (10.8 kbp), S2 (~6 kbp), and S3 (~3.5 kbp). Analysis of the DNA from sucrose gradient fractionated TIV revealed that the fractions corresponding to partially filled virions have less of L relative to S1. All S components are related to L. S1 and S2 are the most similar, while S3 is a subset of S2. Three recombinant plasmids were isolated from a TIV DNA bank constructed in the BamHI site of pBR322. Two plasmids, pTB19-76 and pTB18-110, are related to S1 and S2 and they contain viral sequences that are repeated in the TIV genome. The third plasmid pTBR1-10 is related to S3. At least part of the variation in TIV genome size (174–246 kbp) obtained by summation of restriction endonuclease derived DNA fragments can be accounted for by the presence of the S components and sequence repetition.