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

Regenerative medicine: current research and perspective in pediatric surgery

The field of regenerative medicine, encompassing several disciplines including stem cell biology and tissue engineering, continues to advance with the accumulating research on cell manipulation technologies, gene therapy and new materials. Recent progress in preclinical and clinical studies may transcend the boundaries of regenerative medicine from laboratory research towards clinical reality. However, for the ultimate goal to construct bioengineered transplantable organs, a number of issues still need to be addressed. In particular, engineering of elaborate tissues and organs requires a fine combination of different relevant aspects; not only the repopulation of multiple cell phenotypes in an appropriate distribution but also the adjustment of the host environmental factors such as vascularisation, innervation and immunomodulation. The aim of this review article is to provide an overview of the recent discoveries and development in stem cells and tissue engineering, which are inseparably interconnected. The current status of research on tissue stem cells and bioengineering, and the possibilities for application in specific organs relevant to paediatric surgery have been specifically focused and outlined.

Ciliary Hedgehog signaling patterns the digestive system to generate mechanical forces driving elongation

How tubular organs elongate is poorly understood. We found that attenuated ciliary Hedgehog signaling in the gut wall impaired patterning of the circumferential smooth muscle and inhibited proliferation and elongation of developing intestine and esophagus. Similarly, ablation of gut-wall smooth muscle cells reduced lengthening. Disruption of ciliary Hedgehog signaling or removal of smooth muscle reduced residual stress within the gut wall and decreased activity of the mechanotransductive effector YAP. Removing YAP in the mesenchyme also reduced proliferation and elongation, but without affecting smooth muscle formation, suggesting that YAP interprets the smooth muscle-generated force to promote longitudinal growth. Additionally, we developed an intestinal culture system that recapitulates the requirements for cilia and mechanical forces in elongation. Pharmacologically activating YAP in this system restored elongation of cilia-deficient intestines. Thus, our results reveal that ciliary Hedgehog signaling patterns the circumferential smooth muscle to generate radial mechanical forces that activate YAP and elongate the gut.

The development and stem cells of the esophagus

The esophagus is derived from the anterior portion of the foregut endoderm, which also gives rise to the respiratory system. As it develops, the esophageal lining is transformed from a simple columnar epithelium into a stratified squamous cell layer, accompanied by the replacement of unspecified mesenchyme with layers of muscle cells. Studies in animal models have provided significant insights into the roles of various signaling pathways in esophageal development. More recent studies using human pluripotent stem cells (hPSCs) further demonstrate that some of these signaling pathways are conserved in human esophageal development. In addition, a combination of mouse genetics and hPSC differentiation approaches have uncovered new players that control esophageal morphogenesis. In this Review, we summarize these new findings and discuss how the esophagus is established and matures throughout different stages, including its initial specification, respiratory-esophageal separation, epithelial morphogenesis and maintenance. We also discuss esophageal muscular development and enteric nervous system innervation, which are essential for esophageal structure and function.

Fibrillin-2 is a key mediator of smooth muscle extracellular matrix homeostasis during mouse tracheal tubulogenesis

Epithelial tubes, comprised of polarised epithelial cells around a lumen, are crucial for organ function. However, the molecular mechanisms underlying tube formation remain largely unknown. Here, we report on the function of fibrillin (FBN)2, an extracellular matrix (ECM) glycoprotein, as a critical regulator of tracheal tube formation.We performed a large-scale forward genetic screen in mouse to identify regulators of respiratory organ development and disease. We identified Fbn2 mutants which exhibit shorter and narrowed tracheas as well as defects in tracheal smooth muscle cell alignment and polarity.We found that FBN2 is essential for elastic fibre formation and Fibronectin accumulation around tracheal smooth muscle cells. These processes appear to be regulated at least in part through inhibition of p38-mediated upregulation of matrix metalloproteinases (MMPs), as pharmacological decrease of p38 phosphorylation or MMP activity partially attenuated the Fbn2 mutant tracheal phenotypes. Analysis of human tracheal tissues indicates that a decrease in ECM proteins, including FBN2 and Fibronectin, is associated with tracheomalacia.Our findings provide novel insights into the role of ECM homeostasis in mesenchymal cell polarisation during tracheal tubulogenesis.

Activated Braf induces esophageal dilation and gastric epithelial hyperplasia in mice

Germline mutations in BRAF are a major cause of cardio-facio-cutaneous (CFC) syndrome, which is characterized by heart defects, characteristic craniofacial dysmorphology and dermatologic abnormalities. Patients with CFC syndrome also commonly show gastrointestinal dysfunction, including feeding and swallowing difficulties and gastroesophageal reflux. We have previously found that knock-in mice expressing a Braf Q241R mutation exhibit CFC syndrome-related phenotypes, such as growth retardation, craniofacial dysmorphisms, congenital heart defects and learning deficits. However, it remains unclear whether BrafQ241R/+ mice exhibit gastrointestinal dysfunction. Here, we report that BrafQ241R/+ mice have neonatal feeding difficulties and esophageal dilation. The esophagus tissues from BrafQ241R/+ mice displayed incomplete replacement of smooth muscle with skeletal muscle and decreased contraction. Furthermore, the BrafQ241R/+ mice showed hyperkeratosis and a thickened muscle layer in the forestomach. Treatment with MEK inhibitors ameliorated the growth retardation, esophageal dilation, hyperkeratosis and thickened muscle layer in the forestomach in BrafQ241R/+ mice. The esophageal dilation with aberrant skeletal-smooth muscle boundary in BrafQ241R/+ mice were recovered after treatment with the histone H3K27 demethylase inhibitor GSK-J4. Our results provide clues to elucidate the pathogenesis and possible treatment of gastrointestinal dysfunction and failure to thrive in patients with CFC syndrome.

Development and stem cells of the esophagus

The esophagus is derived from the anterior portion of the developmental intermediate foregut, a structure that also gives rise to other organs including the trachea, lung, and stomach. Genetic studies have shown that multiple signaling pathways (e.g. Bmp) and transcription factors (e.g. SOX2) are required for the separation of the esophagus from the neighboring respiratory system. Notably, some of these signaling pathways and transcription factors continue to play essential roles in the subsequent morphogenesis of the esophageal epithelium which undergoes a simple columnar-to-stratified squamous conversion. Reactivation of the relevant signaling pathways has also been associated with pathogenesis of esophageal diseases that affect the epithelium and its stem cells in adults. In this review we will summarize these findings. We will also discuss new data regarding the cell-of-origin for the striated and smooth muscles surrounding the esophagus and how they are differentiated from the mesenchyme during development.

Enteric co-innervation of striated muscle in the esophagus: still enigmatic?

The existence of a distinct ganglionated myenteric plexus between the two layers of the striated tunica muscularis of the mammalian esophagus has represented an enigma for quite a while. Although an enteric co-innervation of vagally innervated motor endplates in the esophagus has been suggested repeatedly, it was not possible until recently to demonstrate this dual innervation. Twenty-two years ago, we were able to demonstrate that motor endplates in the rat esophagus receive dual innervation from both vagal nerve fibers originating in the brain stem and from varicose enteric nerve fibers originating in the myenteric plexus. Meanwhile, a considerable amount of data has been gathered on enteric co-innervation and its occurrence in the esophagus of a variety of species including humans, its neurochemistry, spatial relationships on motor endplates, ontogeny and possible functional roles. These data underline the significance of this newly discovered innervation component, although its function in vivo is still largely unknown. The aim of this review, which is an update of our previous paper (Wörl and Neuhuber in Histochem Cell Biol 123(2):117-130. doi: 10.1007/s00418-005-0764-7 , 2005a), is to summarize the current knowledge about enteric co-innervation of esophageal striated muscle and to provide some hints as to its functional significance.

Embracing change: striated-for-smooth muscle replacement in esophagus development

The esophagus functions to transport food from the oropharyngeal region to the stomach via waves of peristalsis and transient relaxation of the lower esophageal sphincter. The gastrointestinal tract, including the esophagus, is ensheathed by the muscularis externa (ME). However, while the ME of the gastrointestinal tract distal to the esophagus is exclusively smooth muscle, the esophageal ME of many vertebrate species comprises a variable amount of striated muscle. The esophageal ME is initially composed only of smooth muscle, but its developmental maturation involves proximal-to-distal replacement of smooth muscle with striated muscle. This fascinating phenomenon raises two important questions: what is the developmental origin of the striated muscle precursor cells, and what are the cellular and morphogenetic mechanisms underlying the process? Studies addressing these questions have provided controversial answers. In this review, we discuss the development of ideas in this area and recent work that has shed light on these issues. A working model has emerged that should permit deeper understanding of the role of ME development and maturation in esophageal disorders and in the functional and evolutionary underpinnings of the variable degree of esophageal striated myogenesis in vertebrate species.

PAX7 is required for patterning the esophageal musculature

Background

The mammalian esophageal musculature is unique in that it makes a transition from smooth to skeletal muscle, with most of this process occurring after birth. In order to better understand the mechanisms that control esophageal musculature development, we investigated the roles in this process of the paired box transcription factor, PAX7, a principal regulator of skeletal myogenic progenitor cells. Previous studies showed that Pax7 is important for determining the esophageal muscle composition.

Results

We characterized the postnatal development of the esophageal musculature in Pax7^−/− mice by analyzing morphology, muscle composition, and the expression of markers of myogenesis, cell proliferation, and apoptosis. Pax7^−/− mice displayed megaesophagus with a severe defect in the postnatal developmental process whereby esophageal smooth muscle is replaced by skeletal muscle. Pax7^−/− esophagi have substantially reduced skeletal muscle, most likely due to diminished proliferation and premature differentiation of skeletal muscle precursor cells. This impaired the proximal-to-distal progression of skeletal myogenesis and indirectly affected the patterning of the smooth muscle-containing portion of the esophageal musculature.

Conclusions

Postnatal patterning of the esophageal musculature appears to require robust, PAX7-dependent cell proliferation to drive the proximal-to-distal progression of skeletal myogenesis. This process in turn influences distal smooth muscle morphogenesis and development of the mature pattern of the esophageal musculature.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0068-0) contains supplementary material, which is available to authorized users.

A Cranial Mesoderm Origin for Esophagus Striated Muscles

The esophagus links the oral cavity to the stomach and facilitates the transfer of bolus. Using genetic tracing and mouse mutants, we demonstrate that esophagus striated muscles (ESMs) are not derived from somites but are of cranial origin. Tbx1 and Isl1 act as key regulators of ESMs, which we now identify as a third derivative of cardiopharyngeal mesoderm that contributes to second heart field derivatives and head muscles. Isl1-derived ESM progenitors colonize the mouse esophagus in an anterior-posterior direction but are absent in the developing chick esophagus, thus providing evolutionary insight into the lack of ESMs in avians. Strikingly, different from other myogenic regions, in which embryonic myogenesis establishes a scaffold for fetal fiber formation, ESMs are established directly by fetal myofibers. We propose that ESM progenitors use smooth muscle as a scaffold, thereby bypassing the embryonic program. These findings have important implications in understanding esophageal dysfunctions, including dysphagia, and congenital disorders, such as DiGeorge syndrome.

Oesophageal and sternohyal muscle fibres are novel Pax3-dependent migratory somite derivatives essential for ingestion

Striated muscles that enable mouth opening and swallowing during feeding are essential for efficient energy acquisition, and are likely to have played a fundamental role in the success of early jawed vertebrates. The developmental origins and genetic requirements of these muscles are uncertain. Here, we determine by indelible lineage tracing in mouse that fibres of sternohyoid muscle (SHM), which is essential for mouth opening during feeding, and oesophageal striated muscle (OSM), which is crucial for voluntary swallowing, arise from Pax3-expressing somite cells. In vivo Kaede lineage tracing in zebrafish reveals the migratory route of cells from the anteriormost somites to OSM and SHM destinations. Expression of pax3b, a zebrafish duplicate of Pax3, is restricted to the hypaxial region of anterior somites that generate migratory muscle precursors (MMPs), suggesting that Pax3b plays a role in generating OSM and SHM. Indeed, loss of pax3b function led to defective MMP migration and OSM formation, disorganised SHM differentiation, and inefficient ingestion and swallowing of microspheres. Together, our data demonstrate Pax3-expressing somite cells as a source of OSM and SHM fibres, and highlight a conserved role of Pax3 genes in the genesis of these feeding muscles of vertebrates.

Smooth muscle fascicular reorientation is required for esophageal morphogenesis and dependent on Cdo

Cdo-deficient mice have defects in smooth muscle fascicular reorientation during esophageal morphogenesis, resulting in structural and functional defects including an aberrantly proximal skeletal–smooth muscle boundary and achalasia.

Postnatal maturation of esophageal musculature involves proximal-to-distal replacement of smooth muscle with skeletal muscle by elusive mechanisms. We report that this process is impaired in mice lacking the cell surface receptor Cdo and identify the underlying developmental mechanism. A myogenic transition zone containing proliferative skeletal muscle precursor cells migrated in a proximal–distal direction, leaving differentiated myofibers in its wake. Distal to the transition zone, smooth muscle fascicles underwent a morphogenetic process whereby they changed their orientation relative to each other and to the lumen. Consequently, a path was cleared for the transition zone, and smooth muscle ultimately occupied only the distal-most esophagus; there was no loss of smooth muscle. Cdo^−/− mice were specifically defective in fascicular reorientation, resulting in an aberrantly proximal skeletal–smooth muscle boundary. Furthermore, Cdo^−/− mice displayed megaesophagus and achalasia, and their lower esophageal sphincter was resistant to nitric oxide–induced relaxation, suggesting a developmental linkage between patterning and sphincter function. Collectively, these results illuminate mechanisms of esophageal morphogenesis and motility disorders.

Genetic and cellular mechanisms regulating anterior foregut and esophageal development

Separation of the single anterior foregut tube into the esophagus and trachea involves cell proliferation and differentiation, as well as dynamic changes in cell-cell adhesion and migration. These biological processes are regulated and coordinated at multiple levels through the interplay of the epithelium and mesenchyme. Genetic studies and in vitro modeling have shed light on relevant regulatory networks that include a number of transcription factors and signaling pathways. These signaling molecules exhibit unique expression patterns and play specific functions in their respective territories before the separation process occurs. Disruption of regulatory networks inevitably leads to defective separation and malformation of the trachea and esophagus and results in the formation of a relatively common birth defect, esophageal atresia with or without tracheoesophageal fistula (EA/TEF). Significantly, some of the signaling pathways and transcription factors involved in anterior foregut separation continue to play important roles in the morphogenesis of the individual organs. In this review, we will focus on new findings related to these different developmental processes and discuss them in the context of developmental disorders or birth defects commonly seen in clinics.

Transdifferentiation: a cell and molecular reprogramming process

Evidence has emerged recently indicating that differentiation is not entirely a one-way process, and that it is possible to convert one cell type to another, both in vitro and in vivo. This phenomenon is called transdifferentiation, and is generally defined as the stable switch of one cell type to another. Transdifferentiation plays critical roles during development and in regeneration pathways in nature. Although this phenomenon occurs rarely in nature, recent studies have been focused on transdifferentiation and the reprogramming ability of cells to produce specific cells with new phenotypes for use in cell therapy and regenerative medicine. Thus, understanding the principles and the mechanism of this process is important for producing desired cell types. Here some well-documented examples of transdifferentiation, and their significance in development and regeneration are reviewed. In addition, transdifferentiation pathways are considered and their potential molecular mechanisms, especially the role of master switch genes, are considered. Finally, the significance of transdifferentiation in regenerative medicine is discussed.

Morphology of the developing muscularis externa in the mouse esophagus

Muscularis externa of mouse esophagus is composed of two skeletal muscle layers in the adult. But less attention is paid to the histogenesis of the muscularis externa of the esophagus, and controversies still exist about the developmental process and the spatio-temporal expression characteristics of muscle-specific proteins during the development of esophageal muscularis externa. To further probe into the developmental pattern of muscularis externa of the mouse esophagus and the expression characteristics of different muscle-specific proteins, immunohistochemical and terminal deoxyribonucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP)-digoxigenin nick-end labeling apoptotic staining methods are used to investigate the expression patterns of different muscle-specific proteins and to elucidate the relationship of these protein expressions with the development of muscularis externa of the mouse esophagus. Thus, an understanding of the developing esophageal muscularis externa may be important for developing therapeutic strategies for the treatment of human esophagus diseases. Serial sections of mouse embryos from embryonic day (ED) 12 to ED18, and full-length esophagi from postnatal first to 5th day were stained with monoclonal antibodies against α-smooth muscle actin (α-SMA), α-sarcomerical actin (α-SCA), desmin, and monoclonal anti-skeletal myosin (MHC), while apoptosis was determined using the terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick-end labeling assay. The expression of α-SMA was started at ED12. During the development of ED14-ED15, α-SMA positive cells were seen extending from the walls of left three, four, and six arch arteries toward the dorsal wall of esophagus. Stronger expression of α-SCA and desmin could be detected at ED14 and ED15, expression intensity in caudal segment and inner layer was stained stronger than that of cranial segment and outer layer, but after ED16, strong expression of α-SCA and desmin was found in the outer layer of muscularis externa. Expression of MHC was first detected in the outer layer of cranial segment of muscularis externa at ED17. At ED18, MHC had extended to the level of thyroid gland, staining intensity in the outer layer and cranial segment was stronger than that of inner layer and caudal segment. One to five days after birth, the thickness of the esophageal muscle layer was obviously increased. Most of the muscle cells in the cranial segment of esophagus showed strong expression of α-SCA and clear cross striations at higher magnification. With progression toward the caudal segment, expression intensity of α-SCA became weaker, but the expression intensity of desmin was the same at different levels of esophagus. The muscle fibers were arranged densely with high expression of MHC in the cranial segment. During the development of esophageal muscularis externa, few apoptotic cells were observed. α-SMA, α-SCA, desmin, and MHC show different expression patterns. The differentiation of outer layer of esophageal muscularis externa is quicker than that of inner layer, and the caudal segment is quicker than that of the cranial segment. Besides, apoptosis may not participate in the development of esophageal muscularis externa. The smooth muscle cells from arch arteries may participate in the development of esophageal muscularis externa.

Members of the TEAD family of transcription factors regulate the expression of Myf5 in ventral somitic compartments

The transcriptional regulation of the Mrf4/Myf5 locus depends on a multitude of enhancers that, in equilibria with transcription balancing sequences and the promoters, regulate the expression of the two genes throughout embryonic development and in the adult. Transcription in a particular set of muscle progenitors can be driven by the combined outputs of several enhancers that are not able to recapitulate the entire expression pattern in isolation, or by the action of a single enhancer the activity of which in isolation is equivalent to that within the context of the locus. We identified a new enhancer element of this second class, ECR111, which is highly conserved in all vertebrate species and is necessary and sufficient to drive Myf5 expression in ventro-caudal and ventro-rostral somitic compartments in the mouse embryo. EMSA analyses and data obtained from binding-site mutations in transgenic embryos show that a binding site for a TEA Domain (TEAD) transcription factor is essential for the function of this new enhancer, while ChIP assays show that at least two members of the family of transcription factors bind to it in vivo.

Loss of Lsc/p115 protein leads to neuronal hypoplasia in the esophagus and an achalasia-like phenotype in mice

BACKGROUND & AIMS

Lsc/p115 originally was described as hematopoietic Ras homologous protein guanine exchange factor (Rho-GEF) regulating leukocyte migration, adhesion, and marginal zone B-cell homeostasis. Here we investigate the expression pattern of lsc/p115 in the gastrointestinal tract and the consequences of lsc/p115 deficiency in lsc/p115-knockout mice.

METHODS

The phenotype of lsc/p115-deficient mice was analyzed in vivo with small-animal computed tomography scans and esophageal manometry. The morphology and myenteric plexus were evaluated with immunohistochemistry, morphometry, Western blot analyses, and quantitative reverse-transcription polymerase chain reaction.

RESULTS

lsc/p115 is expressed in the gastrointestinal tract, sparing the segment of the small intestine. Immunohistochemical staining detects lsc/p115 in the muscle layer and the glial fibrillary acidic protein-positive glia in the esophagus. Esophageal manometry uncovers a severe motor dysfunction in lsc/p115-deficient mice. This achalasia-like phenotype is characterized by disturbed peristalsis, hypertension of the lower esophageal sphincter, and impaired relaxation of the lower esophageal sphincter. Lsc/p115-deficient mice develop a progressive dilatation of the esophagus and decrease of the muscle layer. The muscle cell differentiation is not altered in lsc/p115-deficient mice. However, the density of inhibitory and excitatory neurons and glia cells in the myenteric plexus and the muscle layer are reduced in morphometric analyses. This reduced number of glia cells is accompanied by reduced expression of the neurotrophic nerve growth factor.

CONCLUSIONS

lsc/p115 deficiency results in impaired neuronal innervation and in motor dysfunction recapitulating several aspects of esophageal achalasia. Reduced expression of nerve growth factor and a reduced number of glia cells most likely contribute to this phenotype.

Smooth-muscle-specific expression of neurotrophin-3 in mouse embryonic and neonatal gastrointestinal tract

Vagal gastrointestinal (GI) afferents are essential for the regulation of eating, body weight, and digestion. However, their functional organization and the way that this develops are poorly understood. Neurotrophin-3 (NT-3) is crucial for the survival of vagal sensory neurons and is expressed in the developing GI tract, possibly contributing to their survival and to other aspects of vagal afferent development. The identification of the functions of this peripheral NT-3 thus requires a detailed understanding of the localization and timing of its expression in the developing GI tract. We have studied embryos and neonates expressing the lacZ reporter gene from the NT-3 locus and found that NT-3 is expressed predominantly in the smooth muscle of the outer GI wall of the stomach, intestines, and associated blood vessels and in the stomach lamina propria and esophageal epithelium. NT-3 expression has been detected in the mesenchyme of the GI wall by embryonic day 12.5 (E12.5) and becomes restricted to smooth muscle and lamina propria by E15.5, whereas its expression in blood vessels and esophageal epithelium is first observed at E15.5. Expression in most tissues is maintained at least until postnatal day 4. The lack of colocalization of beta-galactosidase and markers for myenteric ganglion cell types suggests that NT-3 is not expressed in these ganglia. Therefore, NT-3 expression in the GI tract is largely restricted to smooth muscle at ages when vagal axons grow into the GI tract, and when vagal mechanoreceptors form in smooth muscle, consistent with its role in these processes and in vagal sensory neuron survival.

Smooth-to-striated muscle transition in human esophagus: an immunohistochemical study using fetal and adult materials

BACKGROUND

A craniocaudal transition from smooth to striated muscle occurs in the fetal mouse esophagus muscularis propria, until finally the entire muscle component becomes striated. Although no such investigation has been conducted using human fetuses, the transition appears to be incomplete.

METHODS

In horizontal sections of 10 human fetuses between 9 and 16 weeks of gestation, we identified immunoreactivity for smooth muscle actin (SMA), striated muscle myosin heavy chain (MyH), desmin, PGP9.5, S100 protein, c-kit, and CD68 in the thoracic esophagus. The TUNEL method was used to identify apoptosis. For comparison, the same immunohistochemistry was conducted using 10 adult esophaguses.

RESULTS

In fetuses at all stages examined, a transition zone was found in the upper thoracic esophagus that was attached to the middle one-third of the trachea. In the transition zone, the MyH-positive longitudinal muscle fibers were surrounded by flat, SMA-positive cells, whereas the MyH-positive circular fibers were sometimes located adjacent to the SMA-positive fibers. However, in adults, smooth muscle tended to be clearly separated from striated muscle. The distribution of cells showing immunoreactivity for PGP9.5, S100 or c-kit did not differ between the oral and anal sides of the transition zone. Desmin was positive in the muscularis propria, but negative in the muscularis mucosae. Neither CD68-positive macrophages nor TUNEL-positive cells were present in the esophagus.

CONCLUSIONS

In the human esophagus, the smooth-to-striated muscle transition appears to stop at the mid-thoracic level. Cell death or transdifferentiation of smooth muscle appears unlikely, but phenotypic transformation into desmin-positive myofibroblasts is a possibility.

Forkhead box M1 transcriptional factor is required for smooth muscle cells during embryonic development of blood vessels and esophagus

The forkhead box m1 (Foxm1 or Foxm1b) transcription factor (previously called HFH-11B, Trident, Win, or MPP2) is expressed in a variety of tissues during embryogenesis, including vascular, airway, and intestinal smooth muscle cells (SMCs). Although global deletion of Foxm1 in Foxm1(-/-) mice is lethal in the embryonic period due to multiple abnormalities in the liver, heart, and lung, the specific role of Foxm1 in SMC remains unknown. In the present study, Foxm1 was deleted conditionally in the developing SMC (smFoxm1(-/-) mice). The majority of smFoxm1(-/-) mice died immediately after birth due to severe pulmonary hemorrhage and structural defects in arterial wall and esophagus. Although Foxm1 deletion did not influence SMC differentiation, decreased proliferation of SMC was found in smFoxm1(-/-) blood vessels and esophagus. Depletion of Foxm1 in cultured SMC caused G(2) arrest and decreased numbers of cells undergoing mitosis. Foxm1-deficiency in vitro and in vivo was associated with reduced expression of cell cycle regulatory genes, including cyclin B1, Cdk1-activator Cdc25b phosphatase, Polo-like 1 and JNK1 kinases, and cMyc transcription factor. Foxm1 is critical for proliferation of smooth muscle cells and is required for proper embryonic development of blood vessels and esophagus.

Deletion of Pax7 changes the tunica muscularis of the mouse esophagus from an entirely striated into a mixed phenotype

The mechanisms responsible for the different amounts of striated muscle in mammalian esophagi are still enigmatic. A recent ultrastructural analysis in mouse esophagus pointed to a particular role of satellite cells during postnatal growth of striated muscle. The aim of this study was to investigate satellite cell development and the influence of Pax7 on this process. Developing and adult esophagi of wild-type and mice carrying a targeted mutation in Pax7 were analyzed by electron microscopy. We found a gene dose-dependent delayed development of striated muscle and a severe loss of satellite cells in Pax7(+/-) and Pax7(-/-) esophagi. In contrast to the entirely striated wild-type esophagus, Pax7(-/-) mutants developed a mixed phenotype with predominantly smooth muscle caudally. We conclude that Pax7-dependent myogenic progenitor cells are of prime importance for striated muscle formation and the degree of smooth-to-striated muscle conversion during esophageal ontogeny.

MyoD- and nerve-dependent maintenance of MyoD expression in mature muscle fibres acts through the DRR/PRR element

BACKGROUND

MyoD is a transcription factor implicated in the regulation of adult muscle gene expression. Distinguishing the expression of MyoD in satellite myoblasts and muscle fibres has proved difficult in vivo leading to controversy over the significance of MyoD expression within adult innervated muscle fibres. Here we employ the MD6.0-lacZ transgenic mouse, in which the 6 kb proximal enhancer/promoter (DRR/PRR) of MyoD drives lacZ, to show that MyoD is present and transcriptionally active in many adult muscle fibres.

RESULTS

In culture, MD6.0-lacZ expresses in myotubes but not myogenic cells, unlike endogenous MyoD. Reporter expression in vivo is in muscle fibre nuclei and is reduced in MyoD null mice. The MD6.0-lacZ reporter is down-regulated both in adult muscle fibres by denervation or muscle disuse and in cultured myotubes by inhibition of activity. Activity induces and represses MyoD through the DRR and PRR, respectively. During the postnatal period, accumulation of beta-galactosidase correlates with maturation of innervation. Strikingly, endogenous MyoD expression is up-regulated in fibres by complete denervation, arguing for a separate activity-dependent suppression of MyoD requiring regulatory elements outside the DRR/PRR.

CONCLUSION

The data show that MyoD regulation is more complex than previously supposed. Two factors, MyoD protein itself and fibre activity are required for essentially all expression of the 6 kb proximal enhancer/promoter (DRR/PRR) of MyoD in adult fibres. We propose that modulation of MyoD positive feedback by electrical activity determines the set point of MyoD expression in innervated fibres through the DRR/PRR element.

Smooth muscle persists in the muscularis externa of developing and adult mouse esophagus

Following initial patterning as differentiated smooth muscle (SM) cells, the muscularis externa of the murine esophagus is replaced by skeletal muscle, but the mechanism underlying this process is controversial. The hypothesis that committed SM cells transdifferentiate into striated muscle is not consistent with fate mapping studies. Similarly, apoptosis does not fully explain the process. Using immunohistochemical techniques and transgenic mice that express eGFP and Cre-recombinase exclusively in SM, we have identified a population of remnant SM cells that persist throughout the developing and mature murine esophagus. These cells display an atypical phenotype, are not associated with microvasculature, but are often apposed to cKit positive, interstitial cells of Cajal. The absolute length of the SM component of the developing esophagus remains constant during a period when total esophageal length increases 4-fold, resulting in a small maintained distal segment of smooth muscle. Esophageal SM cells fail to express myogenin during development, and striated muscle cell precursors expressing myogenin fail to express specific SM cell markers, indicating that they did not transdifferentiate from SM cells. Moreover, smooth muscle-specific myogenin inactivation has no effect on esophageal skeletal myogenesis. Taken together, our results provide an alternative hypothesis regarding the fate of SM cells in the developing murine esophagus, which does not invoke apoptosis or transdifferentiation.

Transdifferentiation and its applicability for inner ear therapy

During normal development, cells divide, then differentiate to adopt their individual form and function in an organism. Under most circumstances, mature cells cannot transdifferentiate, changing their fate to adopt a different form and function. Because differentiated cells cannot usually divide, the repair of injuries as well as regeneration largely depends on the activation of stem cell reserves. The mature cochlea is an exception among epithelial cell layers in that it lacks stem cells. Consequently, the sensory hair cells that receive sound information cannot be replaced, and their loss results in permanent hearing impairment. The lack of a spontaneous cell replacement mechanism in the organ of Corti, the mammalian auditory sensory epithelium, has led researchers to investigate circumstances in which transdifferentiation does occur. The hope is that this information can be used to design therapies to replace lost hair cells and restore impaired hearing in humans.

Cellular reprogramming

The concept of reprogramming a cell is very intriguing and has immense therapeutic potential. Examples from physiology and developmental biology suggest that it may well be possible. Experimental approaches are beginning to suggest this also, in particular the initially astonishing accomplishment of somatic cell nuclear transfer and cloning. This chapter reviews current strategies and describes emerging methods for the proposition of reprogramming cells with cell extracts.

Ultrastructural analysis of the smooth-to-striated transition zone in the developing mouse esophagus: emphasis on apoptosis of smooth and origin and differentiation of striated muscle cells

The exact mechanism of smooth-to-striated muscle conversion in the mouse esophagus is controversial. Smooth-to-striated muscle cell transdifferentiation vs. distinct differentiation pathways for both muscle types were proposed. Main arguments for transdifferentiation were the failure to detect apoptotic smooth and the unknown origin of striated muscle cells during esophageal myogenesis. To reinvestigate this issue, we analyzed esophagi of 4-day-old mice by electron microscopy and a fine-grained sampling strategy considering that, in perinatal esophagus, the replacement of smooth by striated muscle progresses craniocaudally, while striated myogenesis advances caudocranially. We found numerous (1) apoptotic smooth muscle cells located mainly in a transition zone, where smooth intermingled with developing striated muscle cells, and (2) mesenchymal cells in the smooth muscle portion below the transition zone, which appeared to give rise to striated muscle fibers. Taken together, these results provide further evidence for distinct differentiation pathways of both muscle types during esophagus development.

Presence of neurotrophic factors in skeletal muscle correlates with survival of spinal cord motor neurons

To determine which combination of skeletal muscle-derived neurotrophic factors may be important for the survival of specific subpopulations of developing spinal cord motor neurons, we used Myf5 and MyoD (myogenic regulatory factors) knockouts, containing differentially committed myogenic precursor cells (MPCc) and immunohistochemistry against several muscle-secreted neurotrophic factors. At the peak of motor neuron cell death, skeletal muscle development is delayed in the back and body wall muscles of Myf5-/- embryos and in the limb muscles of MyoD-/- embryos. We hypothesized that, if the skeletal muscle was indeed an important source of survival factors for motor neurons, the back, the abdominal wall, and the forelimb MPCs of Myf5-/- or MyoD-/- embryos should produce at least some neurotrophic factors necessary for the survival of motor neurons. In this report, we demonstrate that (1) different MPCs lacking Myf5, MyoD, or Myf5/MyoD have different capabilities in providing factors potentially required for the survival of motor neurons and intramuscular nerve branching, (2) MPCs in double-mutant embryos do not contain neurotrophic factors in the absence of myogenic specification, and (3) different subpopulations of MPCs contain different combinations of neurotrophic factors potentially required for the survival of the specific subpopulations of innervating motor neurons.

Enteric co-innervation of motor endplates in the esophagus: state of the art ten years after

The existence of a distinct ganglionated myenteric plexus between the two layers of the striated tunica muscularis of the mammalian esophagus represented an enigma for quite a while. Although an enteric co-innervation of vagally innervated motor endplates in the esophagus has been repeatedly suggested, it was not possible until recently to demonstrate this dual innervation. Ten years ago, we were able to demonstrate that motor endplates in the rat esophagus receive a dual innervation from both vagal nerve fibers originating in the brain stem and from varicose enteric nerve fibers originating in the myenteric plexus. Since then, a considerable amount of data could be raised on enteric co-innervation and its occurrence in a variety of species, including humans, its neurochemistry, spatial relationships on motor endplates, ontogeny, and possible roles during esophageal peristalsis. These data underline the significance of this newly discovered innervation component, although its function is still largely unknown. The aim of this review is to summarize current knowledge about enteric co-innervation of esophageal striated muscle and to provide some hints as to its functional significance.

Developing Rat Lung Has a Sided Pacemaker Region for Morphogenesis-Related Airway Peristalsis

Prenatal airways from diverse species are capable of spontaneous peristaltic contractions in each trimester. The function of this smooth muscle activity is unknown. We demonstrate that peristalsis of the embryonic airway originates from a sided pacemaker focus, is stimulated in a calcium-dependent fashion by the pulmonary morphogen fibroblast growth factor-10 (FGF-10), and appears coupled to lung growth. Airway peristalsis may be crucial for lung development (thereby providing a physiologic role for airway smooth muscle) and play a hitherto unanticipated role in reported transgenic mutant lung phenotypes.

From fibroblasts and stem cells: implications for cell therapies and somatic cloning

Pluripotent embryonic stem cells (ESCs) from the inner cell mass of early murine and human embryos exhibit extensive self-renewal in culture and maintain their ability to differentiate into all cell lineages. These features make ESCs a suitable candidate for cell-replacement therapy. However, the use of early embryos has provoked considerable public debate based on ethical considerations. From this standpoint, stem cells derived from adult tissues are a more easily accepted alternative. Recent results suggest that adult stem cells have a broader range of potency than imagined initially. Although some claims have been called into question by the discovery that fusion between the stem cells and differentiated cells can occur spontaneously, in other cases somatic stem cells have been induced to commit to various lineages by the extra- or intracellular environment. Recent data from our laboratory suggest that changes in culture conditions can expand a subpopulation of cells with a pluripotent phenotype from primary fibroblast cultures. The present paper critically reviews recent data on the potency of somatic stem cells, methods to modify the potency of somatic cells and implications for cell-based therapies.

Evidence for the involvement of neurotrophins in muscle transdifferentiation and acetylcholine receptor transformation in the esophagus ofMyf5−/−:MyoD−/−andNT-3−/−embryos

The primary aim of our study was to determine whether the esophageal innervation (i.e., vagal and enteric) and the skeletal muscle-secreted neurotrophins have a role in smooth-to-skeletal muscle transdifferentiation and in the muscarinic-to-nicotinic acetylcholine receptor type transition. To that end, we used genetically engineered embryos and immunohistochemistry. We found that, in the absence of Myf5 and MyoD, the esophageal muscle cells failed to develop the striated phenotype of acetylcholine receptors. In addition, the development of vagal and enteric innervation was delayed in Myf5(-/-):MyoD(-/-) and NT-3(-/-) mutants, but it was reestablished 2 days before the end of gestation. The smooth muscle cells in the esophagus appeared to be a distinct subpopulation of cells and their ability to transdifferentiate was based on their competence to express neurotrophins and their receptors. Finally, our data suggest a role for NT-3 in the esophageal muscle transdifferentiation.

Transdifferentiation, metaplasia and tissue regeneration

Transdifferentiation is defined as the conversion of one cell type to another. It belongs to a wider class of cell type transformations called metaplasias which also includes cases in which stem cells of one tissue type switch to a completely different stem cell. Numerous examples of transdifferentiation exist within the literature. For example, isolated striated muscle of the invertebrate jellyfish (Anthomedusae) has enormous transdifferentiation potential and even functional organs (e.g., tentacles and the feeding organ (manubrium)) can be generated in vitro. In contrast, the potential for transdifferentiation in vertebrates is much reduced, at least under normal (nonpathological) conditions. But despite these limitations, there are some well-documented cases of transdifferentiation occurring in vertebrates. For example, in the newt, the lens of the eye can be formed from the epithelial cells of the iris. Other examples of transdifferentiation include the appearance of hepatic foci in the pancreas, the development of intestinal tissue at the lower end of the oesophagus and the formation of muscle, chondrocytes and neurons from neural precursor cells. Although controversial, recent results also suggest the ability of adult stem cells from different embryological germlayers to produce differentiated cells e.g., mesodermal stem cells forming ecto- or endodermally-derived cell types. This phenomenon may constitute an example of metaplasia. The current review examines in detail some well-documented examples of transdifferentiation, speculates on the potential molecular and cellular mechanisms that underlie the switches in phenotype, together with their significance to organogenesis and regenerative medicine.

Esophageal muscle physiology and morphogenesis require assembly of a collagen XIX-rich basement membrane zone

Collagen XIX is an extremely rare extracellular matrix component that localizes to basement membrane zones and is transiently expressed by differentiating muscle cells. Characterization of mice harboring null and structural mutations of the collagen XIX (Col19a1) gene has revealed the critical contribution of this matrix protein to muscle physiology and differentiation. The phenotype includes smooth muscle motor dysfunction and hypertensive sphincter resulting from impaired swallowing-induced, nitric oxide–dependent relaxation of the sphincteric muscle. Muscle dysfunction was correlated with a disorganized matrix and a normal complement of enteric neurons and interstitial cells of Cajal. Mice without collagen XIX exhibit an additional defect, namely impaired smooth-to-skeletal muscle cell conversion in the abdominal segment of the esophagus. This developmental abnormality was accounted for by failed activation of myogenic regulatory factors that normally drive esophageal muscle transdifferentiation. Therefore, these findings identify collagen XIX as the first structural determinant of sphincteric muscle function, and as the first extrinsic factor of skeletal myogenesis in the murine esophagus.

Development of neuromuscular junctions in the mouse esophagus: Morphology suggests a role for enteric coinnervation during maturation of vagal myoneural contacts

The time course of establishment of motor endplates and the subsequent developmental changes in their enteric and vagal innervation were examined in esophageal striated muscle of perinatal and adult C57/Bl6 mice by using immunocytochemistry and confocal laser scanning microscopy. Nicotinic acetylcholine receptors were visualized with alpha-bungarotoxin; vagal motor nerve terminals with antisera against vesicular acetylcholine transporter; and enteric nerve fibers with antisera against neuronal nitric oxide synthase, vasoactive intestinal peptide, and galanin. Because the various stages of esophageal striated myogenesis advance caudocranially, i.e., more mature stages are found cranial to immature stages, longitudinal cryosections through the esophagus were investigated. Synaptogenesis was divided into several distinct stages. 1) Mononucleated cells express acetylcholine receptors over their entire surface. 2) They start to cluster receptors without nerve fiber contacts. 3) The first nerve contact on a growing receptor cluster is made by a vagal nerve terminal, followed by an enteric terminal. 4) Vagal terminals grow until they match the size of endplate areas, and one to three enteric terminals intertwine with them on every receptor cluster. 5) After vagal terminals have covered the whole endplate area, enteric terminals are withdrawn from the majority of motor endplates. In a minority of endplates, enteric coinnervation persists through adulthood. The enteric innervation of all developing motor endplates, shortly after vagal terminals have contacted them, and the removal of enteric nerve fibers from the majority of mature motor endplates suggest a major role of enteric nerve fibers during maturation of esophageal neuromuscular junctions.

Adult stem cell plasticity: fact or artifact?

There has been unprecedented recent interest in stem cells, mainly because of the hope they offer for cell therapy. Adult stem cells are an attractive source of cells for therapy, especially in view of the recent claims that they are remarkably plastic in their developmental potential when exposed to new environments. Some of these claims have been either difficult to reproduce or shown to be misinterpretations, leaving the phenomenon of adult stem cell plasticity under a cloud. There are, however, other examples of plasticity where differentiated cells or their precursors can be reprogrammed by extracellular cues to alter their character in ways that could have important implications for cell therapy and other forms of regenerative treatment.

Molecular insights into congenital disorders of the digestive system

Recent work is providing new insights into molecular mechanisms of digestive system development and their alteration in clinically significant disorders. An understanding of these mechanisms has largely been gained through the use of animal models, because many of the basic processes required in embryogenesis are functionally conserved among species. Such conserved factors include cell-cell signaling pathways and the regulation of gene expression. Disruption of these pathways have been implicated in several congenital disorders of the digestive system, including Hirschsprung disease, malrotation, altered sphincter development, Meckel diverticulum, biliary atresia, Alagille syndrome, pancreatic heterotopias, and pancreatic agenesis. In this review, we highlight recent studies in digestive system development, which elucidate mechanisms underlying congenital disorders of the human digestive system.

The regeneration process of the striated urethral sphincter involves activation of intrinsic satellite cells

The regeneration of adult skeletal muscle is mediated by satellite cells. Classically, these are considered to be somitically derived cells that colonize the limbs during early embryogenesis. The striated urethral sphincter presents specific developmental characteristics that distinguish it from skeletal muscles, such as the non-somitic origin of its precursor cells and the late formation of its myofibers. This prompted us to determine whether the striated urethral sphincter can regenerate after injury by the same mechanism as skeletal muscles. By means of the single myofiber explant culture technique we investigated the presence of satellite cells in the striated urethral sphincter of male mice and evaluated their ability to recapitulate a myogenic program. In addition, a myotoxic substance (notexin) was injected into the sphincter in order to provoke rapid destruction of the myofibers; the regeneration process was studied by means of electrophysiological and histological techniques. Satellite cells expressing pax7 were found attached to the sphincteric myofibers. They proliferated and expressed MyoD, Myf5 and desmin after 2 days in culture. After 10 days, they formed multinucleated myotubes expressing alpha-actinin-2. In vivo, complete recovery of the striated urethral sphincter, as assessed by normalization of muscle strength and of myofiber number and diameter, was observed after 3 weeks, and resulted from the fusion of myogenic cells. These results demonstrate that the striated urethral sphincter can regenerate by means of a myogenic program involving intrinsic satellite cells. The therapeutic implications of this knowledge and the possible origin of the sphincteric satellite cells are discussed.

Interstitial Cells in the Musculature of the Gastrointestinal Tract: Cajal and Beyond

Expression of the receptor tyrosine kinase KIT on cells referred to as interstitial cells of Cajal (ICC) has been instrumental during the past decade in the tremendous interest in cells in the interstitium of the smooth muscle layers of the digestive tract. ICC generate the pacemaker component (electrical slow waves of depolarization) of the smooth musculature and are involved in neurotransmission. By integration of ICC functions, substantial progress has been made in our understanding of the neuromuscular control of gastrointestinal motility, opening novel therapeutic perspectives. In this article, the ultrastructure and light microscopic morphology, as well as the functions and the development of ICC and of neighboring fibroblast-like cells (FLC), are critically reviewed. Directions for future research are considered and a unifying concept of mesenchymal cells, either KIT positive (the "ICC") or KIT negative "non-Cajal" (including the FLC and possibly also other cell types) cell types in the interstitium of the smooth musculature of the gastrointestinal tract, is proposed. Furthermore, evidence is accumulating to suggest that, as postulated by Santiago Ramon y Cajal, the concept of interstitial cells is not likely to be restricted to the gastrointestinal musculature.

Development of the enteric nervous system

Development of the ENS requires the function of a diverse set of genes encoding transcription factors, signaling molecules, and their receptors. Mutations of these genes result in altered ENS function in animals and humans. In particular, such mutations have been shown to contribute to many cases of Hirschsprung's disease. Elucidation of the mechanisms of ENS development and function allow the development of new approaches to the diagnosis, therapy, and prevention of human disorders of gastrointestinal motility.

Muscle satellite cells are multipotential stem cells that exhibit myogenic, osteogenic, and adipogenic differentiation

Muscle satellite cells are believed to represent a committed stem cell population that is responsible for the postnatal growth and regeneration of skeletal muscle. However, the observation that cultured myoblasts differentiate into osteocytes or adipocytes following treatment with bone morphogenetic proteins (BMPs) or adipogenic inducers, respectively, suggests some degree of plasticity within the mesenchymal lineage. To further investigate this phenomenon, we explore the osteogenic and adipogenic potential of satellite cells isolated from adult mice. Our experiments clearly demonstrate that satellite cell-derived primary myoblasts, expressing myogenic markers such as MyoD, Myf5, Pax7 and desmin, differentiated only into osteocytes or adipocytes following treatment with BMPs or adipogenic inducers, respectively However, satellite cells on isolated muscle fibers cultured in Matrigel readily differentiated into myocytes as well as osteogenic and adipogenic lineages, whereas primary myoblasts did not. Satellite cell-derived primary myoblasts isolated from mice lacking the myogenic transcription factor MyoD (MyoD-/-) differentiate into myocytes poorly in vivo and in vitro (Megeney et al., Genes Dev. 1996; Sabourin et. al, J. Cell Biol., 1999). Therefore, we tested whether MyoD-/- primary myoblasts display increased plasticity relative to wild type cells. Unexpectedly, the osteogenic or adipogenic differentiation potential of MyoD-/- primary myoblasts did not increase compared to wild-type cells. Taken together, these results strongly suggest that muscle satellite cells possess multipotential mesenchymal stem cell activity and are capable of forming osteocytes and adipocytes as well as myocytes.

Cardiomyocytes induce endothelial cells to trans-differentiate into cardiac muscle: implications for myocardium regeneration

The concept of tissue-restricted differentiation of postnatal stem cells has been challenged by recent evidence showing pluripotency for hematopoietic, mesenchymal, and neural stem cells. Furthermore, rare but well documented examples exist of already differentiated cells in developing mammals that change fate and trans-differentiate into another cell type. Here, we report that endothelial cells, either freshly isolated from embryonic vessels or established as homogeneous cells in culture, differentiate into beating cardiomyocytes and express cardiac markers when cocultured with neonatal rat cardiomyocytes or when injected into postischemic adult mouse heart. Human umbilical vein endothelial cells also differentiate into cardiomyocytes under similar experimental conditions and transiently coexpress von Willebrand factor and sarcomeric myosin. In contrast, neural stem cells, which efficiently differentiate into skeletal muscle, differentiate into cardiomyocytes at a low rate. Fibroblast growth factor 2 and bone morphogenetic protein 4, which activate cardiac differentiation in embryonic cells, do not activate cardiogenesis in endothelial cells or stimulate trans-differentiation in coculture, suggesting that different signaling molecules are responsible for cardiac induction during embryogenesis and in successive periods of development. The fact that endothelial cells can generate cardiomyocytes sheds additional light on the plasticity of endothelial cells during development and opens perspectives for cell autologous replacement therapies.

Skeletal muscle formation in vertebrates

Research in the past year has added to our understanding of the signalling systems that specify myogenic identity in the embryo and of the regulation and roles of MyoD family members. New insights into the movement of muscle precursor cells include the demonstration that Lbx1 is essential for their migration from the somite to some but not all sites of muscle formation elsewhere. Later in development, ras as well as calcineurin signalling is now implicated in the definition of slow versus fast fibre types. The myogenic identity of precursor cells in the adult depends on Pax7, the orthologue of Pax3 which is required for early myogenesis; this finding is of major importance for muscle regeneration and the active field of stem cell research.

Two upstream enhancers collaborate to regulate the spatial patterning and timing of MyoD transcription during mouse development

MyoD is a member of the basic-helix-loop-helix (bHLH) transcription factor family, which regulates muscle determination and differentiation in vertebrates. While it is now well established that the MyoD gene is regulated by Sonic hedgehog, Wnts, and other signals, it is not known how MyoD transcription is initiated and maintained in response to these signals. We have investigated the cis control of MyoD expression to identify and characterize the DNA targets that mediate MyoD transcription in embryos. By monitoring lacZ reporter gene expression in transgenic mice, we show that regulatory information contained in 24 kb of human MyoD 5' flanking sequence is sufficient to accurately control MyoD expression in embryos. Previous studies have identified two muscle-specific regulatory regions upstream of MyoD, a 4-kb region centered at -20 kb (designated fragment 3) that contains a highly conserved 258-bp core enhancer sequence, and a more proximal enhancer at -5 kb, termed the distal regulatory region (DRR), that heretofore has been identified only in mice. Here, we identify DRR-related sequences in humans and show that DRR function is conserved in humans and mice. In addition, transcriptional activity of MyoD 5' flanking sequences in somites and limb buds is largely a composite of the individual specificities of the two enhancers. Deletion of fragment 3 resulted in dramatic but temporary expression defects in the hypaxial myotome and limb buds, suggesting that this regulatory region is essential for proper temporal and spatial patterning of MyoD expression. These data indicate that regulatory sequences in fragment 3 are important targets of embryonic signaling required for the initiation of MyoD expression.

Regeneration as an evolutionary variable

Regeneration poses a distinctive set of problems for evolutionary biologists, but there has been little substantive progress since these issues were clearly outlined in the monograph of T. H. Morgan (1901). The champions at regeneration among vertebrates are the urodele amphibians such as the newt, and we tend to regard urodele regeneration as an exceptional attribute. The ability to regenerate large sections of the body plan is widespread in metazoan phylogeny, although it is not universal. It is striking that in phylogenetic contexts where regeneration occurs, closely related species are observed which do not possess this ability. It is a challenge to reconcile such variation between species with a conventional selective interpretation of regeneration. The critical hypothesis from phylogenetic analysis is that regeneration is a basic, primordial attribute of metazoans rather than a mechanism which has evolved independently in a variety of contexts. In order to explain its absence in closely related species, it is postulated to be lost secondarily for reasons which are not understood. Our approach to this question is to compare a differentiated newt cell with its mammalian counterpart in respect of the plasticity of differentiation.

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

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