Wnt signalling regulates myogenic differentiation in the developing avian wing

Involvement of Wnt4 signaling during myogenic proliferation and differentiation of skeletal muscle

The direct effects of Wnt4 on myogenic proliferation and differentiation of skeletal muscle precursors are examined. Wnt4 cDNA was misexpressed in the presumptive limb fields on the right side of stage 16 chick embryos. Muscle development was evaluated at stage 37 with hematoxylin-eosin staining and immunohistochemical staining for fast and slow types of the myosin heavy chain (MyHC). Overexpression of Wnt4 resulted in up-regulation of Pax7 and MyoD1 expression. The muscle mass showed a significant increase compared with that of the control limb. The area for fast MyHC-expressing cells showed a significant increase, whereas a slight decrease was observed for slow MyHC-expressing cells. Wnt4 acted as a stimulator during myogenic proliferation and differentiation, especially, for fast-type muscle in C2C12 cells. The present results are identical to those of myostatin knockout, suggesting that Wnt4 is acting against myostatin as an antagonizing signal for myostatin.

Sox6 is required for normal fiber type differentiation of fetal skeletal muscle in mice

Sox6, a member of the Sox family of transcription factors, is highly expressed in skeletal muscle. Despite its abundant expression, the role of Sox6 in muscle development is not well understood. We hypothesize that, in fetal muscle, Sox6 functions as a repressor of slow fiber type-specific genes. In the wild-type mouse, differentiation of fast and slow fibers becomes apparent during late fetal stages (after approximately embryonic day 16). However, in the Sox6 null-p(100H) mutant mouse, all fetal muscle fibers maintain slow fiber characteristics, as evidenced by expression of the slow myosin heavy chain MyHC-beta. Knockdown of Sox6 expression in wild-type myotubes results in a significant increase in MyHC-beta expression, supporting our hypothesis. Analysis of the MyHC-beta promoter revealed a Sox consensus sequence that likely functions as a negative cis-regulatory element. Together, our results suggest that Sox6 plays a critical role in the fiber type differentiation of fetal skeletal muscle.

A highly conserved Wnt-dependent TCF4 binding site within the proximal enhancer of the anti-myogenic Msx1 gene supports expression within Pax3-expressing limb bud muscle precursor cells

The product of the Msx1 gene is a potent inhibitor of muscle differentiation. Msx1 is expressed in muscle precursor cells of the limb bud that also express Pax3. It is thought that Msx1 may facilitate distal migration by delaying myogenesis in these cells. Despite the role played by Msx1 in inhibiting muscle differentiation, nothing is known of the mechanisms that support the expression of the Msx1 gene within limb bud muscle precursor cells. In the present study we have used a combination of comparative genomics, mouse transgenic analysis, in situ hybridisation and immunohistochemistry to identify a highly conserved and tissue-specific regulatory sub-domain within the previously characterised Msx1 gene proximal enhancer element that supports the expression of the Msx1 gene in Pax3-expressing mouse limb pre-muscle masses. Furthermore, using a combination of in situ hybridisation, in vivo ChIP assay and transgenic explant culture analysis we provide evidence that Msx1 expression in limb bud muscle precursor cells is dependent on the canonical Wnt/TCF signalling pathway that is important in muscle shape formation. The results of these studies provide evidence of a mechanistic link between the Wnt/TCF and the Msx1/Pax3/MyoD pathways within limb bud muscle precursor cells.

Differential expression of CaMK-II genes during early zebrafish embryogenesis

CaMK-II is a highly conserved Ca(2+)/calmodulin-dependent protein kinase expressed throughout the lifespan of all vertebrates. During early development, CaMK-II regulates cell cycle progression and "non-canonical" Wnt-dependent convergent extension. In the zebrafish, Danio rerio, CaMK-II activity rises within 2 hr after fertilization. At the time of somite formation, zygotic expression from six genes (camk2a1, camk2b1, camk2g1, camk2g2, camk2d1, camk2d2) results in a second phase of increased activity. Zebrafish CaMK-II genes are 92-95% identical to their human counterparts in the non-variable regions. During the first three days of development, alternative splicing yields at least 20 splice variants, many of which are unique. Whole-mount in situ hybridization reveals that camk2g1 comprises the majority of maternal expression. All six genes are expressed strongly in ventral regions at the 18-somite stage. Later, camk2a1 is expressed in anterior somites, heart, and then forebrain. Camk2b1 is expressed in somites, mid- and forebrain, gut, retina, and pectoral fins. Camk2g1 appears strongly along the midline and then in brain, gut, and pectoral fins. Camk2g2 is expressed early in the midbrain and trunk and exhibits the earliest retinal expression. Camk2d1 is elevated early at somite boundaries, then epidermal tissue, while camk2d2 is expressed in discrete anterior locations, steadily increasing along either side of the dorsal midline and then throughout the brain, including the retina. These findings reveal a complex pattern of CaMK-II gene expression consistent with pleiotropic roles during development.

RBP-J (Rbpsuh) is essential to maintain muscle progenitor cells and to generate satellite cells

In the developing muscle, a pool of myogenic progenitor cells is formed and maintained. These resident progenitors provide a source of cells for muscle growth in development and generate satellite cells in the perinatal period. By the use of conditional mutagenesis in mice, we demonstrate here that the major mediator of Notch signaling, the transcription factor RBP-J, is essential to maintain this pool of progenitor cells in an undifferentiated state. In the absence of RBP-J, these cells undergo uncontrolled myogenic differentiation, leading to a depletion of the progenitor pool. This results in a lack of muscle growth in development and severe muscle hypotrophy. In addition, satellite cells are not formed late in fetal development in conditional RBP-J mutant mice. We conclude that RBP-J is required in the developing muscle to set aside proliferating progenitors and satellite cells.

Wnt Signaling Pathway in Mammary Gland Development and Carcinogenesis

The signaling pathway mediated by Wingless-type (Wnt) proteins is highly conserved in evolution. This pivotal pathway is known to regulate cell fate decisions, cell proliferation, morphology, migration, apoptosis, differentiation and stem cell self-renewal. It currently includes the canonical or Wnt/beta-catenin pathway in which Wnt proteins bind to 'frizzled' receptors, which leads to downstream activation of gene transcription by beta-catenin. Second, the noncanonical or beta-catenin-independent pathways are now known to be mediated by three possible mechanisms: (1) the Wnt/Ca(2+) pathway, (2) the Wnt/G protein signaling pathway, and (3) the Wnt/PCP or planar cell polarity pathway. Wnt signaling is implicated at several stages of mammary gland growth and differentiation, and possibly in the involution of mammary gland following lactation. Recent evidence suggests the role of Wnt signaling in human breast cancer involves elevated levels of nuclear and/or cytoplasmic beta-catenin using immunohistochemistry, overexpression or downregulation of specific Wnt proteins, overexpression of CKII and sFRP4, downregulation of WIF-1 and sFRP1, as well as amplification of DVL-1. Further research is required to determine how Wnt signaling is involved in the development of different histological types of breast cancer and whether it promotes the viability of cancer stem cells or not.

Amniote somite derivatives

Somites are segments of paraxial mesoderm that give rise to a multitude of tissues in the vertebrate embryo. Many decades of intensive research have provided a wealth of data on the complex molecular interactions leading to the formation of various somitic derivatives. In this review, we focus on the crucial role of the somites in building the body wall and limbs of amniote embryos. We give an overview on the current knowledge on the specification and differentiation of somitic cell lineages leading to the development of the vertebral column, skeletal muscle, connective tissue, meninges, and vessel endothelium, and highlight the importance of the somites in establishing the metameric pattern of the vertebrate body.

Tbx1 regulation of myogenic differentiation in the limb and cranial mesoderm

The T-box transcription factor Tbx1 has been implicated in DiGeorge syndrome, the most frequent syndrome due to a chromosomal deletion. Gene inactivation of Tbx1 in mice results in craniofacial and branchial arch defects, including myogenic defects in the first and second branchial arches. A T-box binding site has been identified in the Xenopus Myf5 promoter, and in other species, T-box genes have been implicated in myogenic fate. Here we analyze Tbx1 expression in the developing chick embryo relating its expression to the onset of myogenic differentiation and cellular fate within the craniofacial mesoderm. We show that Tbx1 is expressed before capsulin, the first known marker of branchial arch 1 and 2 muscles. We also show that, as in the mouse, Tbx1 is expressed in endothelial cells, another mesodermal derivative, and, therefore, Tbx1 alone cannot specify the myogenic lineage. In addition, Tbx1 expression was identified in both chick and mouse limb myogenic cells, initially being restricted to the dorsal muscle mass, but in contrast, to the head, here Tbx1 is expressed after the onset of myogenic commitment. Functional studies revealed that loss of Tbx1 function reduces the number of myocytes in the head and limb, whereas increasing Tbx1 activity has the converse effect. Finally, analysis of the Tbx1-mesoderm-specific knockout mouse demonstrated the cell autonomous requirement for Tbx1 during myocyte development in the cranial mesoderm.

Shaping leg muscles in Drosophila: role of ladybird, a conserved regulator of appendicular myogenesis

Legs are locomotor appendages used by a variety of evolutionarily distant vertebrates and invertebrates. The primary biological leg function, locomotion, requires the formation of a specialised appendicular musculature. Here we report evidence that ladybird, an orthologue of the Lbx1 gene recognised as a hallmark of appendicular myogenesis in vertebrates, is expressed in leg myoblasts, and regulates the shape, ultrastructure and functional properties of leg muscles in Drosophila. ladybird expression is progressively activated in myoblasts associated with the imaginal leg disc and precedes that of the founder cell marker dumbfounded. The RNAi-mediated attenuation of ladybird expression alters properties of developing myotubes, impairing their ability to grow and interact with the internal tendons and epithelial attachment sites. It also affects sarcomeric ultrastructure, resulting in reduced leg muscle performance and impaired mobility in surviving flies. The over-expression of ladybird also results in an abnormal pattern of dorsally located leg muscles, indicating different requirements for ladybird in dorsal versus ventral muscles. This differential effect is consistent with the higher level of Ladybird in ventrally located myoblasts and with positive ladybird regulation by extrinsic Wingless signalling from the ventral epithelium. In addition, ladybird expression correlates with that of FGF receptor Heartless and the read-out of FGF signalling downstream of FGF. FGF signals regulate the number of leg disc associated myoblasts and are able to accelerate myogenic differentiation by activating ladybird, leading to ectopic muscle fibre formation. A key role for ladybird in leg myogenesis is further supported by its capacity to repress vestigial and to down-regulate the vestigial-governed flight muscle developmental programme. Thus in Drosophila like in vertebrates, appendicular muscles develop from a specialised pool of myoblasts expressing ladybird/Lbx1. The ladybird/Lbx1 gene family appears as a part of an ancient genetic circuitry determining leg-specific properties of myoblasts and making an appendage adapted for locomotion.

Pitx2 promotes development of splanchnic mesoderm-derived branchiomeric muscle

Recent experiments, showing that both cranial paraxial and splanchnic mesoderm contribute to branchiomeric muscle and cardiac outflow tract (OFT) myocardium, revealed unexpected complexity in development of these muscle groups. The Pitx2 homeobox gene functions in both cranial paraxial mesoderm, to regulate eye muscle, and in splanchnic mesoderm to regulate OFT development. Here, we investigated Pitx2 in branchiomeric muscle. Pitx2 was expressed in branchial arch core mesoderm and both Pitx2 null and Pitx2 hypomorphic embryos had defective branchiomeric muscle. Lineage tracing with a Pitx2cre allele indicated that Pitx2 mutant descendents moved into the first branchial arch. However, markers of both undifferentiated core mesoderm and specified branchiomeric muscle were absent. Moreover, lineage tracing with a Myf5cre allele indicated that branchiomeric muscle specification and differentiation were defective in Pitx2 mutants. Conditional inactivation in mice and manipulation of Pitx2 expression in chick mandible cultures revealed an autonomous function in expansion and survival of branchial arch mesoderm.

The eventful somite: patterning, fate determination and cell division in the somite

The segmental somites not only determine the vertebrate body plan, but also represent turntables of cell fates. The somite is initially naive in terms of its fate restriction as shown by grafting and rotation experiments whereby ectopically grafted or rotated tissue of newly formed somites yielded the same pattern of normal derivatives. Somitic derivatives are determined by local signalling between adjacent embryonic tissues, in particular the neural tube, notochord, surface ectoderm and the somitic compartments themselves. The correct spatio-temporal specification of the deriving tissues, skeletal muscle, cartilage, endothelia and connective tissue is achieved by a sequence of morphogenetic changes of the paraxial mesoderm, eventually leading to the three transitory somitic compartments: dermomyotome, myotome and sclerotome. These structures are specified along a double gradient from dorsal to ventral and from medial to lateral. The establishment and controlled disruption of the epithelial state of the somitic compartments are crucial for development. In this article, we give a synopsis of some of the most important signalling events involved in somite patterning and cell fate decisions. Particular emphasis has been laid on the issue of epithelio-mesenchymal transition and different types of cell division in the somite.

The Wnt/beta-catenin pathway regulates Gli-mediated Myf5 expression during somitogenesis

Canonical Wnt/beta-catenin signaling regulates the activation of the myogenic determination gene Myf5 at the onset of myogenesis, but the underlying molecular mechanism is unknown. Here, we report that the Wnt signal is transduced in muscle progenitor cells by at least two Frizzled (Fz) receptors (Fz1 and/or Fz6), through the canonical beta-catenin pathway, in the epaxial domain of newly formed somites. We show that Myf5 activation is dramatically reduced by blocking the Wnt/beta-catenin pathway in somite progenitor cells, whereas expression of activated beta-catenin is sufficient to activate Myf5 in somites but not in the presomitic mesoderm. In addition, we identified Tcf/Lef sequences immediately 5' to the Myf5 early epaxial enhancer. These sites determine the correct spatiotemporal expression of Myf5 in the epaxial domain of the somite, mediating the synergistic action of the Wnt/beta-catenin and the Shh/Gli pathways. Taken together, these results demonstrate that Myf5 is a direct target of Wnt/beta-catenin, and that its full activation requires a cooperative interaction between the canonical Wnt and the Shh/Gli pathways in muscle progenitor cells.

Regulation of ectodermal Wnt6 expression by the neural tube is transduced by dermomyotomal Wnt11: a mechanism of dermomyotomal lip sustainment

Ectodermal Wnt6 plays an important role during development of the somites and the lateral plate mesoderm. In the course of development, Wnt6expression shows a dynamic pattern. At the level of the segmental plate and the epithelial somites, Wnt6 is expressed in the entire ectoderm overlying the neural tube, the paraxial mesoderm and the lateral plate mesoderm. With somite maturation, expression becomes restricted to the lateral ectoderm covering the ventrolateral lip of the dermomyotome and the lateral plate mesoderm. To study the regulation of Wnt6 expression, we have interfered with neighboring signaling pathways. We show that Wnt1 and Wnt3a signaling from the neural tube inhibit Wnt6 expression in the medial surface ectoderm via dermomyotomal Wnt11. We demonstrate that Wnt11 is an epithelialization factor acting on the medial dermomyotome, and present a model suggesting Wnt11 and Wnt6 as factors maintaining the epithelial nature of the dorsomedial and ventrolateral lips of the dermomyotome, respectively,during dermomyotomal growth.

Expression of avian C-terminal binding proteins (Ctbp1 and Ctbp2) during embryonic development

C-terminal binding proteins (CtBPs) are transcriptional corepressors of mediators of Notch, Wnt, and other signalling pathways. Thus, they are potential players in the control of several developmentally important processes, including segmentation, somitogenesis, and neural tube and limb patterning. We have cloned the avian orthologues of Ctbp1 and Ctbp2 and examined their expression pattern by whole-mount in situ hybridization between Hamburger and Hamilton (HH) stages 3 and 24. Both Ctbp genes show similar expression patterns during embryonic development, and both are detected from HH stage 3 in the developing central nervous system, by HH stage 7 in the paraxial mesoderm and later in the limb bud. In most places, Ctbp1 and Ctbp2 are expressed in overlapping domains. However, there are interesting domains and/or temporal expression patterns that are specific to each Ctbp gene. For instance, Ctbp1 is predominantly expressed in the epiblast, whereas Ctbp2 is in the primitive streak at HH stage 3. However, by HH stage 4, both genes are found in the primitive streak and in the ectoderm. Similarly, although both genes display similar expression patterns in early somitogenesis, in mature somites, Ctbp1 transcripts are located in myotomal cells, whereas Ctbp2 transcripts are observed in dermomyotomal cells. Finally, we found that emigrating neural crest cells express Ctbp2, whereas dorsal root ganglia express Ctbp1. These data suggest that Ctbp1 and Ctbp2 may be functionally redundant in some tissues and have unique functions in other tissues.

Novel expression patterns of Pax3/Pax7 in early trunk neural crest and its melanocyte and non-melanocyte lineages in amniote embryos

Neural crest cells are considered a key vertebrate feature that is studied intensively because of their relevance to development and evolution. Here we report the expression of Pax7 in the dorsal non-neural ectoderm and in the trunk neural crest of the early chick embryo. Pax7 is expressed in the trunk neural crest migrating along the ventral and dorsolateral routes. Pax7 is first downregulated in the neural crest-derived neuronal precursors, secondly in the glial, and finally in the melanocyte precursors. Conserved developmental expression in the melanocyte lineage of both Pax3 and Pax7 was evidenced in chick and quail, but only Pax3 in mouse and rat.

Spatial relations between avian craniofacial neural crest and paraxial mesoderm cells

Fate maps based on quail-chick grafting of avian cephalic neural crest precursors and paraxial mesoderm cells have identified the majority of derivatives from each population but have not unequivocally resolved the precise locations of and population dynamics at the interface between them. The relation between these two mesenchymal tissues is especially critical for the development of skeletal muscles, because crest cells play an essential role in their differentiation and subsequent spatial organization. It is not known whether myogenic mesoderm and skeletogenic neural crest cells establish permanent relations while en route to their final destinations, or later at the sites where musculoskeletal morphogenesis is completed. We applied beta-galactosidase-encoding, replication-incompetent retroviruses to paraxial mesoderm, to crest progenitors, or at the interface between mesodermal and overlying neural crest as both were en route to branchial or periocular regions in chick embryos. With respect to skeletal structures, the results identify the avian neural crest:mesoderm boundary at the junction of the supraorbital and calvarial regions of the frontal bone, lateral to the hypophyseal foramen, and rostral to laryngeal cartilages. Therefore, in the chick embryo, most of the frontal and the entire parietal bone are of mesodermal, not neural crest, origin. Within paraxial mesoderm, the progenitors of each lineage display different behaviors. Chondrogenic cells are relatively stationary and intramembranous osteogenic cells move only in transverse planes around the brain. Angioblasts migrate invasively in all directions. Extraocular muscle precursors form tightly aggregated masses that en masse cross the crest:mesoderm interface to enter periocular territories, while branchial myogenic lineages shift ventrally coincidental with the movements of corresponding neural crest cells. En route to the branchial arches, myogenic mesoderm cells do not maintain constant, nearest-neighbor relations with adjacent, overlying neural crest cells. Thus, progenitors of individual muscles do not establish stable, permanent relations with their connective tissues until both populations reach the sites of their morphogenesis within branchial arches or orbital regions.

Inhibitors of CXCR4 affect the migration and fate of CXCR4+ progenitors in the developing limb of chick embryos

Chemokines and their receptors play major roles in numerous physiological and pathological processes during development and disease. CXCR4 is the most abundantly expressed chemokine receptor during development. In contrast to other chemokine receptors, CXCR4 binds and is activated exclusively by its ligand stromal derived factor-1 (SDF-1) or CXCL12. SDF-1 signaling has a wide range of effects on CXCR4-expressing cells depending on the cell type ranging from cell growth to adhesion, chemotaxis, and migration. CXCR4 also serves as a co-receptor for HIV-1 entry into T-cells and has been implicated in the pathogenesis of rheumatoid arthritis and cancer growth and invasion. Numerous inhibitors and antagonists of CXCR4 have been produced and are being tested for their efficiency to target its role in pathogenesis. Our initial expression analysis revealed that CXCR4 is expressed by the migrating myogenic and angiogenic precursors in the developing chick limb. In this study, we used the most specific peptidic inhibitors of CXCR4, T140 and its analog TN14003, to analyse the effect of blocking CXCR4/SDF-1 signaling on the undetermined bioptent migratory progenitors in the developing chick limb. Our results point to defects in migration and an altered differentiation program of these CXCR4-expressing progenitor pool in the limb.

Ectodermal Wnt-6 promotes Myf5-dependent avian limb myogenesis

Limb muscles of vertebrates are derived from precursor cells that migrate from the lateral edge of the dermomyotome into the limb bud. Although several signaling molecules have been reported to be involved in the process of limb myogenesis, none of their activities has led to a consolidate idea about the limb myogenic pathway. Particularly, the role of ectodermal signals in limb myogenesis is still obscure. Here, we investigated the role of the ectoderm and ectodermal Wnt-6 during limb muscle development. We found that ectopic expression of Wnt-6 in the limb bud specifically extends the expression domains of Pax3, Paraxis, Myf5, Myogenin, Desmin and Myosin heavy chain (MyHC) but inhibits MyoD expression. Ectoderm removal results in a loss of expression of all of these myogenic markers. We show that Wnt-6 can compensate the absence of the ectoderm by rescuing the expression of Pax3, Paraxis, Myf5, Myogenin, Desmin and MyHC but not MyoD. These results show that, in chick, at least two signals from the limb ectoderm are necessary for muscle development. One of the signals is Wnt-6, which plays a unique role in promoting limb myogenesis via Pax3/Paraxis-Myf5, whereas the other putative signaling pathway involving MyoD expression is negatively regulated by Wnt-6 signaling.

Distinctive morphological and gene/protein expression signatures during myogenesis in novel cell lines from extraocular and hindlimb muscle

Skeletal muscles are not created equal. The underutilized concept of muscle allotypes defines distinct muscle groups that differ in their intrinsic capacity to express novel traits when exposed to a facilitating extrinsic environment. Allotype-specific traits may have significance as determinants of the preferential involvement or sparing of muscle groups that is observed in a variety of neuromuscular diseases. Little is known, however, of the developmental mechanisms underlying the distinctive skeletal muscle allotypes. The lack of appropriate in vitro models, to dissociate the cell-autonomous and non-cell-autonomous mechanisms behind allotype diversity, has been a barrier to such studies. Here, we derived novel cell lines from the extraocular and hindlimb muscle allotypes and assessed their similarities and differences during early myogenesis using morphological and gene/protein expression profiling tools. Our data establish that there are fundamental differences in the transcriptional and cellular signaling pathways used by the two myoblast lineages. Taken together, these data show that myoblast lineage plays a significant role in the divergence of the distinctive muscle groups or allotypes.

Dkk1, -2, and -3 expression in mouse craniofacial development

The Dickkopf family is important for embryogenesis and postnatal development and growth. Dkk1 is a strong head inducer and knockout of this gene leads to absence of anterior head structures, which are predominantly formed through neural crest migration. During early craniofacial development, Dkk1 to Dkk3 show developmentally regulated expression in a number of elements. However, their expression and roles in late times of craniofacial development are largely unknown. This study focuses on the expression profile of Dkk1-3 on late embryonic and early postnatal stages. It was found that Dkks were involved in a variety of craniofacial developmental processes, including facial outgrowth, myogenesis, osteogenesis, palatogenesis, olfactory epithelium and tooth development; and the expression persisted to postnatal stage in the muscles and bones. Their expression patterns suggest important roles in these processes; further study is warranted to elucidate these roles.

Wnt/beta-catenin signaling activates growth-control genes during overload-induced skeletal muscle hypertrophy

Beta-catenin is a transcriptional activator shown to regulate the embryonic, postnatal, and oncogenic growth of many tissues. In most research to date, beta-catenin activation has been the unique downstream function of the Wnt signaling pathway. However, in the heart, a Wnt-independent mechanism involving Akt-mediated phosphorylation of glycogen synthase kinase (GSK)-3beta was recently shown to activate beta-catenin and regulate cardiomyocyte growth. In this study, results have identified the activation of the Wnt/beta-catenin pathway during hypertrophy of mechanically overloaded skeletal muscle. Significant increases in beta-catenin were determined during skeletal muscle hypertrophy. In addition, the Wnt receptor, mFrizzled (mFzd)-1, the signaling mediator disheveled-1, and the transcriptional co-activator, lymphocyte enhancement factor (Lef)-1, are all increased during hypertrophy of the overloaded mouse plantaris muscle. Experiments also determined an increased association between GSK-3beta and the inhibitory frequently rearranged in advanced T cell-1 protein with no increase in GSK-3beta phosphorylation (Ser9). Finally, skeletal muscle overload resulted in increased nuclear beta-catenin/Lef-1 expression and induction of the transcriptional targets c-Myc, cyclin D1, and paired-like homeodomain transcription factor 2. Thus this study provides the first evidence that the Wnt signaling pathway induces beta-catenin/Lef-1 activation of growth-control genes during overload induced skeletal muscle hypertrophy.

Comparative analysis of the expression patterns of Wnts during chick limb development

Wnts control a number of processes during limb development-from initiating outgrowth and controlling patterning, to regulating cell differentiation in a number of tissues. We analyzed the expression pattern of various Wnts (4, 5a, 5b, 6, 11, and 14) in whole mount in situ hybridization during chick wing development. From HH stage 26, expression of Wnt 4 is observed in the central elbow region and wrist-forming regions, and during later stages, expression is seen in the joint-forming regions of the whole limb. Wnt 5a is expressed throughout the limb mesenchyme during early limb developmental stages, and later, at HH stage 23, it becomes predominantly confined to the distal tip, leaving low expression levels proximally. At HH stage 29, expression at the distal tip is restricted to the interdigital regions, and at day 8, expression is seen in the region surrounding the phalanges. Wnt 5b expression is first observed in the AER at HH stage 20 and later in the dorsal and ventral mesenchyme surrounding the cartilage elements of the limb. Expression of Wnt 6 is observed from HH stage 17 until day 8 in the dorsal and ventral ectoderm and also in the dorsoventral limb boundaries. Expression of Wnt 11 is observed in the proximal dorsal mesenchyme of the limb from HH stage 23 onward and later in the dorsal and ventral subectodermal mesenchyme and in the regions adjacent to the digits at day 8. Weak expression of Wnt 14 is observed at the proximal mesenchyme of the limb at HH stage 23; later, it extends as a transverse strip surrounding the cartilage elements as well as in the interdigital mesenchyme.

Wnt10b deficiency promotes coexpression of myogenic and adipogenic programs in myoblasts

Adult myoblasts retain plasticity in developmental potential and can be induced to undergo myogenic, adipogenic, or osteoblastogenic differentiation in vitro. In this report, we show that the balance between myogenic and adipogenic potential in myoblasts is controlled by Wnt signaling. Furthermore, this balance is altered during aging such that aspects of both differentiation programs are coexpressed in myoblasts due to decreased Wnt10b abundance. Mimicking Wnt signaling in aged myoblasts through inhibition of glycogen synthase kinase or through overexpression of Wnt10b resulted in inhibition of adipogenic gene expression and sustained or enhanced myogenic differentiation. On the other hand, myoblasts isolated from Wnt10b null mice showed increased adipogenic potential, likely contributing to excessive lipid accumulation in actively regenerating myofibers in vivo in Wnt10b-/- mice. Whereas Wnt10b deficiency contributed to increased adipogenic potential in myoblasts, the augmented myogenic differentiation potential observed is likely the result of a compensatory increase in Wnt7b during differentiation of Wnt10b-/- myoblasts. No such compensation was apparent in aged myoblasts and in fact, both Wnt5b and Wnt10b were down-regulated. Thus, alteration in Wnt signaling in myoblasts with age may contribute to impaired muscle regenerative capacity and to increased muscle adiposity, both characteristic of aged muscle.

Distinct Wnt members regulate the hierarchical morphogenesis of skin regions (spinal tract) and individual feathers

Skin morphogenesis occurs in successive stages. First, the skin forms distinct regions (macropatterning). Then skin appendages with particular shapes and sizes form within each region (micropatterning). Ectopic DKK expression inhibited dermis formation in feather tracts and individual buds, implying the importance of Wnts, and prompted the assessment of individual Wnt functions at different morphogenetic levels using the feather model. Wnt 1, 3a, 5a and 11 initially were expressed moderately throughout the feather tract then were up-regulated in restricted regions following two modes: Wnt 1 and 3a became restricted to the placodal epithelium, then to the elongated distal bud epidermis; Wnt 5a and 11 intensified in the inter-tract region and interprimordia epidermis or dermis, respectively, then appeared in the elongated distal bud dermis. Their role in feather tract formation was determined using RCAS mediated misexpression in ovo at E2/E3. Their function in periodic feather patterning was examined by misexpression in vitro using reconstituted E7 skin explant cultures. Wnt 1 reduced spinal tract size, but enhanced feather primordia size. Wnt 3a increased dermal thickness, expanded the spinal tract size, reduced interbud domain spacing, and produced non-tapering "giant buds". Wnt 11 and dominant negative Wnt 1 enhanced interbud spacing, and generated thinner buds. In cultured dermal fibroblasts, Wnt 1 and 3a stimulated cell proliferation and activated the canonical beta-catenin pathway. Wnt 11 inhibited proliferation but stimulated migration. Wnt 5a and 11 triggered the JNK pathway. Thus distinctive Wnts have positive and negative roles in forming the dermis, tracts, interbud spacing and the growth and shaping of individual buds.

Six1 and Eya1 expression can reprogram adult muscle from the slow-twitch phenotype into the fast-twitch phenotype

Muscle fibers show great differences in their contractile and metabolic properties. This diversity enables skeletal muscles to fulfill and adapt to different tasks. In this report, we show that the Six/Eya pathway is implicated in the establishment and maintenance of the fast-twitch skeletal muscle phenotype. We demonstrate that the MEF3/Six DNA binding element present in the aldolase A pM promoter mediates the high level of activation of this promoter in fast-twitch glycolytic (but not in slow-twitch) muscle fibers. We also show that among the Six and Eya gene products expressed in mouse skeletal muscle, Six1 and Eya1 proteins accumulate preferentially in the nuclei of fast-twitch muscles. The forced expression of Six1 and Eya1 together in the slow-twitch soleus muscle induced a fiber-type transition characterized by the replacement of myosin heavy chain I and IIA isoforms by the faster IIB and/or IIX isoforms, the activation of fast-twitch fiber-specific genes, and a switch toward glycolytic metabolism. Collectively, these data identify Six1 and Eya1 as the first transcriptional complex that is able to reprogram adult slow-twitch oxidative fibers toward a fast-twitch glycolytic phenotype.

Hedgehog can drive terminal differentiation of amniote slow skeletal muscle

BACKGROUND

Secreted Hedgehog (Hh) signalling molecules have profound influences on many developing and regenerating tissues. Yet in most vertebrate tissues it is unclear which Hh-responses are the direct result of Hh action on a particular cell type because Hhs frequently elicit secondary signals. In developing skeletal muscle, Hhs promote slow myogenesis in zebrafish and are involved in specification of medial muscle cells in amniote somites. However, the extent to which non-myogenic cells, myoblasts or differentiating myocytes are direct or indirect targets of Hh signalling is not known.

RESULTS

We show that Sonic hedgehog (Shh) can act directly on cultured C2 myoblasts, driving Gli1 expression, myogenin up-regulation and terminal differentiation, even in the presence of growth factors that normally prevent differentiation. Distinct myoblasts respond differently to Shh: in some slow myosin expression is increased, whereas in others Shh simply enhances terminal differentiation. Exposure of chick wing bud cells to Shh in culture increases numbers of both muscle and non-muscle cells, yet simultaneously enhances differentiation of myoblasts. The small proportion of differentiated muscle cells expressing definitive slow myosin can be doubled by Shh. Shh over-expression in chick limb bud reduces muscle mass at early developmental stages while inducing ectopic slow muscle fibre formation. Abundant later-differentiating fibres, however, do not express extra slow myosin. Conversely, Hh loss of function in the limb bud, caused by implanting hybridoma cells expressing a functionally blocking anti-Hh antibody, reduces early slow muscle formation and differentiation, but does not prevent later slow myogenesis. Analysis of Hh knockout mice indicates that Shh promotes early somitic slow myogenesis.

CONCLUSIONS

Taken together, the data show that Hh can have direct pro-differentiative effects on myoblasts and that early-developing muscle requires Hh for normal differentiation and slow myosin expression. We propose a simple model of how direct and indirect effects of Hh regulate early limb myogenesis.

A proliferative role for Wnt-3a in chick somites

The proper patterning of somites to give rise to sclerotome, dermomyotome, and myotome involves the coordination of many different cellular processes, including lineage specification, cell proliferation, cell death, and differentiation, by intercellular signals. One such family of secreted signaling proteins known to influence somite patterning is the Wnt family. Although the participation of Wnt-3a in the patterning of dorsal structures in the somite is well established, no clear consensus has emerged about the cellular processes that are governed by Wnt-3a in the somite. The recent demonstration that Wnt-3a has a proliferative role in the neural tube [Development 129 (2002) 2087] suggested that Wnt-3a might also act to regulate proliferation in somites. To test this hypothesis, we first analyzed the effects of Wnt-3a on segmental plate and somite explants (from Hamburger and Hamilton stage 10 chick embryos) grown in culture. These studies indicate that Wnt-3a is capable of maintaining and/or inducing expression of both Pax-3 and Pax-7, transcription factors that have been implicated in proliferation. To directly test for a role in proliferation, explants were immunostained with antibodies against phospho-histone H3. Explants treated with Wnt-3a show an increase in the percentage of cells expressing phospho-histone H3 as compared to controls. To test the proliferative effect of Wnt-3a in vivo, we ectopically expressed Wnt-3a in chick neural tubes via electroporation. Consistent with previous studies, ectopic expression of Wnt-3a in vivo results in a mediolateral expansion of the dermomyotome and myotome. We now show that proliferation of dorsal/dermomyotomal cells is significantly enhanced by ectopic Wnt-3a. Collectively, our explant and in vivo studies indicate that an increase in proliferation plays an important role in the expansion of the dermomyotome and myotome in Wnt-3a-treated embryos. Furthermore, our results demonstrate that small changes in proliferation can dramatically influence patterning and morphogenesis.

Wnts and wing: Wnt signaling in vertebrate limb development and musculoskeletal morphogenesis

In the past twenty years, secreted signaling molecules of the Wnt family have been found to play a central role in controlling embryonic development from hydra to human. In the developing vertebrate limb, Wnt signaling is required for limb bud initiation, early limb patterning (which is governed by several well-characterized signaling centers), and, finally, late limb morphogenesis events. Wnt ligands are unique, in that they can activate several different receptor-mediated signal transduction pathways. The most extensively studied Wnt pathway is the canonical Wnt pathway, which controls gene expression by stabilizing beta-catenin in regulating a diverse array of biological processes. Recently, more attention has been given to the noncanonical Wnt pathway, which is beta-catenin-independent. The noncanonical Wnt pathway signals through activating Ca(2+) flux, JNK activation, and both small and heterotrimeric G proteins, to induce changes in gene expression, cell adhesion, migration, and polarity. Abnormal Wnt signaling leads to developmental defects and human diseases affecting either tissue development or homeostasis. Further understanding of the biological function and signaling mechanism of Wnt signaling is essential for the development of novel preventive and therapeutic approaches of human diseases. This review provides a critical perspective on how Wnt signaling regulates different developmental processes. As Wnt signaling in tumor formation has been reviewed extensively elsewhere, this part is not included in the review of the clinical significance of Wnt signaling.

A Tcf4-Positive Mesodermal Population Provides a Prepattern for Vertebrate Limb Muscle Patterning

Nai;ve myogenic cells migrate from the somites into the developing vertebrate limb, where they simultaneously differentiate into myotubes and form distinct anatomical muscles. Limb signals have been hypothesized to direct the pattern of muscles formed, but the molecular nature of these signals and the identity of the cells that produce them have remained unclear. We have identified a population of lateral plate-derived limb mesodermal cells in both chick and mouse that expresses the transcription factor Tcf4 in a muscle-specific pattern independently of the muscle cells themselves. Functional experiments in the chick demonstrate that TCF4 and the Wnt-beta-catenin pathway in these limb mesodermal cells are critical for muscle patterning. We propose that Tcf4-expressing cells establish a prepattern in the limb mesoderm that determines the sites of myogenic differentiation and thus establishes the basic pattern of limb muscles.

A second canon. Functions and mechanisms of beta-catenin-independent Wnt signaling

More is becoming known about so-called noncanonical Wnt pathways that signal independently of beta-catenin. Here we review recent developments in both the functions and mechanisms of noncanonical Wnt signaling. We also discuss some unresolved and vexing questions. How many noncanonical Wnt pathways are there? How extensive are the parallels between Drosophila planar polarization and vertebrate convergence and extension? Last, we will outline some challenges and difficulties we foresee for this exciting but still very young field.